Presumed Ocular 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. In the United States, it is most commonly found in the Mississippi-Ohio River Valley region. Depending on location, 60-90% of the adult population living in this region (called the “Histo Belt”) are carriers. It affects men and women equally. Histoplasmosis is often so mild that it produces no 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 mild cases, can later cause a serious eye disease called presumed ocular histoplasmosis syndrome (POHS), a leading cause of vision loss in Americans ages 20 to 40. The fungus causes small areas of inflammation and scarring of the retina. These are called “histo spots” and may be found in both eyes. The affect on vision depends on the location of the scars. Scarring in the peripheral area of the retina may have little or no impact on vision, while a central scar affecting the macula may cause a prominent blind spot.
Although only a tiny fraction of the people infected with the histo fungus ever develops POHS, any person who has had histoplasmosis should be alert for any changes in vision similar to those described above. Studies have shown the POHS patients usually test positive for previous exposure to histoplasmosis.
|Peripheral spots of chorioretinal atrophy commonly called “histo spots.”|
Most people with histo spots in the retina are totally unaware of their presence unless the central vision is affected. Studies indicate that only about 5% of those with histo spots are at risk of losing vision.
|A patient with an old laser scar [black circular area] and a new area of choroidal neovascularization [central white spot].|
POHS causes ocular symptoms in two ways. Small, self-limited areas of inflammation can occur leaving behind the typical atrophic “histo spot.” A more serious problem is 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. When this scar tissue forms, visual signals 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 involve the center of the macula, they may significantly affect visual acuity. Damage to the fovea can severely impair, and even destroy, central vision. Early treatment of POHS is essential because once the abnormal blood vessels affect the fovea, regaining central vision is more difficult. POHS rarely if ever affects peripheral vision, so the disease does not cause complete blindness.
Signs and Symptoms
Many patients with histo spots in their eyes have no symptoms. Others may experience the following:
- Distorted vision
- Blind spots
- Scars in the retina, ranging in severity
Detection and Diagnosis
Ocular histoplasmosis is detected with a dilated examination of the retina. It is usually diagnosed based on its distinctive appearance and characteristics.
- The presence of peripheral histo spots, which indicate previous exposure to the histo fungus spores; and
- Atrophy or thinning of the retina around the optic nerve
- Swelling of the retina, which signals the growth of new, abnormal blood vessels
If fluid, blood, or abnormal blood vessels are present, we will perform a diagnostic procedure called fluorescein angiography. In this procedure, a dye, injected into the patient's arm, travels to the blood vessels of the retina. The dye allows a better view of the CNV, 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.
Ocular histoplasmosis requires no treatment except when abnormal blood vessels develop in the central retina. For these patients, laser treatment, intraocular injections, or vitrectomy surgery may be necessary. Regular eye exams and routine use of an Amsler Grid to monitor central vision is recommended for anyone with histo spots.
There are two proven laser treatments for POHS at this time. The first uses a small, powerful beam of light to destroy the abnormal blood vessels as well as the overlying retinal tissue. Although the destruction of retinal tissue during the procedure itself causes some loss of vision, this is done to try to protect the fovea from an expanding membrane. Controlled clinical trials, sponsored by the National Eye Institute, have shown that photocoagulation can reduce future vision loss from PPOHS by more than half. The treatment is most effective when:
- The CNV has not grown into the center of the fovea, where it can affect vision.
- The eye care professional is able to identify and destroy the entire area of CNV.
Laser photocoagulation usually does not restore lost vision. However, it does reduce the chance of further CNV growth and any resulting vision loss.
The second form of laser is called photodynamic therapy (PDT or OPT). Initially developed for CNV due to macular degeneration, its indications have been expanded to include POHS. This treatment uses a light-sensitizing dye injected into a vein to more selectively destroy the vessels and spare the overlying retinal tissue. This is generally used for vessels that involve the direct center of the retina or fovea. The dye, called verteporfin (Visudyne), is infused into the patient's arm over a ten minute period. Five minutes later a laser is used to treat the vessels for an 83 seconds. Because this photosensitizing dye is injected into a patient's vein, the dye travels throughout their body and there are strict restrictions to avoid direct sunlight for five days. Like thermal laser, this treatment does not eliminate the original cause of the vessel growth, and patients generally need more than one treatment. Patients are seen on an every three month basis with repeat Fluorescein Angiograms. If there is leakage on the angiogram, the laser treatment is repeated. In rare cases where both eyes are affected by these vessels, a treatment can be performed sequentially with the same dye infusion.
Triamcinolone acetate (Kenalog) is a long-acting anti-inflammatory steroid which is used in ocular histoplasmosis for its action against blood vessel growth and retinal swelling. It may be difficult to differentiate between inflammatory and early neovascular lesions, but kenalog has activity against both. Four milligrams are injected through the white part of the eye directly into the vitreous cavity. Topical and subconjunctival anesthesia make this a quick and painless procedure. The drug lasts in the eye for three months on average. Patients are followed monthly to monitor intraocular pressure which may rise after this injection.
Intravitreal Bevacizumab (Avastin)
Bevacizumab is an anti-VEGF antibody that was originally approved by the FDA for treatment of colorectal cancer. Through pioneering work at the Bascom Palmer Eye institute in Miami, Dr. Phil Rosenfeld and colleagues have shown this to be a very effective treatment for exudative macular degeneration. Based on a similar mechanism of choroidal neovascularization, patients with POHS have also been successfully treated with this medication. Because this is considered by most insurance carriers to be experimental, this involves out-of-pocket expenses for patients in Indiana.
In select cases, the abnormal vessels can be surgically removed to improve vision. Two of my mentors, Dr. Matthew Thomas and Dr. Henry Kaplan, are credited with developing the technique at the Barnes Retina Institute and Washington University in St. Louis. The technique involves removing the vitreous gel, creating a small retinal detachment, gently pulling the abnormal vessels from under the retina, and reattaching the retina. Unfortunately, this also does not address the underlying trigger of the CNV and recurrences can be quite common. The National Eye Institute (NEI) supported a multicenter clinical study 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.
Despite significant advances in treatments, POHS still cannot be cured. Once contracted, POHS remains a threat to a person's sight for their lifetime. People with POHS who experience one bout of abnormal blood vessel growth may have recurrent episodes. Each recurrence can damage vision and may require additional therapy. It is crucial to detect and treat POHS as early as possible before it causes significant visual impairment.
A person with POHS 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. We advise patients with POHS to check their vision daily with the Amsler grid one eye at a time. If there are changes in the grid from baseline, call within 24 hours to set up an appointment.
Low Vision Aids
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 create 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.