Presumed ocular histoplasmosis syndrome (POHS) is a distinct clinical entity that is characterized by peripheral atrophic chorioretinal scars, peripapillary scarring, and maculopathy.  Vitritis is typically absent. This condition is believed to be secondary to exposure to Histoplasma capsulatum, although this fungus rarely has been isolated or cultured from an eye with the typically associated clinical findings.
Epidemiologic findings link the fungus to this condition. Based on skin tests in the United States, a similar geographic distribution of the fungal infection and POHS exists. Histoplasmin skin testing may exacerbate the ocular condition. Visual loss in POHS is secondary to the development of macular choroidal neovascularization (CNV).
H capsulatum is a dimorphic pathogenic fungus that often grows in soil around old chicken houses and areas harboring bats, such as caves. Large numbers of spores are dispersed into the air when contaminated soil is disturbed (eg, spelunking, raking). Exposure occurs when spores are inhaled.
In a normal host, the initial infection is usually asymptomatic or feels like influenza. In a few patients, a chronic cavitary pulmonary disease may follow. In immunocompromised patients, a progressive, life-threatening, disseminated form can occur. Following initial infection, hematogenous spread to the rest of the body, including the eye, can occur. A focal granulomatous choroiditis is thought to occur. This phase seldom is observed in humans.
In an experimental nonhuman primate model, inoculation of the organism via the carotid artery results in an active choroiditis. These foci are observed as discrete, round, yellowish choroidal lesions. Six weeks following inoculation of the organisms, isolating the organisms by any histologic techniques was not possible. The inflammatory response probably destroyed the invading organisms. With time, the lesions resolve, leaving the typical "punched-out" atrophic scars that disrupt the Bruch membrane (see Physical). Reexposure to the histoplasmin antigen may account for the enlargement of old scars and the emergence of new scars.
Visual loss in POHS is secondary to the development of CNV. Pigment epithelium derived factor (PEDF) was found to have an inhibitory effect on ocular neovascularization. Another peptide, vascular endothelium growth factor (VEGF), is a well-known ocular angiogenic factor. The balance between antiangiogenic factors (eg, PEDF) and angiogenic factors (eg, VEGF) may determine the growth of CNV. This CNV usually grows in the subretinal space in a sheetlike fashion, not in the sub–retinal pigment epithelium (RPE) space. As the CNV grows, reactive hyperplastic RPE tries to surround and envelop the CNV. If successful, the CNV usually involutes.
Why CNV arises is unclear. Based on a case of a pregnant woman who developed a macular detachment during the third trimester, some propose that a type of vascular decompensation caused the CNV.  A lymphocytic choroidal infiltrate usually is found near histo spots. Some hypothesize that an allergic reaction to Histoplasma antigens is an important stimulus for CNV growth. A few propose that the infectious granuloma secondary to the fungus is responsible for CNV growth. Others state that damage to the Bruch membrane by itself is a strong stimulus for CNV.
Presumed ocular histoplasmosis syndrome occurs in endemic areas of the United States, including the Ohio and Mississippi River Valleys (Indiana, Ohio, Illinois, Kentucky, Tennessee, and Mississippi) and parts of the mid-Atlantic region (Maryland, West Virginia, and Virginia). Approximately, 200,000-500,000 new infections occur annually. 
Peripheral atrophic scars were present in 2% of people living in endemic areas; disciform scars were found in 0.1% of people. 
A clinical entity indistinguishable from POHS has been reported in a series of 10 patients from a nonendemic area, the Pacific Northwest. These patients tested negative to a lymphocyte stimulation assay with H capsulatum. Unlike series from the Midwest, all patients were female; 50% of them were myopic. The authors speculate that an atypical mycobacteria might be responsible for this entity. 
H capsulatum is endemic in the Caribbean, Central America, and South America. No reports of POHS from these areas have appeared in the literature. However, certain areas of India are endemic for systemic histoplasmosis.  Three cases of POHS were recently described in India. 
Patients with clinical findings indistinguishable from POHS have been reported in areas where H capsulatum is not found. These reports suggest that other agents can cause similar fundus findings as POHS. In the Netherlands and the United Kingdom, a clinical syndrome indistinguishable from POHS has been described. In the Dutch series by Suttorp-Schulten et al, a large number of female and myopic patients were reported.  Given that H capsulatum is not found in Europe, some European colleagues have proposed to change the name from POHS to multifocal choroidopathy. Other cases from Brazil and Morocco have also been described. [12, 13]
POHS is an uncommon cause of visual loss.
The incidence and the prevalence in the blind population in Tennessee were reported to be 2.8% and 0.5%, respectively.
Histoplasmin skin testing reveals that African Americans and Caucasians living in endemic areas are infected equally by H capsulatum. No difference is apparent between the prevalence of peripheral histo spots between these 2 groups. Caucasian patients are more likely than African Americans to develop macular CNV.
Some reports suggest an increased prevalence of POHS in patients with human leukocyte antigen B7 (HLA-B7) and human leukocyte antigen DRw2 (HLA-DRw2). Patients with HLA-DRw2 are more likely to have peripheral histo spots and disciform scarring, whereas patients with HLA-B7 are more likely to have peripheral histo spots.
No apparent gender predilection exists in the Midwest and the mid-Atlantic series according to Smith et al. 
Presumed ocular histoplasmosis syndrome usually affects patients aged 20-50 years; the average age is 35 years.
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