Clinical Presentation
POHS continues to be one of the leading causes of severe irreversible vision loss in middle-aged adults located in the endemic parts of the USA, including Ohio and Mississippi River valleys, or the 'Histo Belt'. It was first described as ocular lesions noted by Reid et al. in 1942 in patients with disseminated Histoplasmosis. In 1959, Woods and Wahlen and later Schlaegel and Kenney demonstrated activation of chorioretinal lesions with histoplasmin skin testing, pulmonary calcifications and atrophy around the optic nerve head, suggesting the link with histoplasmosis. Classically, the ocular manifestations of POHS include discrete atrophic choroidal scars or 'punched-out' lesions, peripapillary atrophy and the absence of inflammation in the anterior chamber and vitreous (Fig. 1). In addition, about 5% of patients can also have linear streaks of atrophic scars in the mid periphery. The choroidal scars are thought to represent lesions caused by dissemination of Histoplasma capsulatum through choroidal circulation. This dissemination is thought to cause a subclinical choroiditis, which is likely why the majority of these patients never present to an ophthalmologist. A small minority of patients (<5%) will go on to develop choroidal neovascularization (CNV) arising from areas of chorioretinal scarring. By the time patients present to the ophthalmologist, they usually have developed foveal-compromising CNV and complain of painless progressive blurring of central vision and metamorphopsia which can be acute or gradual.
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Figure 1.
Choroidal scars of ocular histoplasmosis. This patient demonstrates the classic atrophic choroidal scars or 'punched-out' chorioretinal lesions seen in ocular histoplasmosis. Also note the peripapillary atrophy, more notable in the left eye.
H. capsulatum, a dimorphic unencapsulated fungus, is the known causative agent of systemic histoplasmosis which is characterized by a self-limited flu-like disease with respiratory symptoms in an immunocompetent host. The course of the disease depends on the number of inhaled spores and the immune status of the host. There have been many reports of ocular involvement in primary disseminated histoplasmosis, with the most recent being a patient with HIV and primary disseminated histoplasmosis who developed ocular and central nervous system disease. This patient responded to antifungal treatment. However, the eradication of the fungi from the anterior segment proved to be quite difficult, as H. capsulatum was cultured from the aqueous humour of this patient after 16 months of antifungal therapy.
A few reports of a nonspecific chorioretinitis that develops in pediatric immunocompetent hosts with acute systemic histoplasmosis exist. A single case report documents acute histoplasmosis choroiditis in two immunocompetent brothers (ages 3 and 5), responding to antifungal therapy. The most recent case is that of a 16-year-old patient presenting with symptomatic active multifocal chorioretinitis prior to developing systemic histoplasmosis symptoms. Interestingly, the patient's symptoms responded drastically to intravenous steroids, not antifungal therapy. Furthermore, 3 months after initial presentation, there were only mild retinal pigment epithelium changes present on dilated fundus examination, no classic chorioretinal scars seen with POHS. Both of these patients were 20/20 on initial examination, which suggests that they may represent the acute manifestations of ocular histoplasmosis which is subclinical in the majority of patients.