An Overview of Ocular Histoplasmosis

A Leading Cause of Vision Loss in Adults

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Imaging the retina
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Histoplasmosis is a lung infection caused by inhaling a type of fungus spores into the lungs. The fungus, known as histoplasma capsulatum, is found throughout the world in soil and in bird or bat droppings. It is spread into the air by plowing fields, sweeping chicken coops, or digging holes.

Although most cases of histoplasmosis are mild and require no treatment, people with weakened immune systems may develop more serious cases of the disease.

Histoplasmosis may progress and spread to other areas of the body, including the heart, brain, spinal cord, and adrenal glands.

In rare cases of histoplasmosis, a serious eye infection develops called ocular histoplasmosis syndrome (OHS). In cases of OHS, fungus spores spread into the eyes. This disease is a leading cause of vision loss in people between the ages of 20 and 40.

Symptoms

Symptoms of a histoplasmosis infection will usually appear within 10 days of histo fungus exposure. Symptoms might include the following:

  • Fever
  • Dry cough
  • Watery eyes
  • Chest pain
  • Joint pain
  • Red bumps on legs

In severe cases, symptoms may include:

  • Sweating
  • Shortness of breath
  • Coughing up blood

In most cases of OHS, there are no symptoms. A person infected with the disease might assume they are throwing off a common cold or flu.

The infection usually disappears over a period of a few days without treatment. Damage to one's vision may not happen right away, as the inflammation caused by the infection may leave behind tiny scars called “histo spots” at the infection site in the eyes.

Histo spots don't usually affect your vision right away, but they can cause problems later in life.

Because the initial histoplasmosis infection usually does not cause symptoms throughout the body, most people never realize they have histoplasmosis scars, or histo spots, in their retina. The scars can later lead to neovascularization in the macula resulting in visual loss.

In some people, several years may go by before any complications arise. Abnormal blood vessels can form and cause changes in vision, including blind spots or straight lines appearing wavy.

Causes

Breathing fungus spores into the lungs can lead to a case of histoplasmosis. The spores can then spread from the lungs to the eyes, where abnormal blood vessels may begin to grow underneath the retina. These blood vessels can cause lesions, and if left untreated, can form scar tissue.

Most scar tissue in the retina does not cause any problems. However, the scar tissue can begin to take the place of healthy retinal tissue in the macula, the central part of the retina that enables our sharp, clear vision.

Scars in the macula can lead to new blood vessel growth, referred to as neovascularization. Neovascularization causes vision loss because the abnormal blood vessels can leak fluid and blood. If left untreated, this neovascularization can form a scar, leaving damaged tissue in place of the normal, living tissue. Scar tissue eventually affects communication between the eyes and the brain, resulting in vision loss.

Types

A case of histoplasmosis can be classified as either acute or chronic, depending on severity and longevity of the disease.

  • Acute histoplasmosis or short-term histoplasmosis is typically mild. It rarely leads to complications.
  • Chronic histoplasmosis, or long-term, histoplasmosis occurs far less often than the acute form. In rare cases, it can spread throughout the body, including the eyes.

Diagnosis

Your eye doctor will be able to diagnose OHS after a dilated eye examination. The eyes will need to be dilated in order for the doctor to be able to better examine the retina. This means that the pupils are enlarged temporarily with special drops, allowing the eye care professional to better examine the retina.

A confirmed diagnosis will include the presence of histo spots, indicating prior exposure to the fungus spores and swelling of the retina.

The retina will appear swollen due to the growth of new, abnormal blood vessels.

Another diagnostic tool for OHS is optical coherence tomography (OCT). OCT is used to detect swelling or damage in the retina. This technique can help the doctor to document the quantity and location of bleeding and help to follow a patient's response rate to treatment.

The doctor may also perform a diagnostic test called fluorescein angiography. In this procedure, a dye is injected into the patient’s arm. The dye travels to the blood vessels of the retina, allowing a clear view of possible lesions.

Treatment

Ocular histoplasmosis generally requires no treatment. Although it is a disease caused by a fungus, antifungal medications are not useful. OHS causes scars to form inside the eye, but no active fungal infection is present in the eye.

Photocoagulation is the main treatment for OHS cases that have progressed to neovascularization. Photocoagulation is a form of laser surgery in which a small beam of light destroys the abnormally growing blood vessels. If bleeding is found close to the center of the eye, certain medications are also used. Currently, the same medications are being used that are used to treat macular degeneration. The medications are given by injection into the eye. In some cases, the injections will need to be repeated.

During the laser procedure, some healthy retinal tissue may be destroyed. Although this may cause some loss of vision, it is considered beneficial in attempting to protect the fovea, which is needed for clear vision.

Although photocoagulation usually does not restore lost vision, it does reduce the chance of further vision loss. Sometimes a person can develop recurrent cases of OHS, which may require additional laser surgery.

A Word From Verywell

OHS is rare. Most people infected with the histo fungus will not ever develop the infection in their eyes.  However, if you are diagnosed with histoplasmosis, be alert for any changes in your vision. While rare, the disease has affected up to 90 percent of the adult population in a region of the US known as the "Histo Belt."

The high-risk region includes the states of Arkansas, Kentucky, Missouri, Tennessee, and West Virginia. If you have ever lived in these areas, you should consider having a doctor examine your eyes for possible histo spots. As with every eye disease, early detection is key to preventing future possible vision loss.

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  • Bakri, Sophie J, MD. "Presumed Ocular Histoplasmosis Syndrome." American Society of Retinal Specialists (ASRS), 2016.