What is the ocular histoplasmosis syndrome (OHS)?

The ocular histoplasmosis syndrome (OHS) is caused by an infection with a fungus called Histoplasma capsulatum. It is most commonly seen in the Ohio and Missippi River Valleys. The fungus is inhaled and causes a flu-like illness. Typically, the immune system kills the fungus, and it usually does not cause any chronic health problems. However, the fungus does leave behind scars, often in the lungs and in the back portion of the eye. Often, these scars are located in the peripheral retina and do not cause any visual loss. In some cases, though, they occur in the central retina (the macula), which is important for detailed vision. The scars, themselves, usually do not cause visual loss, but they increase the risk of abnormal blood vessel growth. These abnormal blood vessels grow beneath the retina and can bleed and leak fluid under the retina, causing the vision to worsen.

How is OHS diagnosed?

The diagnosis is made based on the appearance of the retina on examination, which is distinct. There are no specific laboratory tests for diagnosing ocular histoplasmosis. If evidence of abnormal blood vessel growth (i.e., blood and fluid under the retina) is noted on the examination, an angiogram can be done to determine the location of the abnormal vessels. This test sometimes involves the injection of a dye, fluorescein, into a vein in the arm. The dye travels through the bloodstream to the eye, where it leaks from the abnormal blood vessels.

How is OHS treated?

Histoplasmosis can result in choroidal neovascular membrane (CNVM) development in the macula which is a serious complication. CNVM involves the growth of abnormal blood vessels under the retina, which can lead to vision loss if left untreated.

The most common and effective treatment is the use of anti-VEGF injections, which help stop the growth of these abnormal vessels and reduce fluid leakage. These medications are injected directly into the eye during a quick, in-office procedure. Multiple treatments are often needed over time, depending on the response.

Early diagnosis and prompt treatment are key to preserving vision. If you notice sudden changes in your vision, such as blurriness, distortion, or dark spots, it’s important to see a retinal specialist as soon as possible.