Year : 1971 | Volume
: 19 | Issue : 1 | Page : 2--6
Intrascleral nerve loops
Yale University School of Medicine, New Haven (Conn.), USA
Yale University School of Medicine, New Haven (Conn.)
|How to cite this article:|
Katz D. Intrascleral nerve loops.Indian J Ophthalmol 1971;19:2-6
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Katz D. Intrascleral nerve loops. Indian J Ophthalmol [serial online] 1971 [cited 2022 Aug 8 ];19:2-6
Available from: https://www.ijo.in/text.asp?1971/19/1/2/35005
In the course of the objective examination of the external surface of the eyeball one occasionally finds anterior to the insertion of a rectus muscle, usually in my experience, the internal or inferior rectus, the projecting loop of an intrascleral nerve. [Figure 1],[Figure 2] One might ask the question, "Anatomically, how does an intrascleral nerve loop come about?" Or of more practical importance, one might pose the question, "Of what clinical significance are intrascleral nerve loops?"
It is elementary knowledge that anatomically all the nerves within the eyeball are ciliary nerves i.e., 10-20 short ciliary nerves and 2 long ciliary nerves. The descriptive terms short and long pertain to their length in the orbit and not in the eyeball. The short and long ciliary nerves loosely imbedded in oblique canals, pass through the sclera, around the optic nerve, and thus gain entrance into the perichereidal space Because it was believed that lymph circulated through these canals they were designated emissaria. We still retain this term although the theory which brought it about has been discredited. The ciliary nerves, having gained entrance into the perichoroidal space, traverse this space in a meridional direction. While doing so they give off fine branches to the sclera and to the choroid. The main mass of ciliary nerve fibers runs forward to the region of the ciliary muscle to form the ciliary plexus.
It was Axenfeld who on microscopic study of some serial sections of an enucleated human eyeball, noted a thick nerve which passed thru the sclera at a right angle, anterior to the insertion of one of the rectii tendons. He was of the opinion that the nerve passed from without inward. Because of this thinking and because of the anterior location of the nerve Axenfeld incorrectly termed it an "anterior ciliary nerve". We new know that there are no "anterior cilirary nerves". Axenfeld did note that when the thick nerve study reached the perichoroidal space it divided into a fairly large anterior and posterior branch. the former going to the ciliary body and the latter passing backward through the perichoridal space to the choroid; so he thought. [Figure 3].
Axenfeld was puzzled by the thickness of the so-called posterior branch for he was aware of the fact that recurrent branches given off of ciliary nerves within the eyeball were always extremely fine.
Subsequently Axenfeld in the microscopical study of serial sections of another enuleated human eyeball again noted a thick nerve passing through the sclera, at a right angle to it, in the region of the pars plana of the ciliary body. In the perichoroidal space this nerve apparently divided into an anterior and posterior branch. In view of the fact that this nerve did not reach the surface of the eyeball but was separated from the episclera by a thin layer of scleral tissue, [Figure 4] Axerfeld came to the realization that in this instance, and perhaps also in his previous case, he was dealing with a nerve which came from behind, bent at an acute angle into the sclera through which it passed for varying distances, even to its outer surface, and then turned on itself and passed inward to the perichoroidal sliace and thence to its termination in the ciliary body. He labelled such nerve loops Intraseleral Nerve Loops. They are now occasionally referred to as Axenfeld's Nerves. [Figure 5].
Subsequent microscopic study of serial sections of enucleated eyes by various investigators proved conclusively that, when present, the intrascleral nerve loops were definitely nomal ciliary nerves for they were able to follow these throughout their entire course as they passed through the sclera in the neighbourhood of the optic nerve to their termination in the ciliary body.
It was then noted that a relatively large artery was almost always present in the emissarium with the neve loop. This is due to the fact that the emissarium utilized by the intrascleral nerve loop is usually that of the perforating branch of an anterior ciliary artery.
Microscopic specimens in which the loop extends partly through the sclera [Figure 6],[Figure 7] and clinical cases in which it just peeps through the external scleral surface [Figure 8] are no, uncommon. In fewer instances the nerve loop extends through the entire thickness of the sclera with its cupola projecting beyond the external scleral surface, covered only by conjunctiva which is freely movable over it. This was well demonstrated in Meller's study of serial sections of a case of keratitis Disciformis. A vessel was present between the intrascleral arms of the loop.
The embryology of the intrascleral nerve loop is in dispute. Ernst Fuchs was of the opinion that the correct explanation for the developments of these loops rests on an abnormal growth tendency, i.e., some ciliary nerves grow more rapidly than the sclera, and their greater length is compensated for by their duplication or loops in the sclera. There are several factors which favour this thinking.
As the loops are quite thick and as the caps of the loops are sometimes made much thicker by the deposition of nuclear rich connective tissue, the exposed portion of some intrascleral nerve loops has the appearance of a grayish-white mushroom shaped, neuroma-like enlargement 1-2 mm. in diameter. [Figure 8]
The uninitiated might incorrectly diagnose such as intrascleral nerve loop with the result that some loops have found their way to the laboratory as this one did. This photograph is from the book, "Ophthalmic Pathology" by Hogan and Zimmerman. The caption stated that the pre-operative diagnosis was conjunctival cyst. Excision of an intrascleral nerve loop results in hyperesthesia of the cornea over a sector corresponding to the site of the loop, but there is no indication that the innervation of the ciliary body and iris is affected.
Occasionally a loop is capped by pigmented cells and presents the picture of a small pigmented mass. Under such circumstances a diagnosis of `neoplasm' might be entertained. The consequence of such a diagnosis can be mot unfortunate.
Intrascleral nerve loops are asymptomatic except in those relatively rare cases in which the extra ocular portion is of such a size and so located that the patient is conscious of the lid rubbing over the eyeball. In some instances this can be quite discomforting. The treatment of choice under such circumstances is the application of the electric cautery to the loop. For some reason this does not produce hyperesthesia of the cornea but does relieve the patient of the discomfort.
It is of interest to note that Axenfeld, who first described these loops from microscopic preparations, was subsequently, the first to report a case observed clinically. The patient complained of intermittent pain from the movement of the lower lid over the eyeball. Examination revealed, anterior to the insertion of the inferior rectus, about 3 ram from the limbus, a sharply circumscribed and demarcated grayish-white flat prominence, 2.5 mm in diameter. The mass was sensitive to palpation and pain was elicited when the prominence was pulled on. Excision showed it to be composed of nerve tissue and this surgical procedure left a hyperesthesia of the cornea almost to the horizontal meridian. Others also have Hyperesthesia by the excision of a loop. These cases are cited as evidence that the recognition of an intrascleral nerve loop is of clinical importance. Indeed, if a point is made of looking for these loops, it is surprising how often they may be seen.
To summarize, the clinical diagnosis of an intrascleral nerve loop is readily suggested by its location, i.e., 3 to 4 mm, from the limbus in the plane of a rectus muscle, by its greyish-white or pigmented nodular .like appearance over -which the conjunctiva moves freely; and certainty of diagnosis is had if, in addition to these, the perforating branch of an anterior ciliary artery is seen entering the involved emissarium and if with time the mass remains of the same size. The latter is clinically important for the extraocular extension of an intraocular tumor in this part of the eyeball frequently takes place thru u the emissaria of the anterior ciliary arteries.
I have called your attention to this anatomical structure so that if, when, and as, encountered, you allow the patient to retain it rather than be responsible for bearing a number in the laboratory.