Case Study - A patient with decreased vision

A confusing case of absent red reflex. By Dr Zafar Iqbal, Dr Sarabjit Soorae and Dr Lynn Sheldrake.

Toxoplasmosis causes scarring of the retina (yellow lesions) (Photograph: SPL)
Toxoplasmosis causes scarring of the retina (yellow lesions) (Photograph: SPL)

A 30-year-old man presented complaining of decreased vision in his right eye.

He had absent red reflex and the fundus could not be visualised properly. Visual acuity was reduced to 6/12.

The patient had had occasional problems with his vision for a long time but did not feel the need for a medical review until now. Further questioning revealed he had always had cats as pets since his childhood.

His age suggested that congenital cataract, retrolental fibroplasia or retinoblastoma were unlikely. These all cause absent red reflex but are mostly bilateral and present in children aged five years and under.

Diagnosis
On the basis of the history and unilateral absent red reflex, the diagnosis of ocular toxocariasis was deemed appropriate and he was referred to ophthalmology, where he was diagnosed with toxoplasma chorioretinitis.

The diagnosis was confirmed with PCR on anterior chamber tap, which was negative for herpes simplex, varicella zoster, fungi and bacteria but was positive for toxoplasma antibodies.

The patient was treated with prednisolone and co-trimoxazole for three months. He recovered well and remains under follow-up with ophthalmology.

Rare in the UK
Toxoplasma gondii is an intracellular parasite that causes zoonotic infections in humans. It is the most common cause of intraocular inflammation in the world and can be congenital or acquired.

The principal mode of transmission of toxoplasmosis is through the ingestion of oocysts shed into the environment in cats' faeces.

Ingestion of tissue cysts from contaminated beef, lamb or pork; ingestion of oocysts from soil or vegetables; and contaminated blood transfusions, organ transplants and accidental inoculation in the laboratory can all result in acquired toxoplasmosis.

The incidence rate in the UK is 0.4 cases per 100,000 persons per year.1 Toxoplasmosis infections seem to be more prevalent in hot and humid climates. In southern Brazil, approximately 18 per cent of the population have been found to have retinal lesions suggestive of ocular toxoplasmosis.

Toxoplasmosis is responsible for 30-50 per cent of all posterior uveitis cases in the US.

Untreated infection can lead to cataract formation, glaucoma, branch retinal artery and vein occlusion, uveitis, optic atrophy and blindness. It is easily treatable with steroids and anti-toxoplasmic agents. Toxoplasma retinitis frequently reactivates, and recurrence rates approach 80 per cent at five years.2

The incidence of toxoplasma chorioretinitis in the UK is very low but differential diagnosis of absent red reflex in adults should include ocular toxocariasis as a possibility, as was demonstrated in this case.

  • Dr Iqbal is a GP registrar and Dr Soorae and Dr Sheldrake are GP trainers in Birmingham

References
1. McCannel CA, Holland GN, Helm CJ, et al. Am J Ophthalmol 1996; 121(1): 35-46.

2. Bosch-Driessen LE, Berendschot TT, Ongkosuwito JV et al. Ophthalmology 2002; 109(5): 869-78.

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