Ocular histoplasmosis syndrome

What is histoplasmosis?

Histoplasmosis is a disease caused when airborne spores of the fungus Histoplasma capsulatum are inhaled into the lungs, the primary infection site. This microscopic fungus, which is found throughout the world in river valleys and soil where bird or bat droppings accumulate, is released into the air when soil is disturbed by plowing fields, sweeping chicken coops, or digging holes.

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What is Ocular histoplasmosis syndrome (OHS)?

Ocular histoplasmosis syndrome (POHS) is a clinical entity that is characterized by peripheral atrophic chorioretinal scars, peripapillary scarring, and maculopathy.

How does histoplasmosis cause ocular histoplasmosis syndrome (OHS)?

Scientists believe that Histoplasma capsulatum (histo) spores spread from the lungs to the eye, lodging in the choroid, a layer of blood vessels that provides blood and nutrients to the retina.

The retina is the light-sensitive layer of tissue that lines the back of the eye. Scientists have not yet been able to detect any trace of the histo fungus in the eyes of patients with ocular histoplasmosis syndrome. Nevertheless, there is good reason to suspect the histo organism as the cause of OHS.

How does OHS develop?

OHS develops when fragile, abnormal blood vessels grow underneath the retina. These abnormal blood vessels form a lesion known as choroidal neovascularization (CNV). If left untreated, the CNV lesion can turn into scar tissue and replace the normal retinal tissue in the macula.

The macula is the central part of the retina that provides the sharp, central vision that allows us to read a newspaper or drive a car. When this scar tissue forms, visual messages from the retina to the brain are affected, and vision loss results.

Vision is also impaired when these abnormal blood vessels leak fluid and blood into the macula. If these abnormal blood vessels grow toward the center of the macula, they may affect a tiny depression called the fovea. The fovea is 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, this straight-ahead vision. Early treatment of OHS is essential; if the abnormal blood vessels have affected the fovea, controlling the disease will be more difficult. Since OHS rarely affects side, or peripheral vision, the disease does not cause total blindness.

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Who is at risk for OHS?

Although only a tiny fraction of the people infected with the histo fungus ever develops OHS, any person who has had histoplasmosis should be alert for any changes in vision similar to those described above. Studies have shown the OHS patients usually test positive for previous exposure to histoplasmosis.

In the United States, the highest incidence of histoplasmosis occurs in a region often referred to as the "Histo Belt," where up to 90 percent of the adult population has been infected by histoplasmosis.

This region includes all of Arkansas, Kentucky, Missouri, Tennessee, and West Virginia as well as large portions of Alabama, Illinois, Indiana, Iowa, Kansas, Louisiana, Maryland, Mississippi, Nebraska, Ohio, Oklahoma, Texas, and Virginia.

Since most cases of histoplasmosis are undiagnosed, anyone who has ever lived in an area known to have a high rate of histoplasmosis should consider having their eyes examined for histo spots.

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