Understanding Ocular Histoplasmosis Syndrome (OHS)
Scientists believe that OHS is caused by Histoplasma capsulatum (H. capsulatum). This fungus is found in the dust and soil in parts of the U.S. (mostly the Mississippi-Ohio River Valley area). If breathed into the lungs, the fungus can cause histoplasmosis. This is typically a mild condition with flu-like symptoms or no symptoms at all. The condition often goes away without treatment. But it is thought that once in the body, H. capsulatum can also affect the eyes. This is OHS.
OHS causes inflammation in the choroid. This is the layer of blood vessels in the retina (the light-sensing layer of tissue at the back of the eye). The inflammation leaves scars called “histo spots.” As OHS progresses, abnormal blood vessels can grow at the histo spots. These blood vessels form lesions called choroidal neovascularization (CNV). If the CNV lesions leak blood and fluid into the macula (the center part of the retina that plays a key role in clear vision), vision can be distorted. More seriously, if the lesions occur in the macula, vision loss can result. If OHS occurs in one eye, it may later occur in the other eye.
Risk Factors for OHS
Anyone who has had histoplasmosis is at risk of OHS. But very few people develop OHS.
Signs and Symptoms of OHS
Early cases often have no symptoms. But as the disease progresses, an eye doctor may find histo spots during an eye exam. And a patient may notice vision changes such as:
- A blind spot in the eye
- Straight lines appearing crooked or wavy
Diagnosis of OHS
An eye doctor can check for OHS. Your pupils (the dark circles in the centers of your eyes) will be dilated (enlarged) with special drops. Your eyes will then be examined. Fluorescein angiography may also be done. With this test, a special dye (fluorescein) is injected into a vein in your arm. The dye travels to your retina. The dye allows the doctor to check blood flow to the retina. This helps the doctor see any histo spots.
Treatment of OHS
If the macula is not affected, OHS does not need treatment. When the macula is affected, there are two types of treatment:
- Photocoagulation. With this surgery, a special laser destroys the abnormal blood vessels in the retina. This procedure can only help protect against future loss of vision. It cannot repair vision loss that has already occurred.
- Medications. These limit abnormal blood vessel growth. The eye is numbed and then injected with medications. These injections need to be repeated every 4 to 12 weeks. The frequency depends on the type of medication used. Your doctor can tell you more about this treatment.
Checking for Macular Damage
Your eye doctor may suggest that you use an Amsler grid. This can be helpful for patients who have already had laser surgery, as well as those with histo spots but no vision loss. An Amsler grid is a simple lined grid with a dot in the middle. Looking at it can help you check for signs of damage to the macula. If damage has occurred, the lines of the grid may look curved, or a blank spot may seem to appear. Your eye doctor can tell you more about the Amsler grid.