Ocular histoplasmosis syndrome: An eye disease that is a leading cause of vision loss, due to the spread of spores of the fungus Histoplasma capsulatum (histo) from the lungs to the eye where they lodge in the choroid (a layer of blood vessels that provides blood and nutrients to the retina).
There the spores cause fragile, abnormal blood vessels to grow underneath the retina. These abnormal blood vessels form a lesion known as choroidal neovascularization (CNV). If left untreated, the CNV can turn into scar tissue and replace the normal retinal tissue in the macula (the central part of the retina that provides sharp central vision. If these abnormal blood vessels grow toward the center of the macula, they may affect a tiny depression called the fovea (the region of the retina with the highest concentration of special retinal nerve cells, called cones, that produce sharp, daytime vision). Damage to the fovea and the cones can severely impair, and even destroy, straight-ahead vision. Since the syndrome rarely affects side, or peripheral vision, the disease does not cause total blindness.
The syndrome usually causes no symptoms in its early stages. The initial infection subsides leaving tiny scars called "histo spots," which remain at the infection sites. Histo spots do not generally affect vision, but they can result in complications years or even decades after the original eye infection. In later stages, symptoms may appear if the abnormal blood vessels cause changes in vision. For example, straight lines may appear crooked or wavy, or a blind spot may appear in the field of vision.
A careful eye examination reveals two conditions: (1) The presence of histo spots, which indicate previous exposure to the histo fungus spores; and (2) Swelling of the retina, which signals the growth of new, abnormal blood vessels. professional to better examine the retina.
Treatment may include injections of anti-VEGF agents, photocoagulation or photodynamic therapy (PDT). Lasers produce small, powerful beam of light that destroy the fragile, abnormal blood vessels, as well as a small amount of the overlying retinal tissue. Although the destruction of retinal tissue during the procedure can itself cause some loss of vision, this is done in the hope of protecting the fovea and preserving the finely-tuned vision it provides.