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ARTICLES |
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Year : 1983 | Volume
: 31
| Issue : 3 | Page : 105-107 |
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Functional anatomy of macula and diagnostic procedures for macular function in clear media
RP Sachdeva
E-96, Ansari Nagar, New Delhi-110029, India
Correspondence Address: R P Sachdeva E-96, Ansari Nagar, New Delhi-110029 India
Source of Support: None, Conflict of Interest: None | Check |
PMID: 6676190
How to cite this article: Sachdeva R P. Functional anatomy of macula and diagnostic procedures for macular function in clear media. Indian J Ophthalmol 1983;31:105-7 |
The focal point of the eye, not only optically but functionally and organically is macula, as it provides the most important sensory channel. It is concerned with the precise visual function of acuity, form sense, colour differentiation, and steropsis. Electron microscopy and advances in biochemical, l,istopathological techniques have led to a better understanding of anotonorny and pathology of macular disease. Great improvements have been made in methods of clinical examination, objective recordings of visible changes by retinal photography and stereophotcgraphy. Sophistication of electrodiagnostic tests have also enhanced our assessment of macular function.
Macula represents shallow concavity lying in thinckened retina 4 m.m. away from the optic disc at the posterior pole and forming a horizontal ellipse approximately 2 m.m. long 1.5 m.m. wide. In the centre of the fovea there is small depression about 0.2 m.m. in diameter called the foveola and it is at this that the greatest concentration of cones occurs and where the focal point of entering rays of light is situated.
Foveala is comprised of entirely of elongated specially adapted cones. They have transversely striped rootlets in their inner segments and their mitochondria are dispresd in the innner segment and outer cone fibre. No ellipsoid is discernible in the inner segments and the there are unusual cytoplasmic complexed in the cone synapses. The Mullers fibres are displaced laterally in the outer plexiform layer (Henles layer) where they run prependicular to the receptor cells. In the foveal region these Muller cells possess a watery cytoplasm, especially in the region surrounding the central cone fibres, thus presenting very little optical hinderence. The displacement of the inner retinal layers away from the foveola leads to an aggregation of bipolar and ganglion cells around the edge of the fovea making this region the thickest part of the fundus.
The retina thus loses its compact nature here and this laxity enables large quantities of extracellular fluid or exudates to be accommoin the macular region, this leads to the characteristic cystoid oedema pattern seen in a number of pathological conditions.
The internal limiting membrane in the foveal region is very thin, having a width of less than 20 nm, and electron-microscopic studies suggest that it is more firmly adherent to the foveal retinal than elsewhere in the posterior pole. The membrane is attached to Mullers cells by plaques and rupture of the attachment may provoke macrophage mobilisation giving rise to preretinal membrane formation.
The thinness and adherenc of the internal limiting membrane in the foveal region they may preferentially disturb the function of the cells which are highly conccatrated around the foveal rim, and also affect the permeability of the macular capillaries.
The retinal pigment epithelium in the macular region is denser than in other areas and shows some spectral variation peaking at an average of 460 nm. It place special role in the phagocytosing of the outermost discs of the photoreceptors when they have been shed. Within the cytoplasm of the pigment epithelial cells there are numerous coated vesicles related to the endoplasmic reticulum and Golgi apparatus, and their function is thought to be concerned in the active transport of proteins between the pigment epithelial cells and the photoreceptors. Microvilli arise from the surface of the pigment cell to embrace the outer segments of the receptors, these are concerned with cohesion between the two layers of cells and with phagocytosis.
Beneath the pigment epithelium lies Bruchs membrane consisting of three basic layers-a trabecular central zone surrounded on either side by a collagenous layer, the basis of the pigment epithelial cells show infoldings to which Bruchs membrance is firmly attached, while the outer layer is continuous with the basement membrane of the choriocapillaris, which presents openings selectively pointing towards the retina. In the macular region the choriocapillaris is supplied by the short posterior ciliary arteries.
Segmental vascular pattern of the human choriocapillaries has been demonstrated. Choriocapillaries does not form a continuous anastomosis network. The location of the macular region is the meeting point of arterial and venous watershed zones of choriod and it makes it more vulnerable to vascular disorders.
The foveola is nourished by the underlying choroidal vasculature, whereas the superficial retina in the macular region receives blood from macular branches of the superior and inferior temporal arteries.
The macula, choroid and pigment epithelium are also the preferential sites for degenerative changes which may be hereditary, toxic or arteriosclerotic in nature, and there is a predisposition for choroidal vascular disease with decompensation and haemorrhage in the central area. There is no doubt that the pigment epithelium in the fovea is very active metabolically being concerned with the excessive vitamin A transport necessary for photopic vision and that this hyperactivity, combined with the special haemodynamic effects of the narrow choroidal capillaries in this region, may be responsible for the susceptibility of this tissue to a large variety of pathological process.
2. Investigation of Macula in clear media.
(a) Although direct ophthalmoscopy still plays the major role in the clinical evaluation of macular changes, it should be augmented by other examinations which are able to add a great deal more information. The two main disadvantages of direct ophthalmoscopy are the lack of a stereoscopic view of structural changes, and the difficulty of recording and transmitting the information.
An excellent stereoscopic view of the macula can be obtained by use of the slit-lamp and some form of fundus-viewing contact lens for convenience a Hruby lens may be used but resolution of fine detail is not so good as with contact lenses and for extensive lesions at the posterior pole indirect ophthalmoscopy may be of value, although magnification is lost.
Retinal photography and stereo photography provide a valuable means of recording macular changes a permanent record and care be compared with follow up studies. Red-freelight has been used successfully to demonstrate changes in the internal limiting membrane and nerve fibre layer. Photography using redfilters or infrared film can be used to distinguish pigmented lesions in the macula and to examine the choroid and orthochromatic film can be used to study details of nerve fibres.
Fluorescein angiography. Without a doubt the largest single factor in the evaluation of macular disease in recent years has been the development of the technique of fluorescein angiography. Fluorescein angiography not only allows us to examine structures in the macular region which are beyond the reach of direct ophthalmoscopy but also enables us to study the haemodynamic changes that occur in the retina and the loealised abnormalities of flow and perfusion. A modern angiogram consists of a series of high-contrast black and white transparencies taken at speeds of up 0.6s intervals.
The great advantage of fluorescein as a contrast medium is that under normal circumstances it does not leak out of the retinal vessels. A second important barrier to the diffusion of fluorescein through the retinal tissues is the pigment epithelium. Choroidal vessels do not have tight endothelial junctions and are, therefore, fully permeable to flucrescein. In the course of an angiogram the choriccapillaris becomes a lake of fluorescent blood, but this is prevented from diffusing into the retina by the pigment epithelium and Bruches membrane, with the main barrier shown histochemically to be situated in the pigment epithelium. these two cellular layers couples with the masking effect of haemoglobin and pigment in the fundus give the angiogram its special diagnostic value, since disturbances of the vascular endothelium or defects in the pigment epithelium give rise to leakage of fluorescein into the surrounding tissue, and it is this abnormal leakage that produces the characteristic angiogram appearances by which the diagnosis can be made.
Abnormal fluorescence n an angiogram may arise:
(I) From disturbances in the retinal vessels.
(2) From defects in the pigment epithelium which allow the normal choroidal fluorescence to be unmasked.
(3) It may be associated with inflammatory foci in the retina or choroid.
(4) It may develope in organised scar tissue, and
(5) In certain tumours.
In addition, new vessels which develop in the retina, either as part of the organisation of retinal and subretinal scars, or as primary neovascularisation, seem to lack the impermeability of their normal counterparts and show profuse fluorescein leakage.
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