Parasitic Retinal Disorders Definition

Inflammation of the retina caused by infection with parasites such as Onchocerca volvulus (the pathogen that causes onchocerciasis), Toxocara canis or Toxocara cati (nematode larvae that are normally intestinal parasites of dogs and cats), Taenia solium, (pork tapeworm), and other parasites.

Epidemiology: Onchocerciasis, like trachoma and leprosy, is one of the most frequent causes of blindness worldwide. However, like the other parasitic diseases discussed here, it is rare in Europe and North America.

Etiology: Onchocerca volvulus is transmitted by the bite of black flies. This allows the larvae (microfilaria) to penetrate the skin, where they form fibrous subcutaneous nodules. There they reach maturity and produce other micro-filaria, which migrate into surrounding tissue. The danger of ocular infiltration is particularly great where there are fibrous nodules close to the eye.

Toxocara canis or Toxocara cati (eggs of nematodes infesting dogs and cats) are transmitted to humans by ingestion of substances contaminated with the feces of these animals. The eggs hatch in the gastrointestinal tract, where they gain access to the circulatory system and may be spread throughout the entire body. The choroid can become infested in this manner.

Taenia solium: The pork tapeworm infestation can occur from eating pork contaminated with larvae or other substances contaminated with tapeworm eggs. Mature tapeworms can also release eggs into the intestine. The larvae travel through the bloodstream to various organs and can also infest the eye.

Diagnostic considerations and findings: Ophthalmoscopy will reveal intraocular inflammation. Onchocerciasis has been known to be associated with posterior uveitis as well as keratitis and iritis. Histologic examination will demonstrate microfilaria in the retina. Visceral larva migrans, Toxocara canis, or Toxocara cati can cause complications involving endophthalmitis and retinal detachment. Subretinal granulomas and larval inflammation of the retina have been known to occur. The larvae of different species of worms can produce diffuse unilateral subacute neuroretinitis with the typical clinical picture of grayish white intraretinal and subretinal focal lesions. Fly larvae can also invade the subretinal space in ophthalmomyiasis.

Differential diagnosis: Other causes of retinal inflammation and subretinal granulomas should be excluded.

Treatment: Laser photocoagulation or surgical removal of the worm larvae may be indicated.

Clinical course and prognosis: It is not uncommon for these disorders to lead to blindness.

12.8 Retinal Tumors and Hamartomas 12.8.1 Retinoblastoma Definition

A retinoblastoma is a malignant tumor of early childhood that develops from immature retinal cells.

Epidemiology: Retinoblastoma is the most common malignant ocular tumor in children, occurring in approximately one of 20000 births. In 30% of all cases, it is bilateral.

Pathogenesis: A somatic mutation is detected in about 95 % of all patients. In the other patients, it is inherited as an autosomal dominant trait. Changes on chromosome 13 q have been observed in germ-cell mutations. Retinoblas-tomas may then occur at several locations in the retina or bilaterally.

H Where retinoblastoma is inherited as an autosomal dominant trait, the siblings of the affected child should be regularly examined by an ophthalmologist.

Symptoms: Retinoblastoma manifests itself before the age of three in 90% of affected children. Parents observe leukocoria (a whitish yellow pupil; Fig. 12.36) in 60% of these children, strabismus in 20%, and a reddened eye in 10%.

H Every child presenting with strabismus should undergo examination of the fundus with the pupil dilated to exclude a retinoblastoma.

Findings and diagnostic considerations: A grayish white, vascularized retinal tumor will be observed on ophthalmoscopy. In its advanced stages, this tumor was formerly referred to as an amaurotic cat's eye. Infiltration of the vitreous body, anterior chamber (pseudohypopyon), and orbit may occur. A retinoblastoma that also involves the fellow eye and pineal body is referred to as a trilateral retinoblastoma.

U A trilateral retinoblastoma is defined as additional manifestation of the tumor in the pineal body.

Calcifications frequently occur in these tumors. Radiographs or CT images that show calcifications can therefore help to confirm the diagnosis in uncertain cases.

Differential diagnosis: Several other disorders should be excluded by oph-thalmoscopy. These include:

Cataract (with leukocoria).

❖ Primary strabismus (with strabismus).

— Leukocoria in the left eye due to a retinoblastoma. -

Fig. 12.36 The whitish gleam of the pupil of the left eye is a typical finding in retino-blastoma.

— Leukocoria in the left eye due to a retinoblastoma. -

Leukocoria

Retinal detachment, persistent hyperplastic primary vitreous (PHPV), and Coats' disease should also be excluded.

Treatment: Tumors less than four pupil diameters may be managed with radiation therapy delivered by plaques of radioactive ruthenium or iodine (brachytherapy) and cryotherapy. Larger tumors require enucleation of the eye.

Prophylaxis: Following the diagnosis, the fellow eye should be examined with the pupil dilated every three months for five years. After that, follow-up examinations may be performed at greater intervals.

Clinical course and prognosis: Left untreated, a retinoblastoma will eventually metastasize to the brain and cause death. Patients frequently develop a second malignant tumor such as an osteosarcoma.

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