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, a fluorescein angiogram can be done to determine the location of the abnormal vessels. This test 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?
If the abnormal vessels are not beneath the very center of the macula (the fovea), they can be treated with thermal laser, which burns both the “good” tissue (the retina) and the “bad” tissue (the abnormal vessels). It might cause a grey area in the vision, but as long as the laser does not burn the fovea, the detailed, central vision should remain good. This treatment can be done in the office with only eye drops for anesthesia. As other treatments are developed, thermal laser treatment is being used less often.
If the abnormal vessels are located beneath the fovea, traditional laser treatment is avoided, because it would destroy the fovea along with the abnormal vessels, causing the vision to worsen immediately. Avastin, a medication that is injected into the eye, can be quite effective for this condition.
Photodynamic therapy (PDT) has been used for abnormal blood vessels beneath the fovea, but it has been used much less often since Avastin became available. PDT involves the injection of a dye into a vein in the arm, followed by shining a low-intensity laser on the abnormal blood vessels. The dye, called Visudyne, collects in the abnormal vessels. The low-intensity laser “triggers” the dye to damage the vessels, causing them to close. Once the vessels close, they stop bleeding and leaking fluid under the retina.
Several important facts about PDT should be remembered:
- PDT is intended to prevent further visual loss. Only about 15% of patients treated with PDT have improvement in their vision. Still, if further visual loss can be prevented, it is certainly worth performing the procedure, since abnormal blood vessels beneath the retina usually cause severe visual loss if untreated.
- PDT often needs to be repeated. In some cases, PDT is repeated several times.
- PDT is not appropriate for all patients with abnormal blood vessels under the center of the retina. The vessels need to be fairly well-defined on the fluorescein angiogram. If they are poorly defined, the treatment is not likely to be of benefit.
- PDT causes extreme sensitivity to sunlight. Therefore, patients undergoing PDT should avoid sunlight for 3 days following the procedure. Exposure to sunlight can cause severe burns.
Rarely, in cases where the abnormal blood vessels have caused severe visual loss and failed to respond to other treatments, vitrectomy surgery can be performed for removal of the abnormal blood vessels.