This information was developed by the National Eye Institute (NEI) to help
patients and their families in searching for general information about
histoplasmosis. An eye care professional who has examined the patient's eyes and
is familiar with his or her medical history is the best person to answer
specific questions.
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.
Histoplasmosis is often so mild 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 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 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.
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 occurred
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, straight 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 an eye care professional.
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.
How is OHS diagnosed?
An eye care professional will usually diagnose OHS if 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. To confirm the
diagnosis, a dilated eye examination must be performed. This means that the
pupils are enlarged temporarily with special 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, injected into the patient's arm, 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.
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 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.
Source: National Institutes of Health: The National Eye Institute
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This information is provided by the Cleveland Clinic and is not intended to replace the medical advice of your doctor or health care provider. Please consult your health care provider for advice about a specific medical condition. This document was last reviewed on: 8/7/2009...#5635
