Histoplasmosis And
The Eye
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.
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. Patients
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 available on the NEI web site at http://www.nei.nih.gov/neitrials/index.htm.
Where
can I obtain information on histoplasmosis of the lungs and
other parts of the body?
Information on systemic
histoplasmosis can be obtained from:
February
1999
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