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.
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 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.
Does laser surgery restore lost vision?
Laser photocoagulation usually does not restore lost vision.
However, it does reduce the chance of further CNV 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 with 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 visual
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.
Many eye care professionals advise patients who have received
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 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.
Keeping in tune with your disease or condition not only makes treatment less intimidating but also increases its chance of success, and has been shown to lower a patients risk of complications. As well, as an informed patient, you are better able to discuss your condition and treatment options with your physician.
A new service available to patients provides a convenient means of staying informed, and ensures that the information is both reliable and accurate. If you wish to find out more about HealthNewsflash's innovative service, take the tour.
