Basics of Cornea and Infectious Keratitis MCQs

Ans. is `b' i.e., Acanthamoeba [Ref Kanski 8th/e p. 197] "Acanthamoeba keratitis is the most common keratitis caused by a protozoan especially in contact lens users".

It is hypothesized that commensal organisms from periocular skin or the ocular surface gain access to the stroma via suture tracks or through compromised unhealthy corneal epithelium. Microorganisms that can produce this type of infection include α haemolytic Streptococcus, Peptostreptococcus, Pseudomonas and Candida, and prolonged treatment with topical vancomycin, moxifloxacin or gatifloxacin eyedrops with discontinuation or minimization of topical steroid use is generally required. Recurrence of infection and stromal scarring are common which may then require keratoplasty. Neisseriae Gonnorhoea is a highly virulent micro-organism that can invade healthy corneal epithelium whereas ICK is caused by low grade (low virulent) commensal micro-organisms that affect damaged corneal epithelium. Reference: Parson’s diseases of the Eye-Chap 12 – Diseases of Cornea.

Marginal/ Catarrhal ulcers occur near the limbus, especially in old people. Aetiology: As an immune reaction to toxins produced by staphylococcal organisms residing in the conjunctival sac or lids in patients with chronic meibomianitis or blepharitis; they also occur in association with a chronic conjunctivitis and may be caused by Moraxella or Haemophilus. Pseudomonas is implicated in development of contact lens induced keratitis. Reference: Parson’s diseases of the Eye-Chap 12 – Diseases of Cornea.

The corneal physiology is designed in such a way to maintain transparency and take care of healing and/or regeneration capacity of the cornea. In case the cornea sustains injury due to any cause such as trauma, infection or surgery, and if the injury is superficial involving only the epithelium, the stratified squamous epithelium covering the anterior surface of the cornea rapidly regenerates. This regeneration of corneal epithelial cells is mainly from stem cells, which are epithelial cells present as palisades of Vogt at the limbus. These mitotically active cells with an increased surface area of basal cells present in folds, and palisades are ideally suited for this purpose. There is very little mitotic activity in the basal cells at the centre of the cornea. Reference: Parson’s diseases of the Eye-Edition 23rd- Chap 12- Diseases of the cornea

Ocular involvement with the herpes simplex virus (HSV) has varied manifestations which include blepharitis, conjunctivitis, keratitis and iridocyclitis. The corneal involvement can be epithelial (dendritic or geographic keratitis), stromal (necrotizing and non-necrotizing stromal keratitis) and endothelial. Reference: Parson’s diseases of the Eye-Chap 12 – Diseases of Cornea.

Deep vascularization occurs, consisting of radial bundles of brush-like vessels, and since they are covered with a layer of hazy cornea, their bright scarlet is toned down to a dull reddish pink (‘salmon patches’), wherein separate vessels can be seen only with difficulty. The opacity extends a little beyond the vessels, which seem to push the opacity in front of them and at the height of the condition the vessels run in radial bundles almost, but seldom quite, to the centre of the cornea. There is often a moderate degree of superficial vascularization, but it never extends far over the cornea and at the limbus the conjunctiva may be heaped up. Reference: Parson’s diseases of the Eye-Chap 12 – Diseases of Cornea

Herpes zoster is caused by the same virus that causes chickenpox (varicella zoster virus). Reference: Parson’s diseases of the Eye-Chap 12 – Diseases of Cornea.

Loss of epithelium is termed as epithelial defect and can be demonstrated clearly by staining with 1% sodium fluorescein dye and viewed with cobalt blue filtered light. If superficial without any inflammation, an epithelial defect is also called an abrasion or erosion and usually heals within 12–24 hours by regeneration of the epithelial cells from the periphery, which slide over to cover the defect. Reference: Parson’s diseases of the Eye-Edition 23rd- Chap 12- Diseases of the cornea