|Year : 1966 | Volume
| Issue : 2 | Page : 55-74
Srinivasan oration- a study of optic atrophy
Grant Medical College and Sir Cowasji Jehangir Ophthalmic Hospital, Bombay, India
|Date of Web Publication||12-Jan-2008|
H D Dastoor
Grant Medical College and Sir Cowasji Jehangir Ophthalmic Hospital, Bombay
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Dastoor H D. Srinivasan oration- a study of optic atrophy. Indian J Ophthalmol 1966;14:55-74
At the outset I am grateful to our Society in honouring me with the Srinivasan Award which I deeply appreciate to cherish the memory of my dear departed friend and colleague, late Dr. E. V. Srinivasan whose absence many of us feel on the days of our annual meetings. His one great desire in his later years was to establish this Award and after him and onwards, for the fulfillment of which I was instrumental, little realising then that the same would be awarded to me this day.
The subject of Optic Atrophy that 1 have chosen is the one that afflicts a high proportion in the world of the blind whose teeming millions we have amongst us where therapy is almost helpless in the majority of this condition. Though the therapy is limited, the only effective cure is to treat the causative factor giving rise to the atrophy. From this point of view the present study is based on the varied etiological factors giving rise to optic atrophies as reviewed from a large number of our cases as well as from references to standard books and literature on this subject.
To understand some of the important aspects pertaining to the disease it is necessary to consider the anatomical, physiological and histopathological aspects for the treatise.
| Anatomical and Physiological Considerations of the Optic Nerve|| |
The reactions of the optic nerve to injury and disease differ in many respects from other sensory nerves owing to its peculiar anatomical and physiological features. The optic nerve is part of the second neurone in the visual pathway which consists of ganglion cells, the retinal nerve fibers, the optic nerve, chiasma and the optic tract upto the geniculate body. It consists of a trunk of about a million axons arising from the ganglion cells of the retina. It is not like a peripheral nerve but a link in the chain of fibers which run from the rods and cones to the occipital cortex. The nerve fibers are really a part of a tract of the central nervous system, since the retina has developed from the optic vesicle, which is an outpouching of the original neural tube. The optic nerve fibers are composed of the usual axon and a myelin sheath, but entirely lack a neurilemmal sheath. Instead of being separated from one another by neurilemma as are other sensory nerves, neuroglial cells are scattered between them and separate them. In this respect the optic nerve fibers again resemble the fiber tracts of the brain and spinal cord, which too are myelinated but are devoid of any neurilemmal sheaths.
Myelination of the optic nerve fibers begins very late, and it is not until about the seventh month of fetal life that this has progressed down the nerve as far as the lamina cribrosa. The deposit of myelin usually stops at this point, but occasionally it extends beyond the lamina, and the nerve fibers in the retina then appear as semiopaque white sheets, in one or more quadrants adjacent to the optic disc. This constitutes the condition of retained medullated nerve sheaths.
It is to be expected that the anatomic optic nerve, which is actually a tract in the path of conduction, should suffer from and react to injuries and diseases which commonly affect neurons of the second order of the central nervous system and not sensory nerves.
A further point of peculiarity of the optic nerve, as compared with other sensory nerves in the body, is the fact that it is surrounded by prolongations of the sheaths which encase the brain, i.e. the dura, pia and arachnoid. The presence of these layers, and the spaces between them and the nerve, account for many of the peculiarities in the pathologic reactions of the optic nerve. The sheaths surrounding the optic nerve are supplied with many sensory nerve fibers. This gives rise to pain in inflamatory conditions of the nerve and the orbit, such as occurs in retrobulbar neuritis in the acute stage. The fact that patients with choked disk suffer no pain, although the nerve head itself is edematous and the fibers pressed upon, proves that the pain in retrobulbar neuritis arises from the sheaths, and not from the nerve fibers themselves. Since both the subarachnoid and the subdural spaces are in direct communication with those of the brain, these spaces are found to be distended with fluid in the presence of increased intracranial pressure.
| The Optic Nerve|| |
The optic nerve which consists of axons from the ganglion cells of the retina extends to the optic chiasma and it is 40 to 50 mm. in length. These axons course towards the optic papilla in the nerve fiber layer of the retina. The nerve emerges from the eye through a short circular opening in the sclera situated 1 mm. below and 3 mm. nasal to the posterior pole of the eye (macula). The nerve fibers become myelinated upon leaving the eye and are supported by neuroglia, thus increasing the diameter from 1.5 mm. within the sclera to 3 mm. within the orbit. It is rounded in section while in the orbit and flattened within the cranium. It is anatomically divisible into four parts, intra-ocular (within the coats of the eyeball), intra-orbital, intra-osseous or canalicular and intracranial.
The relations of the optic nerve are very important from the clinical point of view.
The intraocular part of the optic nerve head merits a special description. As the nerve fibers converge from the retina they pile up at the margin of the disc to form the optic papilla,-a name given by Briggs in 1688. Since they pile up on the nasal side more than on the temporal, the prominence is more marked nasally. This gives rise to a central depression which is pushed temporally due to massing of nasal fibers.
The fibers which originate in the retina turn at a right-angle abruptly to traverse through the scleral canal (1 mm. in length) to form the nerve trunk.
The intraorbital part of the nerve measures about 25 mm. and proceeds backwards and medially in a sinuous manner to allow play of eye movements. Here it lies within the cone of recti muscles and is in close proximity of the superior and medial recti at the apex of the orbit, which is the cause of pain on moving the eyes up and in, in cases of optic neuritis. This part of the nerve is surrounded by the extension of the meningeal coverings, and spaces. A supravaginal lymph space encircles the dura itself except at the optic foramen, where the dura is firmly anchored to the overlying bone. Fine pial septa hearing blood vessels and capillaries pass into the nerve substance, dividing it into polygonal bundles.
Between the dural sheath and the muscle cone lies a variable mass of adipose tissue with many vessels, nerves and the ciliary ganglion embedded in the mass. [Figure - 1]
The opthalmic artery at first inferolateral, crosses under the optic nerve along with the nasociliary nerve and superior ophthalmic vein to gain a position lateral to the nerve after it gives off the central retinal branch which runs along the under surface of the nerve and enters it a cm. behind the sclera. Its passage into the nerve is characterised by two right-angled bends between which the nerve traverses the subarachnoid space and it is at this spot that it is subjected to pressure in increased intracranial tension.
Case No. 1.
An accident of unusual and tragic nature I had the occasion to see with my colleague Dr. Cooper, which brings out the applied anatomy of this region [Figure - 2], was when a general surgeon tried to inject alcohol into the spheno-palatine ganglion for spheno-palatine neurelgia. Immediately after the injection there was complete loss of vision with total paralysis of all ocular movements, without any relief of the spheno-palatine neurelgia. Evidently, as can be seen from the figure, the needle must have slipped past the maxillary nerve and entered the orbit through the inferior orbital fissure.
The intra osseous or intra canalicular part lies in the optic foramen and is 5 to 9 mm. in length. The ophthalmic artery lies inferolateral to it. The sphenoidal and posterior ethmoidal sinuses are separated from it medially by only a thin bony lamina. Thus sinus infection is quite prone to affect the nerve bringing about optic neuritis and subsequently optic atrophy, as was seen in two of my cases.
In the cranium (intra cranial) from the optic foraman to the chiasma it is about 10 mm. in length. The optic nerves lie above the cavernous sinus and converge over the diaphragma sellae that overlies the pituitary body to form the chiasma. Superior relations are the anterior perforated substance, the medial root of the olfactory tract and the anterior cerebral artery (which crosses from without inwards). Inferolaterally lies the internal carotid artery where it gives off its ophthalmic branch. It will he seen that the optic nerve here is surrounded all round by important structures. Any space occupying lesion in this area, either a tumour of the pituitary or an aneurysm of the internal carotid, would press over the optic nerve and cause atrophy.
It may be noted that besides the visual fibers, the optic nerve carries afferent pupillary reflex fibers, retinomotor (from brain to retina) and possibly also some inter-retinal and autonomic trophic fibers.
The arrangement of nerve fibers in the optic nerve and chiasma are too well-known to bear description here and can be seen in any text-book of ophthalmology.
| Blood Supply of the Optic Nerve and Chiasma|| |
The optic nerve and chiasma are nourished by the pial felt-work of capillaries and arterioles which reach down to the deeper layers in the interlacing septa as elsewhere in the central nervous system. This constitutes the peripheral system of vessels.
There is also an axial system of vessels running through the center of the nerve. Anterior to the entrance of the central retinal artery this function is taken up by the collaterals of the central artery which run forward towards the eyes, while the central retinal artery itself reserves blood for the retina. As the visual fibers pass backwards the nourishment is provided first by the ophthalmic arteries, then at the chiasma by the anterior cerebral and internal carotids while adjacent branches of the circle of Willis may also contribute.
Francois and Neetens (1955) drew special attention to the central optic nerve artery, distinct from the central retinal artery, both originating from the ophthalmic artery, entering the nerve independent of each other. This central optic nerve artery, after reaching the center of the nerve divides into an anterior and posterior branch which constitutes the major axial supply system of the optic nerve.
Thus, in the optic nerve there are two distinct supply lines, (1) the central retinal artery serves the visual elements of the inner layers of the retina right upto the periphery, and (2) the central artery of the optic nerve which serves mainly the supporting structure in the optic nerve and the papilla and a mm. or so of the retina beyond the edge of the disc. A cilioretinal artery from it may reach the macula in some cases and save macular vision in obstruction of the central retinal artery.
At the papilla a rich vascular network is formed by vessels derived from (a) the circle of Zinn, (b) pial vessels, (c) short posterior ciliary vessels, (d) anterior division of the central artery of the optic nerve. In this network the central retinal artery appears to contribute little blood if at all.
This concept of two independent supplies, one for the elements of the retina and the other for the papilla explains many points of pathological ambiguity. For example, if the effects of increased intracranial tension are due to pressure on the central retinal vein as it courses through the subarachmoid space, as is ordinarily supposed, why should the oedema and haemorrhages be confined to the disc area only in the initial stages and not to the whole retina as in thrombosis of the central retinal vein if this vein is pressed upon during its passage in the arachnoidal space? If we adopt the concept of two supply lines, if we consider the central vein of the optic nerve as independent smaller and at a lower pressure than the central retinal vein, the early inflammatory changes confined to the disc can be well understood.
Similarly, in primary type of optic atrophy, the central retinal blood-vessels remain unaffected at least in the early part whereas the vessels on the disc get lost, probably because of secondary changes in the supporting elements of the papilla which is supplied by the central vessels of the optic nerve.
One can explain on similar lines a pallor of the disc with a normally functioning optic nerve and vice versa a pink disc in the presence of atrophied nerve fibres. In this connection let me quote two cases from my clinical diary.
Case No. 2.
Boy aged 17, studying in college suddenly developed progressive dimness of vision which dropped down to less than 6/60 in both eyes. Fundal examination revealed a typical secondary optic atrophy which presented an alarming outlook. Clinical investigations revealed focal sepsis in the throat. Under treatment with massive doses of antibiotics, the vision gradually improved within a period of two months with 6/6 and J 1 vision in each eye. Even with regaining of normal vision the fundal picture of optic disc remained the same as before and even after a duration of almost 15 years it presents the same picture as before with perfectly normal vision.
Case No. 3.
Patient (male) aged 75 with bilateral aphakia having normal vision with glasses for over 7 years since the operation developed gradual dimness of vision in both eyes to below 6/60. Fundal examination revealed a classical picture of secondary optic atrophy. All clinical, neurological and pathological investigations proved negative. With the help of the general physician and neurologist the treatment was given with Iodides, B. Complex Vitamins and massive doses of antibiotics on emperical lines. There was gradual improvement of vision to 6/9 and J 2 in each eye but the optic atrophy presented the same picture as before. The vision remained good for over a period of 5 years during which he had periodic checkups.
| Degeneration and Regeneration|| |
The optic nerve fibers, like all other nerves in the body, will degenerate when their connections with the cell bodies which govern their nutrition have been severed. When it comes to regeneration, since the optic nerve fibers lack a neurilemmal sheath and resemble the white matter of the brain, their attempt at regeneration fails. Hence one cannot expect any recovery of function in cases in which the fibers have been severed, or in which the ganglion cells in the retina have been destroyed. Once the fiber has atrophied, it can never recover. On the other hand, apparent cessation of function may not mean that the nerve has degenerated. Pressure on the nerve from tumors, either in the orbit or inside the skull, may cause great functional embarrassment to the point of total loss of sight, yet when the pressure is removed, recovery may be rapid.
In the optic nerve, the medullary sheath and the axis cylinders may show degenerative changes, but in the absence of the neurilemmal sheath the nerve fibers do not regenerate. Thus optic atrophy is characterized by disappearance of axis cylinders and the myelin sheaths and a resultant profound shrinkage of the optic nerve. There is always a slight orderly proliferation of the astrocytes and an increase in the collagenous tissue, contained in the pial septum. Such is the case with simple optic atrophy. [Figure - 4] a, b However, in secondary and postneuritic optic atropy there are pronounced reactive alterations in the glial and mesenchymal tissues of the nerve head and hence the essential degenerative features may be obscured by the proliferation of astrocytes, fibrous connective tissue and blood vessels. [Figure - 4] c.
In animals, however, regeneration of some fibers can be seen after experimental sectioning of the nerve, indicating presence of centrifugal fibers from the brain e.g. in the cat. In more primitive animals e.g. the newt, a sectioned optic nerve regenerates completely.
It is necessary to emphasise that all non-traumatic types of damage to the optic nerve are vascular in nature. Deprivation of the blood-supply primarily depresses only the functioning of the nerve as demonstrated by depression of the visual field (contraction for smaller size test-objects). Such fields and lesions are capable of recovery if the vascular supply is restored. Only when this deprivation is continued over a considerable period that the function is totally lost as shown by contraction of the field for larger targets.
| Diagnosis of Optic Atrophy|| |
A survey of the available literature of the diagnosis of optic atrophy brings forth the fimiliar triad of symptoms
(1) Pallor of the disc, (2) Reduced visual acuity, (3) Defects in the visual field.
The reduced vision and the defects in the visual field are dependant upon the stage of the disease as well as on the type of nerve fibers implicated and to the extent to which they have undergone atrophy.
In a case of diminished vision diagnosis of optic atrophy depends on (a) pallor of the disc and (b) other changes in the fundus, which may or may not be present.
For reasons explained under blood-supply of the optic nerve, pallor alone is not sufficient for the diagnosis of optic atrophy, but pallor associated with loss of vision and/or demonstrable changes in the visual fields is sufficient evidence for actual or potential optic atrophy.
The line separating an apparent pallor and an actual one is thin. Pallor of the disc not associated with optic atrophy may be found in myopia and in infants. The disc is normally paler in advancing years than in youth because of the gradual narrowing of the blood vessels as age advances. Again, patients who have suffered from encephalomyelitis or disseminated sclerosis and who have temporarily had a reduction in visual acuity may exhibit discs which appear pale in their entirety or only in the temporal one third, but the visual acuity as well as the fields of such individuals may be normal and hence they cannot be said to suffer from optic atrophy. In such cases, the axis cylinders during the acute episodes have been depressed but not destroyed. As a converse of this, optic atrophy may be present when the nerve head appears grey red, especially after head injury in which the optic nerve is damaged, when the atrophy can be ophthalmoscopically recognised only a fortnight or 3 weeks after the injury.
Causes of Pallor
The normal colour of the disc is mainly dependent upon its blood vessels. The temporal aspects of the nerve is always relatively pale in the normal nerve. However, normally the temporal side has a lustre and an appreciable pink edge which is absent in the atrophic nerve.
In the central nervous system, loss of function is constantly associated with reduction of blood supply so is the case with the optic nerve which has ceased to function normally. There is loss of normal capillarity of the disc due to closure of the finer vessels. In addition to the reduction of the blood supply there is deposition of fibrin or filial tissue which replaces the nerve fibers as they disappear and which further accounts for the pallor associated with optic atrophy.
Various workers attribute the pallor to different causes. Adler has summarized the opinions of numerous observers that pallor of the disc may be due to one or all of the following factors; (a) low haemoglobin content of the blood, (b) diminution in the number of capillaries on the disc, (c) overgrowth of filial fibers or deposition of fibrin, and (d) actual loss of nerve fibers.
Of these factors the number of capillaries of the disc facilitates ready and precise ophthalmoscopic evaluation. Kestenbaum in 1946 pointed out their possible utility in the diagnosis of primary optic atrophy by what he calls the Kestenbaum Capillary Number Test. In order to set a numerical value on the degree of atrophy, the vessels which pass over the margin of the disc may be counted. One starts at the twelve O'Clock position and counts all the vessels crossing the margin, counting separately the arteries, veins and the small vessels. "Small vessels" mean the vessels which cannot be recognised as arteries or veins. The number of vessels passing over the margin in normal eyes is fairly constant. Usually 9 large vessels (4 or 5 veins and 4 or 5 arteries) and about 10 small vessels can be seen. Of course, there are many exceptions in normal eyes. Sometimes the arteries and veins branch repeatedly on the disc so that more than 9 large vessels pass over the disc's edge. In discs with irregular or inverse distribution of the vessels, the number test is unreliable. In a large majority of cases however the arrangement of the vessels is regular enough to permit the use of this test.
In primary optic atrophy, the number of arteries and veins remain unchanged but the number of small vessels is diminished to 6 or even less so that an approximate numerical measure of the degree of pallor is possible. The test may be of value in observation of the development of disease. [Figure - 5]
The different colours which characterize various forms of optic atrophy may be caused by the relative preponderance of the two factors viz. loss of capillaries and overgrowth of filial tissue. The chalky white disc of tabetic optic atrophy, with exposure of the lamina cribrosa, is due to a marked disappearance of the capillaries on the disc. The dirty white disc of post-papillitic atrophy, with filled-in cup and blurred disc margins is due to loss of capillaries with remaining oedema and deposits of fibrin and cellular debris, [Figure - 6],[Figure - 7] while the waxy disc seen in primary pigmentary degeneration of the retina is due to an overgrowth of glial tissue.
Hence one must not be hasty in judging the condition of the optic nerve from the appearance of the disc alone; for a pale disc due to loss of capillaries may not necessarily indicate a true atrophy of the optic nerve fibers and one cannot always tell the state of degeneration of the fibers from the colour of the disc. So often we see patients in whom the colour of the disc is extremely pale due to loss of capillaries, but who retain good vision and fields. This is particularly true in cases of advanced arteriosclerosis and in anemic patients.
| Classification of Optic Atrophies|| |
Different authors have tried to classify optic atrophies in various ways. Some try to classify on the ophthalmoscopic findings viz., Clinical Classification, whereas others try to base classification on the eetiological factors. These classifications are not ideal and there is a lot of intermingling of the clinical findings in both of these classifications. There is also a third classification based on pathological changes.
The Classical subdivision of optic atrophy into "Primary" and "Secondary" categories has no etiologic significance but they merely refer to the appearance of the disc. Primary optic atrophy is said to result when degeneration and replacement are orderly so that the glial replacement equals the nerve fiber degeneration. The nerve head slowly loses its normal pink colour and turns grey-white or chalkwhite, with retention of sharp margins. Secondary optic atrophy results when the degeneration of nerve fibers is very rapid and glial replacement is excessive and disorganised. The nerve head developes indistinct margins and the glial tissue fills the physiologic cup and extends past the disc margins. This terminology in general indicates the location of the lesion which has produced the change in the optic nerve. It does not indicate the type of lesion causing the atrophy but merely points out whether the atrophy occured in a nerve head which was previously normal or one which was previously inflamed or choked. For example, brain tumours may produce either type of atrophy of the optic nerve. Those tumours that produce internal hydrocephalus will invariably give rise to papilicedema and the resulting atrophy in these cases will be secondary atrophy. On the other hand, tumours like pituitary adenomas that cause pressure on the chiasma or the optic nerve, but no internal hydrocephalus, will give rise to a primary atrophy.
| Clinical Classification|| |
1. Simple (Primary) Optic Atrophy
it is characterised by a uniformly greyish white disc, which has sharply cut margins and clearly visible lamina cribrosa in an otherwise normal appearing fundus with normal blood vessels. It occurs characteristically in tabes dorsalis. [Figure - 5] It also occurs as a result of tumours which exert pressure on the optic nerve or the tracts without causing internal hydrocephalus. The tumour may be in the optic nerve sheaths or inside the skull, particularly a pituitary tumour or a basal meningioma. Basal arachnoiditis also frequently gives rise to primary atrophy, by causing an encystment which exerts pressure on the nerve or chiasma.
So closely does bilateral atrophy of the optic nerves due to pressure from the pituitary resemble tabetic atrophy that in every case of primary type of optic atrophy an X-Ray of the skull to show the pituitary fossa is an absolute necessity.
A very illustrative case came my way in which an elderly Muhamedan was elsewhere given 3 courses of anti-syphilitic line of treatment in spite of a negative Wassermann and was allowed to run blind till an X-Ray was taken when it was found that the atrophy was due to a large pituitary tumour, and it was too late to improve vision.
2. Consecutive Atrophy
It is the type associated with lesions of the retina and choroid. [Figure - 6]. The lesions may be inflammatory or degenerative. Changes in the surrounding retina are mostly found to account for this atrophy. The disc shows a yellowish waxy appearance and there is extreme attenuation of retinal vessels in advanced cases. This group includes optic atrophy following chorioretinitis and primary pigmentary degeneration of the retina. However one feels that in the former it is the local toxins that secondarily affect the optic nerve causing its atrophy and therefore could be included under the group having a toxic aetiology; whereas in the latter. where the process is an ill-understood abiotrophy, could be included in the group of unknown aetiology.
3. Postneuritic Atrophy
It is characterised by the deposition of fibrous tissue on the disc. It is also associated with narrowing of the retinal vessels. It follows optic neuritis or papilloedema. [Figure - 7]
4. Temporal (Partial) Atrophy
It is characterized by atrophy (pallor) of the temporal side of the disc. It is the result of the involvement of the papillomacular bundle and occurs characteristically in toxic amblyopia due to tobacco and alcohol and also in disseminated sclerosis.
5. Glaucomatous Atrophy
Besides the atrophy, the main features are deep cupping and displacement of the vessels to the nasal side of the disc.
6. Vascular Atrophy
It occurs following embolism or thrombosis of central retinal artery and in so-called "Arteriosclerotic Optic Atrophy". Its essential feature is marked attenuation of the retinal vessels with pallor of the disc. There is total absence of any primary retinal pathology. Obstruction of the arterial lumen may occur under the following circumstances.
(a) Local arteriosclerotic lesion extends further into the lumen until it is completely obstructed.
(b) A partial arteriosclerotic narrowing of the lumen is completed by a thrombus forming at the site.
(c) Associated with an atherosclerosic plaque extending into the lumen, there is added local vasomotor constriction of the vessel, causing complete obstruction. Thus gradual narrowing of the blood vessels leads to progressive diminution of blood supply and ultimately atrophy.
| Aetiological Classification|| |
1. Circulatory: Occlusion of the central retinal vein or artery, arteriosclerotic changes within the optic nerve itself disturbing its normal nutrition, or post-haemorrhagic due to sudden massive blood loss e.g. bleeding peptic ulcer, accident trauma, etc. The fundus, besides the optic atrophy. shows extreme degree of attenuation of blood vessels.
2. Degenerative : Consecutive atrophy secondary to retinal disease with destruction of ganglion cells e.g. in retinitis or chorioretinitis, or as part of a systemic degenerative disease e.g. Cerebromacular degeneration.
3. Post - oedematous : Secondary to papiloedema. [Figure - 6]
4. Post-inflammatory: Secondary to inflammatory lesions of the nerve itself e.g. neuritis, perineuritis, tuberculosis, syphilis, etc. and also following retrobulbar neuritis.
5. Pressure and Traction : Due to pressure on the nerve fibers or due to traction, e.g. from bony pressure at the optic foramen (osteitis deformans), pressure of calcified and sclerosed arteries, aneurysms of the internal carotid artery and of the arterial circle of Willis, intraorbital or intra-cranial tumours.
6. Toxic : From endogenous and exogenous toxemia.
7. Traumatic: Intracranial or intraorbital injury to the nerve.
8. Glaucomatous optic atrophy. 9. Idiopathic and genetically determined:
(a) Leber's disease: considered to be sex-linked recessive gene.
(b) Behr's hereditary optic atrophy: rare autosomal recessive disease.
(c) Congenital or infantile hereditary optic atrophy. There is a severe autosomal recessive form and a milder autosomal dominant one.
| Pathological Classification|| |
Though not commonly used it is based on the usually occurring three varieties of pathological changes that are observed.
1. Where the degeneration of nerve fibers occurs fast with irregular fibrosis thereby leading to lack of any definite pattern (secondary optic atrophy).
2. Where the degeneration of nerve fibers is slow and the replacing fibrous tissue gets arranged in the form of columns thereby giving rise to Columnar Gliosis.
3. Where the atrophy of nerve fibers occurs without the replacement of fibrous tissue thereby giving rise to visible empty spaces and produces the Cavernous Atrophy or Gliosis.
Optic Atrophy may follow injury or infammation of the optic nerve or raised intracranial tension as in cases of intracranial tumours or raised intraocular pressure as in glaucoma or from local pathological lesions of the retina such as chorio-retinitis and primary pigmentary degenerations of the retina. Optic atrophy can be ascending or descending, depending upon the site of the primary lesion which caused the atrophy: e.g. a local lesion such as chorioretinitis or retinal degeneration like primary pigmentary degeneration of the retina or glaucoma would give rise to an ascending optic atrophy; whereas intraorbital or intracranial lesions would give rise to descending optic atrophy. [Figure - 8],[Figure - 9]
One can see from the above discussion that the classification of optic atrophies is far from clear and not at all uniform. It is best to realise that there is a confusion here in order to appreciate the confusion so that one can accommodate to the man opposite, talking about any kind of classification he may be using.
| Optic Atrophy Following Injury|| |
The nerve may be affected in a severe head injury. It may be divided if it is involved in a fracture of the anterior cranial fossa passing through the optic canal. More commonly, it is pressed by an accompanying haematoma of the intervaginal space around the nerve which comes from the vessels which cross this space, or there is actually an intraneural hemorrhage. At the site of the injury the changes are (I) an acute necrosis resulting in a complete breakdown of the axis cylnders, (2) fragmentation of the medullary sheaths, (3) degeneration of the neuroglia and invasion of the polymorphonuclear leucocytes. These are later replaced by fatty granular cells and finally there results a glial scar. It is interesting to note, that beginning optic atrophy can be made out with the ophthalmoscope in about fourteen days after the injury and is well marked after six weeks. In a diagnosis of this condition one is often at the mercy of the radiologist.
(Case 5) In a recent case which was referred to me from Junagadh, the patient had suffered a peculiar injury 20 days ago. He was injured by a branch of a tree, the point of which hit him between the lower part of the eyeball and the inferior orbital margin. According to the history there was a "black-eye", the eyeball had become completely immobile and the vision had diminished considerably.
When we examined him for the first-time, twenty days after the injury, there were no signs of external injury, no hoematoma and the movements of the eyeball were restricted upwards and downwards. There was just light perception. The disc was pale but the margin was slightly hazy. It appeared impossible that the nature of the injury as could be gauged from the history and the clinical findings could cause a break in the optic nerve fibers. The X-Ray findings as reported by the radiologist were "nothing abnormal." We were inclined to make a diagnosis of hemorrhage in the optic nerve sheath with damage to the superior and inferior rectus muscles.
On referring the case to the neurosurgeon for a decompression of the optic nerve from the frontal route, the keen eye of the neurosurgeon detected a break in the optic canal in the X-Ray, and argued that it was a complete break of the nerve with injury to the 3rd and 4th C. Nerves. He expressed the futility of an operation. How such an injury could have caused a damage of this nature is anybody's guess and an exercise in the practice of dynamics.
| Optic Atrophy Following Inflammation|| |
Inflammations of the optic nerve are usually classified into four groups. (1) Perineuritis, (2) Periaxial Neuritis, (3) Axial Neuritis, (4) Transverse Neuritis.
(1) PERINEURITIS: Here the inflammatory process first affects the septal system, the nerve fibers being secondarily attacked. The inflammatory process may be an extension from the primary meningitis of the brain along the meningeal coverings; or it may be a direct extension of the localised inflammation of the paranasal sinuses, bony canal, orbit or the eye itself. The nerve fibers are first intact, but owing to the contracting connective tissue and the resulting diminished blood supply or from the spread of the inflammatory process, they eventually degenerate. The medullary sheath degenerates first, then the axis cylinder breaks down into myelin droplets and is absorb: d. Unlike tabes, the degeneration is irregular, so that some portions will stain with Weigert, while neighbouring parts will not. Later the destruction of nerve fibers is followed by glial proliferation.
(2) PERIAXIAL NEURITIS: Here the meningeal inflammation extends along the pial septa into the parenchyma of the optic nerve. Hence it is also known as parenchymatous degeneration. The characteristic example of this group is tabetic optic neuritis and atrophy. The degeneration of the nerve may occur with or without inflammatory signs, such as oedema of the nerve fibers and cellular infiltration. Spirochetes have been demonstrated, but in very small numbers mostly in the pia. The degeneration passes backwards to the chiasma and the optic tract (Wolff).
Changes in the sheaths and septa are always seen. They are largely secondary to the loss of volume of the nerve resulting from degeneration of the nerve fibers. Without any increase in cells, there is a thickening of the main septal fibers, especially around the vessels. The pia is thickened owing to the diminution of the volume of the whole nerve which causes the elastic fibers in it to contract. In the arachnoid, since they occupy a smaller space, the cells appear to have multiplied. The dura appears to retract less than the pia and tends to lie loosely as the nerve shrinks.
This type of atrophy is definitely becoming rarer with more effective treatment of syphilis in the early stages.
(3) AXIAL NEURITIS: The classical example of this group is multiple sclerosis. The toxic factors on the degenerative processes due to malnutrition may cause selective affection of the inner portion of the nerve,-axial neuritis. In multiple sclerosis since the axis cylinders remain intact, secondary degeneration is not usually seen. Hence the nerve fibers of the papilla are largely retained and the ganglion cells of the retina are hardly if at all altered. However, frequent attacks of neuritis may ultimately lead to atrophy.
It is now an established fact that disseminated sclerosis is almost nonexistent in India, so also the neuritis associated with it.
(Case 6) 1 remember an experience in my early days of practice at an age when youth is synonymous with rashness, of having seen a case of optic neuritis with a central scotoma first in one eye then in the other. Fresh from Europe I made a hasty diagnosis of optic neuritis gave a serious prognosis and was rash enough to suggest that he would develop disseminated sclerosis in the course of time. The boy recovered full vision and is now 50 years old and has no neurological signs.
On the other hand in India, what we get frequently is viral optic neuritis usually bilateral, mostly during the monsoon and just before it, which recovers completely with broad spectrum antibiotics and corticosteroids.
(4) TRANSVERSE NEURITIS: Axial and periaxial neuritis together constitute transverse neuritis, which may totally destroy the optic nerve as in Devic's disease also known as neuromyelitis optica. Here, there is destruction of grey and white matter as well as of the axis cylinders.
OPTIC ATROPHY FOLLOWING INTRACRANIAL TUMOURS: Atrophy of the optic nerve follows mechanical pressure on the nerve by tumours, usually anterior to the chiasma or in the neighbourhood of the chiasma. These may produce a secondary atrophy with the same ophthalmoscopic picture as the primary variety. As aready noted, a brain tumour will produce a primary type of atrophy if it presses upon the chiasma or optic nerve, and it may produce a postpapillooedemic atrophy if it results from papilloedema due to increased intracranial pressure. Pituitary tumours belong to the former category. Rarely, pituitary tumours may cause a choked disc which may further give rise to post-papilloedemic type of atrophy- as is the case with other intracranial tumours. In this type of atrophy, the capillaries which are first distended, later become occluded and are converted into strands. The glial and connective tissue sclerosis causes a shrinking of the tissues which further obliterates the capillaries on the disc. Hence the factors producing the opaque white appearance of the secondary atrophy are the glial and connective tissue sclerosis together with obliteration of the capillaries.
| Optic Atrophy in Glaucoma|| |
Owing to increased intraocular tension, the lamina cribrosa begins to yield backwards. The amount of cupping and its early or late onset depend on not only the amount of intra-ocular tension, but also on the toughness of the lamina cribrosa. Atrophy of the nerve fibers occurs as a result of pressure as well as due to vascular sclerosis causing diminution of blod supply as in arteriosclerotic type of optic atrophy. In case of glaucomatous optic atrophy or that following central retinal artery occlusion, the degeneration is not only of nerve fibers and ganglion cells, but also of inner plexiform and inner nuclear layers.
According to Goldmann, the glucomatous excavation is not due to pressure on the optic disc, but it is due to an atrophy of the nerve and the disappearance of the supporting glial tissue. The duration of pressure is the chief factor rather than the height of pressure in causing the cupping.
It is also interesting to note that the ERG in glaucoma remains normal even in a case of absolute glaucoma as pointed out by Dhanda, suggesting that the pressure atrophy is of the nerve fibers of the retina which form the optic nerve and not of the percepient elements.
| Optic Atrophy in Primary Pigmentary Degeneration of the Retina|| |
There is a progressive degeneration of the neuroepithelium, primarily of the rods, to be followed by gradual atrophy of the whole tissue, associated with filial overgrowth, and obliterative sclerosis of the retinal vessels. There is migration of pigment into the retina
| Optic Atrophy in Chorioretinitis|| |
During the final stages of the disease there is complete disappearance of the neural elements and their replacement by fibrous tissue deprived from the vascular elements and the glial supportive tissue, thus leading to optic atrophy.
| Schnabel's Cavernous Atrophy|| |
Schnabel put forward the theory that the cupping of the disc in glaucoma was not due to tension. This type of atrophy is seen in glaucoma without raised intraocular tension. He stated that minute cavities appeared in the nerve, both in front and behind the lamina cribrosa. Those in front of the lamina, enlarged and coalesced together to form a large cavity. Those behind the lamina collapsed and the lamina cribrosa thus drawn back by contracting glial (scar) tissue, formed the glaucomatous cup. In the early stages the nerve fibers degenerate and disappear entirely, leaving empty spaces. However, the glial network and septa remain unchanged. Later as the process of cavitation advances, larger cavities are formed. Thus there is progressive diminution of bond supply of the nerve bringing about the disappearance of the nerve fibers.
Histologically, it is characterised by the mucoid degeneration of the glia in association with the disappearance of the optic nerve fibers. Neither the glia nor the connective tissue elements exhibit any proliferative reaction on this condition.
| Optic Atrophy from Exogenous Toxins (Toxic Amblyopias)|| |
Toxic amblyopias comprise a group of diseases caused by the absorption of exogenous poisonous substances which have a specific effect of destroying nerve tissue elements near or in the optic nerve. In some of them, like tobacco and alcohol the disease is primarily retinal and follows poisoning of ganglion cells of the retina which results in degeneration of the nerve fibers, seen only after they have obtained their medullary sheaths behind the lamina cribrosa. In others it is due to a direct action on the nerve fibers themselves. In general these substances attack the nerve fibers or the ganglion cell layer of the retina and cause either papillomacular scotomas or peripheral visual field loss. The exact mechanism of destruction by those substances is not known but they are believed to be either neurotoxic or vasoconstrictive giving rise to degeneration.
The toxic agents belong to two main groups. The first group includes agents that produce central field defects, while the toxins of the second group produce a peripheral contraction of the visual fields. The group that produces central field defects mainly comprises of agents like tobacco and alcohol besides the rarer substances like carbon disulphide (as seen in rayon industry), iodoform, lead, thallium and inorganic arsenic preparations. Pathologically there is extreme degeneration of the retinal ganglion cells and the nerve fibers, which may extend upto the external geniculate body. The other group that causes peripheral contraction of fields contains drugs like quinine, ethyl-hydrocuprein (optochin), salicylates, barbituric compounds, felixmas, organic arsenic preparations and aniline dyes. Here the pathology reveals chromatolysis and degeneration of ganglion cells and nerve fibers. In later stages there is thickening of the walls of the blood vessels and obliteration of the lumen.
Recently with the advent of broad spectrum antibiotics, some toxic changes in the nerve have been observed, especially when streptomycin is given over a long period in cases of tuberculous meningitis. More recently optic atrophy has been reported following chloromycetin also.
As regards streptomycin in tuberculous meningitis and blindness, there has been a controversy whether optic nerve involvement is due to streptomycin or is it due to the tuberculous process involving the meninges near the cysterna chiasmatis. Others believe that "Isonex" is the cause of damage to the optic nerve and not streptomycin.
The following two instances in our experiences prove conclusively the mat effect of streptomycin, in some cases at least, on the optic nerve.
(Case 7) A nurse from the Turner Sanatorium for Tuberculous patients was being treated for phlyctenular keratitis in one eye with streptomycin and other anti-tuberculous line of treatment. She was developing defective vision when pas-isonex were discontinued and she still continued to lose her vision. It was at this stage that we saw her first when her vision was reduced to finger counting only in both the eyes. The anti-tuberculous line of treatment was not doing her keratitis the slightest good. Her optic disc in the eye that had no keratitis was very pale. The fundus in the other eye was not visible due to the keratitis. On discontinuing streptomycin her vision improved rapidly. Desensitising therapy with tuberculin improved her keratitis and she could resume her work as a nurse within a month. This case not only proves the toxic effect of streptomycin on the optic nerves in this particular case but also shows that phlyctenular keratitis was not a manifestation of tuberculosis but was only an allergic manifestation, to tuberculin in this case, which got improved with desensitization to tuberculin.
(Case 8) The second case is a sad case and even more convincing. The patient, a young man of 16 years was consulted for complete bilateral optic atrophy in both the eyes with no perception of light. The history was that he had suffered from tuberculous meningitis for which he was treated with streptomycin and 'pas-isonex.'
The boy recovered but he got a recurrence. This time when the treatment was restarted he complained of misty vision. The treatment was continued in spite of this, when he complained of further loss of vision. The boy refused to take any more streptomycin as he thought he was getting blind. The boy said that his vision improved on discontinuing streptomycin. Some residual effects of the tuberculous infection remained and somehow the physician in charge could not be convinced that streptomycin was causing the blurring of vision. He said that streptomycin was known to affect the acoustic nerve but not the ophthalmic. He was inclined to put the blurring of vision down as functional. He proposed to give more streptomycin but the boy refused, but he was induced to take it after being assured that it was not streptomycin but some other injection. The boy, ignorant that he was getting streptomycin under a false assurance complained of immediate amaurosis after receiving this single dose of streptomycin. When we saw him at this stage for the first time, the vision was irretrievably lost, the fundus presenting the appearance as in quinine amblyopia--chalky white discs with attenuated vessels.
This case teaches us one useful lesson besides driving the nail in the fact that streptomycin does cause optic atrophy. It shows that courses of streptomycin injections separated by intervals of rest, progressively become more damaging. In this case the very first injection in the third course precipitated complete and irreversible blindness. It is a form of Herxheimer reaction, which is due to individual susceptibility which seems to get built up in some individuals with every course separated by an interval.
A severe deficiency of vitamins in the diet, particularly of thiamine as in severe starvation and in extreme degree of pellagra may cause optic neuritis usually of the axial type with loss of central vision just as similar lesions in the mid-brain give rise to various types of ophhalmoplegias like that of Wernicke's acute haemorrhagic anterior polioencephalitis. A partial optic atrophy which may eventually develop into a complete one leads to permanent visual defect.
The essential TREATMENT of these amblyopias is to recognise and stop the toxic agent completely and to give high doses of the B-Complex group of vitamins, particularly, thiamine and B 12. Also found useful is the administration of peripheral vaso dilators like amyl nitrite, sodium nitrite and retrobulbar injections of acetylcholine, etc.
| Course-Prognosis-treatment|| |
In optic atrophy disease, the changes in visual function usually occur very slowly over weeks or months, except in cases of severe injury and acute toxoemia like quinine and methyl alcohol. It is difficult to assess prognosis on the basis of ophthalmoscopic findings alone. Atrophic cupping, attenuation and reduced number of vessels on the disc, and pallor with papilloedema are all signs of unfavourable prognostication. Optic atrophy secondary to vascular, traumatic, degenerative and some of the severe toxic causes, usually has a very bad prognosis. Visual loss due to optic atrophy secondary to pressure against the optic nerve may be restored, particularly if the cause is relieved early. No treatment is effective for optic atrophy except to treat the underlying cause and on the general line of treatment as done for toxic amblyopias.
From the series of cases under review we have found that glaucomatous optic atrophy was the commonest followed next in order by vascular, toxic and syphilitic. Due to efficient and energetic antisyphilitic treatment, the last has become very infrequent. The least common causes were those due to intracranial tumours, trauma, meningitis and septic foci of infection, particularly from the posterior ethmoidal and sphenoidal sinuses. Finally, there was a group of cases where the etiology could not be determined.
| Conclusions|| |
In conclusion it may be noted that there is much that is unpredictable about the course of lesions of the optic nerve. Some lesions are acute and irreversible, others appear progressive at first but are completely curable functionally although a tell-tale optic atrophy may prevail. There are still others which gain in severity with every repetition of the noxious lesion. One is tempted to explain these on pathological basis.
The acute lesions, traumatic or toxic are neuropathic and primarily damage the ganglion cells and the nerve fibers.
The second variety of lesions primarily affect the vascular supply which at first are of a spastic nature, functional and capable of recovery. They are clinically recognised by a "depression" of the field and fall in line with Traquair's postulation that all field defects could be explained on a vascular basis. If the noxious agent is removed the function improves but some auxiliary blood vessels may get obliterated and result in some permanent pallor of the disc resulting in disproportion between visual function and disc colour, as illustrated in cases 1 and 2 on page 57. Fortunately, an appreciable amount of time can elapse (about 3 weeks) before the nerve begins to show signs of degeneration after the initial impairment of the blood supply.
In the third variety the noxious toxin is neurovascular, first affecting mainly the blood vessels but during the resting phase the nerve gets progressively sensitized to the noxious agent so that with subsequent attacks or administration of the same toxin the nerve becomes more vulnerable and the subsequent lesions become neuropathic, acute and permanent as described under streptomycin toxicity.
Arguing on these lines, the unpredictability of optic nerve lesions referred to above, will not appear so unpredictable, the prognosis not so indefinite and the treatment not so irrational. I hope this presentation will help us to face our cases of optic nerve lesions with greater confidence.
| Summary|| |
Anatomical, physiological, pathological and etiological considerations in optic nerve atrophies of various kinds are considered. These are illustrated by clinical experiences wherever applicable.
Three types of noxious agents can cause damage to the optic nerve: (1) Neuropathic, (2) vascular, (3) vasculoneuropathic. The prognosis is best in the second group where a time limit of about three weeks is available to meet the challenge. In the third group a progressive sensitization of the optic nerve takes place with every repetition of the toxic agent after an interval.
I thank my colleague and friend Dr. S. N. Cooper for allowing me to use some of his clinical material which we have seen together and for some of the diagrams in the text. I also thank him for his editorial suggestions.
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[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6], [Figure - 7], [Figure - 8], [Figure - 9], [Figure - 10]