Year : 1983 | Volume
: 31 | Issue : 5 | Page : 662--665
The pathology of cornea (A histopathological study)
Department of Ophthalmology, KG's. Medical College, Lucknow, India
P K Agrawal
Department of Ophthalmology, KG«SQ»s. Medical College, Lucknow
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Agrawal P K. The pathology of cornea (A histopathological study).Indian J Ophthalmol 1983;31:662-665
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Agrawal P K. The pathology of cornea (A histopathological study). Indian J Ophthalmol [serial online] 1983 [cited 2021 Feb 26 ];31:662-665
Available from: https://www.ijo.in/text.asp?1983/31/5/662/36625
Histology of corneal buttons received after keratoplasty has given an opportunity to study the different pathological changes occuring in different corneal conditions. This paper aims at presenting different histopathological changes seen in corneal buttons.
It will not be out of place to discuss the normal histological pattern of the cornea.
(a) Corneal Opacities
Clinically the scar of the cornea are classified according to their size and density. The three grades of the corneal opacities are (i) nebular (ii) macular and (iii) leucoma. The nebular opacities are the slight scar in the cornea which are missed sometimes on clinical examination unless some special care is taken.
Any local change in the index of refraction of the cornea leads to turbidity, with the result that some minute opacities which are easily recognized in the living state may give little or no histologic evidence of their presence. Some corneal scars which are seen readily may on microscopic examination be characterized only by slight irregularity in the arrangement of the corneal fibres. Routine histological stains often do not differentiate between the chondroitin sulphate of fibrous tissue and the keratosulphate of the normal cornea. Moreover, any new form of connective tissue in the cornea has a tendency to take on the characteristic of the normal corneal stroma. The fibres gradually become thicker, their arrangement more regular, and eventually even a certain degree of transparency may be acquired. Such complete regeneration of tissue is of course rare.
Histopathological Changes in Corneal Scar
As has already been indicated that it may be quite difficult to distinguish histologically nebular or even macular type of opacities. However, many changes may be quite evident of corneal scar. The corneal epithelium may be regular or irregular showing changes like atropy, hyperplasia, rate peg formation, oedema bullae, [Figure 1],[Figure 2] calcification and changes like keratinization etc. All these changes in corneal epithelium indicate secondary changes due to causes of scar formation. [Figure 1],[Figure 2],[Figure 3],[Figure 4],[Figure 5],[Figure 6].
The basement membrane in case of corneal scar may be normal or thickened, distorted or may produce hypersecretion. [Figure 3],[Figure 4]
The Bowman's membrane is usually absent or destroyed by vascularisation which seems to be a very important feature in most of the cases of corneal scars. Sometimes a very intense vascular membrane may replace the whole of the Bowman's membrane. [Figure 1],[Figure 6] with all this other changes in Bowman's membrane may be splitting and reduplication of the membrane. It should be noted that presence of Bowman's membrane does not exclude the possibility of a corneal perforation as it has got a tremendous power of regeneration. The changes in the substantia propria may be a thickening, irregularity acellularity, sclerosis and vascularisation of the corneal stroma with evidence of fibroblastic activity. The vascularisation may be mild or quite thick, superficial deep or in whole extent. Secondary changes like hyalinisation or calcification may occur in such disc. An incarceration of uveal pigment indicate a perforation. These may be an extensive proliferation of the fibrous tissue forming a thickened cornea, an ectasia may lead to a staphyloma formation. There is usually an evident thickening of the corneal nerve in cases of old degenerated corneal opacities.
The Descemet's membrane may give an indication as to a perforation has taken place or not as it never regenerates changes in the form of perforation, coiling, folds or duplication may occur. The Descemet's tissue may present deep in the stroma which might have been placed or incarcerated after a descemetocele. However, in few cases curious active granulomatous reaction in the deepest layers of cornea and around the Descemet's membrane have been observed by me (author; in cases of healed corneal ulcer with clinically no evidence of activity for even 9 to 12 months. Three of such cases were earlier diagnosed cases of fungal keratitis. This granulomatous reaction might be the result of an alteration in the chemical composition of the Descemet's membrane perhaps an acquired autosensitisation to this structure. The reason for making this suggestion is that microscopically one sees a massing of inflammatory cells, mainly monocytes, epitheloid cells and multinucleated giant cells about Descemets' membrane. [Figure 9],[Figure 10] The Descemet's membrane in the sections looks as if it were a foreign body, provoking in the deepest layers of the cornea a foreign body granulomatous reaction. The patterns seem to be similar to that observed about lens remanents in phacoanaphylaxis or about necrotic collagen in rheumatic scleritis which are also supposed to be due to acquired autosensitisation. This lesion seems to be associated with healed long standing cases of fungal keratitis where some time has been elapsed. Such peculiar unexplained granulomatous lesions have also been seen by Hogan et al.  and Green and Zimmerman  . The association with fungus was first of all described by Agrawal and Bhradwaj,  Recently we have also seen such reaction in rabbits cornea of healed fungal ulcer produced experimentally, Descemet's endothelium may be normal in superficial corneal opacities. However, this may be completely replaced by uveal tissue. I will like to discuss few other very common conditions of cornea which are related to the sequele of corneal ulcers most frequently complicated corneal opacities.
Ectatic cicatrix or keratectasia
An ectatic cicatrix is a bulging of a thinned, scarred cornea usually following ulceration. It differs from a staphyloma in that the uveal tissue does not line the scar presumably the young scar cannot withstand the intraocular pressure and gradually gives way.
Descemetocele is a herniation of Descemet's membrane through the floor of a corneal ulcer. This membrane may act as a strong barrier to prevent loss of the anterior chamber. Histological examination of a descemetocele shows this membrane into the ulcer. It usually is covered by a thin layer of fibrinous exudate, debris, a few young connective tissue cells, and corneal epithelium. The membrane is thinned, and often the endothelium is missing in this area.
Adherent Leukoma-is a corneal scar which has fibrous tissue adherent to its deeper surface. It always indicate a perforation unless an adherent leukoma of congenital origin.
Corneal ulcers and perforating wounds often are accompanied by prolapse or incarceration of iris. If the prolapse is not replaced there is gradual conversion of the iris stroma to fibrous tissue by activity of fibroblasts and those from adjacent corneal lamellae. Eventually the entire area becomes covered by epithelium. The anterior chamber angle often is obstructed and the resultant secondary glaucoma causes the weak corneal scar to bulge, producing a staphyloma. The type of staphyloma which forms depends on the extend of the wound or ulcer. Conical staphyloma result from small perforations, and spherial staphylomas from broad perforation.
|1||Hogan, H.J., Kimura, S.J., and Thygeson, P., Trans. Amer. Ophth. Society, 61, 75, 1963.|
|2||Green, W.R., Zimmerman, L.E., Amer. J, Ophthal., 64, No. 3, Part 11 555/37, 1967.|
|3||Agrawal, P.K., and Bharadwaj, T.P., Proc. A.l. Oph. Soc., XXXI, 125-132,1971.|