RETINAL DISORDERS

Retina is a layer of photoreceptors cells and glial cells lining the posterior segment, which captures incoming photons and transmits them along neuronal pathways as both electrical and chemical signals for the brain to perceive a visual picture.

Retinopathy of prematurity (ROP), earlier termed as Retrolental fibroplasia, is an important retinal disorder, characterized by variable severity of “arrested or abnormal vascularization of retina”, almost exclusively seen in preterms.

Pathogenesis: Till 16^th week, retina is an avascular structure, when gradual vascularization begins from optic disc toward periphery to reach outer rim of retina (ora serrata) by 36-40^th week. Any insult to this process leads to—(a) sudden cessation of vascularization, (b) cellular hypoxia to non-vascularized segment, and (c) release of endothelial growth factors, leading to neovascularization.

This hypoxic tissue injury, coupled with neo­vascularization, leads to scarring and cicatrisation of retinal tissue, proliferation of extra-retinal fibrovascular tissue and retinal detachment due to vitreous traction.

Risk factors: Prematurity (lt;32 weeks) and prolonged hyperoxia due to high concentration oxygen therapy in neonatal period are two most important risk factors for development of ROP, though other contributory factors include hypoxia, infection, acidosis and severe anemia in newborns.

Classification: ROP is variably classified according to the location, extent and severity of disease, though the most common classification is given in Table 26.11.

Clinical course: In gt;90% of infants, ROP arrests spon­taneously with no significant residual effect on vision.

Diagnosis depends on follow-up ophthalomoscopy, (Fig. 26.1A) which is recommended in all extreme preterms (lt;32 weeks or 1.5 kg) or more mature newborns with stormy neonatal course, at ~ 6 weeks and then at ~6 months.

Treatment depends on severity and course of disease. Cryosurgery or laser photocoagulation of avascular retina is recommended in progressive and severe cases, to prevent further damage. Vitroretinal surgery to reattach detached retina is so far not very successful.

Prevention of ROP depends on: (a) prevention of pre­maturity, (b) rational use of oxygen supplementation, and (c) use of antioxidants, e.g. vit E, which is not proven. Persistent hyperplastic primary vitreous (PHPV) denotes a developmental immaturity with persistence of variable remnants of hyaloid vascular system. Fetal hyaloid artery, which extends from optic disc to the posterior vascular capsule of lens, gradually regresses by 7-8th months. Small remnants of this vascular system may remain attached to the disc (Bergmeister's papilla) or lens (Mittendorf's dots) in normal children with no vision problem. However, larger remnants are termed as

PHPV, presenting with leukocoria (cat-eye reflex), visual impairment and spontaneous intraocular hemorrhage. Posterior PHPV (attached to the optic disc) may also cause retinal detachment.

Surgical removal of lens with attached tissue is indicated in severe cases, followed by lens implant and correction of refractive error.

Retinitis pigmentosa (RP) is a progressive retinal degenerative disorder, characterized by: (a) abnormal retinal pigmentation, and (b) arteriolar attenuation/ narrowing, leading to progressive loss of vision (Fig. 26.1B). Other ocular defects, e.g. cataract, glaucoma, optic atrophy, etc. are common.

Etiologically, Common causes of RP include:

(a) intrauterine infections, (b) neurodegenerative disorders, (c) metabolic disorders, e.g.

Clinically, visual impairment begins as night­blindness and progresses gradually to loss of peripheral vision (Tubular vision). Central vision is usually spared. Electroretinography is typically abnormal in these cases. Cherry-red spot over macula is an important ophthalmo­scopic finding in various neurodegenerative disorders, e.g. sphingolipidoses, gangliosidoses and metachromatic leukodystrophy. It appears as a bright to dull red spot in the center of macula surrounded by a grayish-white or yellowish halo, due to opacification of surrounding retinal ganglion cells (Fig. 26.1C). As fovea does not have ganglion cells, it remains transparent and transmits the red color of underlying vascular choroid (red-spot). Cherry-red spot may also develop in peripheral retina (d/d macular lesion) due to retinal ischemia following vascular lesions.

Fig. 26.1: Important fundus abnormalities: (A) Retinopathy of prematurity; (B) Retintis pigmentosa; (C) Cherry-red spot over macula; (D) Papilledema.

Retinal phakoma are pathognomonic lesions of various neurocutaneous disorders, e.g. Tuberous sclerosis (mulberry lesion), Neurofibromatosis, and Sturge-Weber syndrome (hemangioma)

Retinal detachment, i.e. separation of outer layer of retina from underlying pigmented epithelium may be: (a) traumatic, following retinal injury that allows vitreous fluid to seep into subretinal space, (b) tractional due to presence of a vitreoretinal membrane that pulls the retina, e.g. in ROP, sickle cell disease and diabetes, and (c) exudative due to collection of exudates in subretinal space in chorioretinitis or retinoblastoma.

These cases present with loss of vision, leukocoria and strabismus/nystagmus. Diagnosis rests on ophthalmo­scopy, though orbital CT/MRI imaging is necessary to establish the cause.

Retinal hemorrhage/infarcts, appear as cotton-wool spots on fundoscopy, seen in hypertensive retinopathy, severe anemia, sickle cell disease, leukemia and infective endocarditis (Roth's spots—hemorrhages with white center), etc.

Acute hypertensive retinopathy is characterized by: (a) generalized constriction of arterioles, (b) retinal edema, (c) flame-shaped hemorrhages, (d) cotton-wool spots, denoting retinal infarcts, and (e) papilledema. In longstanding cases, thickening of vessels may produce silver-wire or copper-wire appearance.

26.2.12