Ocular larva migrans (OLM) is a rare but potentially disabling condition. It is also difficult to diagnose because of the negative laboratory test results. In most cases the diagnosis is presumed to be OLM upon the basis of eye examination. Histopathology may also sometimes be confirmatory.
Mechanism of OLM
Ocular toxocariasis is typically due to a single toxocara larva invading the eye. The affected individual has a very low worm load, and other signs of infestation. It is thought that this leads to a low-level immune response which is inefficient in accomplishing larval death or isolation. This allows the larva to persist in the body for years, to migrate through the body and enter the eye.
The larva reaches the posterior segment of the eye through the blood vessels of the retina and choroid, and occasionally through the optic nerve. The larva has no particular predilection for the eye, and it seems to be a matter of chance that it takes up its residence in ocular tissues.
Even after reaching the eye, all larvae do not produce inflammation. Thus the host immune reaction, the larval secretion of proteins like the Toxocara excretory-secretory antigen, and the degree of inflammation produced, all play a role in the final degree of damage that occurs.
Most symptomatic larvae in the eye produce acute or chronic granulomas. Even when these are fibrotic, it is possible that the larvae are not dead and can still penetrate the surrounding tissue to leave the reaction site.
Ocular larva migrans may be due to a variety of different organisms:
- Toxocara canis (T. canis) or cutis is the most common cause of typical OLM
- Baylisascaris causes a more severe and chronic form because of the larger size of the larvae coupled with their ability to grow inside different organs causing more damage
- Angiostrongyloides species
Children with toxocarial ocular larva migrans rarely have any other signs or symptoms of the infestation. In a few cases, they may have the clinical features of covert toxocariasis or visceral larva migrans. The major risk factors are contact with dogs and cats, especially puppies and kittens which have a higher rate of infestation with toxocara.
In the majority of cases (90%), ocular larva migrans affects only one eye. Most children with ocular larva migrans are in the age group of 3-11 years, though recently the incidence among young teenagers has been reported to be higher. The most common symptoms include:
- Loss of visual acuity
- Leukocoria or white pupillary reflex
- Vitreous inflammation
- Bloodshot conjunctiva
- Ocular pain
- Diffuse subacute retinitis
- Blindness of one eye
In some children the retinal symptoms may be associated with those due to central nervous system infestation.
Visual loss in cases of OLM is due to the morbidity caused by:
- Vitreous inflammation
- Cystoid macular edema
- Retinal macular detachment due to traction
- Optic nerve head atrophy in a few cases
In some children the infection is diagnosed based only on routine eye examination, because of the absence of any symptoms. On the other hand, severe visual disability may be present.