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Ocular Histoplasmosis Syndrome (OHS)


What is histoplasmosis?

Histoplasmosis is a disease cause 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 release into the air when soil is disturbed by plowing fields, sweeping chicken coops, or digging holes.

Histoplasmosis is often so mile, that it produces no apparent symptoms. Any symptoms that might occur are often similar to those from a common cold. In fact, if you had histoplasmosis symptoms, you might dismiss them as those from a cold or flu, since the body's immune system normally overcomes the infection in a few days, without treatment.

However, histoplasmosis, even in mild cases, can later cause a serious eye disease called ocular histoplasmosis syndrome (OHS), a leading cause of vision loss in Americans ages 20 to 40.

How does histoplasmosis cause ocular histoplasmosis syndrome?

Scientists believe that Histoplasma capsulatum (histo) spores spread from the lungs to the eye, lodging in the choroid, a layer of blood vessels that provideds blood and nutrients to the retina. Scientists have not yet been able to detect any trace of the histo fungus in the eyes of patients with ODS. 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 ocular histoplasmosisuntreated, the macula 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 fromt he retina to the brain are affected, and vision loss results.

Vision is also impaired with these abnormal blood vessels leak fluid and blood into the macula. If these abnormal 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 peripheral (side) vision, the disease does not cause total blindness.


What are the symptoms of OHS?

OHS usually has no symptoms in its early stages; the initial OHS infection usually subsides without the need for treatment. This is true for other histo infections; in fact, often the only evidence that the inflammation ever occured are tiny scars called "histo spots", which remain at the infection sites. Histo spots do not generally affect vision, but for reasons that are still not well understood, they can result in complications years, sometimes even decades, after the original eye infection. Histo spots have been associated with the growth of the abnormal blood vessels underneath the retina.

In later stages, OHS symptoms may appear if the abnormal blood vessels cause changes in vision. For example, strain lines may appear crooked or wavy, or a blind spot may appear in the field of vision. Because these symptoms indicate that OHS has already progressed enough to affect vision, anyone who has been exposed to histoplasmosis and perceives even slight changes in vision, should consult and eye care professional.

Who is at risk for OHS?

Although only a tiny fraction of the people infected with the histo fungus ever develop 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 Alabama, Illinois, Kentucky, Missouri, Tennessee, and West Virginia, as well as large portions of Alabama, Indiana, Iowa, Kansas, Louisiana, Maryland, Mississippi, Nebraska, Ohio, Oklahoma, Texas, and Virginia. Since most case 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".

How is OHS diagnosed?

An eye care professional will usually diagnose OHS if a careful eye examination reveals two conditions: (1) The presenced 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. To confirm the diagnosis, a dilated eye examination must be performed. This means that the pupils are enlarged temporarily with drops, allowing the eye care professional to better examine the retina.

If fluid, blood, or abnormal blood vessels are present, an eye care professional may want to perform a diagnostic procedure called fluorescein angiography. In this procedure, a dye is injected into the patient's arm, and travels to the blood vessels of the retina. The dye allows a better view of the CNV lesion, and photographs can document the location and extent to which it has spread. Particular attention is paid to how close the abnormal blood vessels are to the fovea.

How is OHS treated?

The only proven treatment for OHS is a form of laser surgery called photocoagulation. A small powerful beam of light destroys 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.

How effective is laser surgery?

Controlled clinical trials, sponsored by the National Eye Institute, have shown that photocoagulation can reduce future vision loss from OHS by more than half. The treatment is more effective when:

Does laser surgery restore lost vision?

Laser photocoagulation usually does not restore lost vision. However, it does reduce the chance of further SNV growth and any resulting vision loss.

Does laser surgery cure OHS?

No. OHS cannot be cured. Once contracted, OHS remains a threat to a person's sight for their lifetime.

People wiht OHS who experience one bout of abnormal blood vessel growth, may have recurrent CNV. Each recurrence can damage vision and may require additional laser therapy. It is crucial to detect and treat OHS as early as possible, before it causes significant visiual impairment.

Is there a simple way to check for signs of OHS damage to the macula?

Yes. A person can check for signs of damage to the macula by looking at a printed pattern called an Amsler grid. If the macula has been damaged, the vertical and horizontal lines of the grid may appear curved, or a blank spot may seem to appear.

Amsler grid normalAmsler grid abnormal



Many eye care professionals advise patients who have recieved treatment for OHS, as well as those with histo spots, to check their vision daily with the Amsler grid one eye at a time. Patients with OHS in one eye are likely to develop it in the other.

What help is available for people who have already lost significant vision from OHS?

Scientists and engineers have developed many useful devices to help people with severe visual impairment in both eyes. These devices, called low vision aids, use special lenses or electronics to creat enlarged visual images. An eye care professional can suggest sources that provide information on counseling, training, and special services for people with low vision. Many organizations for people who are blind, also serve those with low vision.

What research is being conducted on the ocular histoplasmosis syndrome?

The National Eye Institute (NEI) supports research aimed at learning more about the relationship between histoplasmosis and OHS, and how to treat OHS effectively. One such multicenter clinical study is called the Submacular Surgery Trials (SST). This clinical study is examining whether CNV in the fovea, which cannot be treated by laser photocoagulation, can be successfully removed through traditional surgery. Patiens with OHS who would like to receive more information about the Submacular Surgery Trials, should call the SST Chairman's Office toll free at 1-888-554-0412. Information on the Submacular Surgery Trials is also availabe on the NEI web site at http://www.nei.nih.gov/neitrials/index.aspx


Source - National Eye Institute (NEI) of the National Institutes of Health (NIH)

Disclaimer: The information contained in this website does not substitute for the advice of a qualified eye care professional and is not intended to constitute medical advice.