Non-infectious Disorders of Cornea MCQs

Pigmentation may occur from the prolonged topical use of silver nitrate (argyrosis). It is due to the permanent impregnation of the elastic fibres, particularly the Descemet's membrane, with metallic silver. Reference: Parson's diseases of the Eye-Chap 12 - Diseases of Cornea

Keratometer: The keratometer mires are malformed, malaligned and malfocussed. Corneal topography: In the early stages, the condition is diagnosed with corneal topography, which demonstrates the cone and typical astigmatic pattern. Corneal thinness can also be measured with ultrasonic pachymetry or the Orbscan II corneal topography system. With the ophthalmoscope or a plane mirror at a distance of 1 m, a ring of shadow, concentric with the margin, is seen in the red reflex (resembling a droplet of oil), which alters its position on moving the mirror. It is due to a zone through which a few rays pass into the observer’s eye, as the emergent rays in the centre are convergent while those on the periphery are divergent. Refraction: irregular myopic astigmatism Reference: Parson’s diseases of the Eye-Chap 12 – Diseases of Cornea

Cadaveric allograft corneal tissue and sclera are made available for transplantation by eye banks by the process of eye donation organized by eye banks. With consent of the legal next of kin (parents/spouse/sibling/progeny/grandparents/grandchildren), the eyes are harvested and tissues processed before releasing for transplantation. Donors are screened for known contraindications by medical and social history, clinical examination and serological screening of a donor blood sample for transmissible infectious diseases such as HIV/AIDS, HbsAG, hepatitis C and syphilis. Reference: Parson’s diseases of the Eye-Chap 12 – Diseases of Cornea

Corneal opacities in Theil-Behnke dystrophy: typically reticular (honeycomb) in pattern . Corneal opacities in Reis-Bucklers dystrophy: irregular, coarse, geographic shaped opacities at the level of the Bowman layer and superficial stroma. Finger print/ dot pattern is seen in Epithelial Basement Membrane dystrophy-EBMD Reference: Parson’s diseases of the Eye-Edition 23rd- Chap 12- Diseases of the cornea

Ans. A. Staph epidermidis Pneumococcus and staphylococcus aureus have been found to be the commonest microorganisms in the developed world, whereas staph epidermidis is the commonest in developing countries, for causing infectious keratitis after corneal transplantations.

Donor corneas are harvested from cadaveric donors within 6–8 hours after death, sometimes up to 12 hours after death in countries with cold climates or if the donor is refrigerated. Reference: Parson’s diseases of the Eye-Chap 12 – Diseases of Cornea

Major causes of graft failure include primary failure due to poor donor endothelium, graft infection, graft rejection and recurrence of the primary disease in the graft. Reference: Parson’s diseases of the Eye-Chap 12 – Diseases of Cornea

The basic design of a keratoprosthesis includes a central optical cylinder made of polymethyl methacrylate (PMMA) with a surrounding fixation device which is of different designs in terms of shape, size and material. Some of the materials used for the latter include methacrylate, Teflon, Dacron mesh, polycarbon, tooth and bone (osteo-odonto-keratoprosthesis), cartilage (chondro-keratoprosthesis) or nail (onycho-keratoprosthesis). PMMA is used for central optical cylinder making. Reference: Parson’s diseases of the Eye-Chap 12 – Diseases of Cornea