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Describe ocular toxoplasmosis with its features and management.

Ophthalmology · 3rd Professional

Toxoplasma gondii is an obligate intracellular protozoan, and ocular toxoplasmosis is the commonest cause of infectious posterior uveitis worldwide
The definitive host is the cat, in whose intestine the sexual cycle occurs and which excretes oocysts in its faeces. Humans are intermediate hosts
Routes of human infection — ingestion of undercooked meat containing tissue cysts (the commonest route in most populations); ingestion of oocysts from cat faeces, contaminated soil, water or unwashed vegetables; and transplacental transmission

The classical appearance is a focal necrotising retinochoroiditis arising at the edge of an old pigmented scar — a "satellite" lesion. This pattern of recurrence adjacent to a previous scar is highly characteristic and is often the whole of the diagnosis
The active lesion is a fluffy, yellowish-white, elevated area of retinal necrosis with indistinct margins, overlying vitritis, and often adjacent retinal vasculitis with segmental arteriolar sheathing (kyrieleis plaques)
The "headlight IN the fog" — the classical description of the active lesion glowing through the overlying vitreous haze.

Investigation	Value
Serology (IgG)	A positive IgG merely indicates past exposure and is common in the general population, so it does not confirm ocular disease. A negative IgG, however, makes the diagnosis very unlikely and is therefore useful for exclusion
Serology (IgM)	Indicates recent infection; important in pregnancy, and in suspected acquired disease
Aqueous or vitreous PCR	Highly specific; used in atypical cases and in the immunosuppressed
Goldmann-WITMER coefficient	Compares the ratio of specific antibody to total IgG in aqueous with that in serum, demonstrating local antibody production
Imaging of the brain	Where congenital or immunosuppressed disease is suspected

Not every lesion needs treatment. A small peripheral lesion in an immunocompetent patient is self-limiting and resolves in 6 to 8 weeks; treatment is reserved for defined indications
Indications to treat — a lesion threatening the macula, the papillomacular bundle or the optic disc; a large lesion; severe vitritis; an immunosuppressed patient; pregnancy; and congenital infection
Classical triple therapy — pyrimethamine (with folinic acid to protect the bone marrow, since pyrimethamine is a folate antagonist), sulfadiazine, and systemic steroid
The rule about steroid is absolute: steroid is given only with antiparasitic cover, and never alone, and started 24 to 48 hours after the antimicrobial. Steroid alone permits fulminant proliferation of the organism

All questions in Diseases of the Uveal Tract