SIGNS AND SYMPTOMS
Presumed ocular histoplasmosis syndrome (POHS) is characterized by the triad of:
1. Disseminated midperipheral choroiditis, consisting of infiltrates and scarring which appears as yellow-white punched-out lesions.
2. A macular or parafoveal subretinal neovascular membrane which appears as a grayish-green patch beneath the retina in the peripapillary and foveal areas, with or without subretinal blood, exudate or disciform scarring.
3. Atrophy or scarring adjacent to the optic disc, which appears as a flat, whitish-brown lesion; the presentation varies depending on the amount of retinal pigmentary epithelial hyperplasia next to the optic disc.
POHS occurs bilaterally in 60 percent of cases. Patients are usually age 20-50. The eye remains remarkably quiet with virtually no aqueous or vitreous cells and minimal flare. In fact, most patients are asymptomatic until a choroiditis or subretinal neovascular membrane develops around the fovea. Optic disc edema is an occasional finding in active disease.
Presumed ocular histoplamosis syndrome is a dimorphic fungal disease endemic to river valley climates. The designation "presumed" ocular histoplasmosis syndrome is used because researchers have not been able to isolate the Histoplasma Capsulatum organism in ocular tissue.
POHS probably results from infection by the blood-borne pathogen H. Capsulatum. This organism has an affinity for choroidal tissue and disseminates there following a systemic infection. The primary route of infection is through the lungs following an encounter with contaminated molds found in soil or from yeasts found in animals. During the initial infection, the patient may complain of fever, chills and malaise.
H. Capsulatum or its by-products produce a lymphocytic infiltration within the choroid. This results in the formation of granulomatous inflammatory masses that disrupt Bruch's membrane and the retinal pigment epithelium and compromise the outer layers of the retina. In most cases, the resulting peripheral choroidopathies are mild, producing only subtly observable clinical signs that heal without complications within a few weeks. Macular lesions possess serious ocular sequelae and progress through stages. Their presentation can lag behind the systemic infection by 30 years. Stress or immunocompromise increases the risk of macular involvement and POHS reactivation.
Stage 1 POHS maculopathy is marked by active choroiditis near the macula appearing as a yellow focus with fuzzy white borders.
Stage 2 POHS is characterized by SRNVM growth through compromised Bruch's membrane, serous sensory retinal detachment and RPE hyperplasia. Here, the RPE attempts to confine and envelop the SRNVM. This stage may abort and resolve leaving only a small residual scotoma.
Stages 3 and 4 consist of continued SRNVM progression, with sub-RPE hemorrhage and exudation.
Stage 5 is considered the end stage and is marked by maculopathy of more than two years and disciform scarring.
The key to successful management of POHS and SRNVM is early detection. Laboratory tests are of no value in diagnosing POHS. Make the diagnosis via the history and clinical observation. Patient education along with Amsler grid and home monitoring of visual acuity help uncover reactivated or macular disease.
Early asymptomatic active retinal lesions are sometimes treated with oral or depot injections of steroids to reduce inflammation and the risk of macular disease. This is controversial given a lack of firm evidence that steroid or antifungal therapies are beneficial.
Numerous studies demonstrate the benefit of photocoagulation in cases with macular involvement. If you suspect an SRNVM, order fluorescein angiography. If angiography reveals an SRNVM, refer the patient for laser treatment. Follow-up care is critical due to the high incidence of recurrence following treatment of SRNVM.
The differential diagnosis of POHS includes both inflammatory and noninflammatory chorioretinal diseases. Consider diagnoses such as angioid streaks, toxoplasmosis, toxocariasis, recurrent multifocal choroiditis, birdshot choroiditis, punctate inner choroidopathy, acute posterior multifocal placoid pigment epitheliopathy, serpiginous choroidopathy and diffuse unilateral subacute neuroretinitis.
If one eye has a disciform scar, there is a 30 percent chance the fellow eye will develop an SRNVM.
Patients who have POHS frequently have a history of exposure to pigeons or chickens. It is frequently thought that avian fecal matter is a vector of spreading disease.