An Overview of Ocular Histoplasmosis

A leading cause of vision loss in adults

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. Histoplasmosis is aersolized into the air by disturbing soil during farming or construction activities or when working with animals like chickens and cleaning their coops.

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, a bloodborne, systemic spread of histoplasmosis can affect the eyes, known as presumed ocular histoplasmosis syndrome (POHS). Complications from this disease are 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

A person with possible histoplasmosis may have flu-like symptoms when symptomatic, however, in most causes of ocular involvement (POHS), there are no symptoms.

POHS only becomes symptomatic if it progresses towards the complication of forming new blood vessels underneath the retina (called neovascularization). This is a rare but severe consequence.

Experts estimate that 60% of the adult population test positive for histoplasmosis via skin antigen testing, yet only 1.5% of those patients have typical retinal lesions. And only 3.8% of those with lesions develop choroidal neovascularization (CNV).

The systemic infection usually disappears over a period of a few days and without intervention. Damage to one's vision may not happen right away. The inflammation caused by the infection may leave behind tiny pock-like scars called “histo spots” at the infection site in the eyes, which are generally asymptomatic.

Because the initial histoplasmosis infection usually does not cause symptoms throughout the body, most people never realize they have histo spots in their retina. The scars can later lead to neovascularization in the macula (when new vessels develop underneath the scars months to years later), resulting in visual loss. 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 a secondary inflammation may take place, and 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 abnormal blood vessels can leak fluid and blood. If left untreated, neovascularization can cause additional scarring in the ocular tissues (the retina) responsible for transmitting vision signals to the brain.

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

  • Acute histoplasmosis or short-term histoplasmosis is typically mild. It rarely leads to complications.
  • Systemic 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 POHS 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, small retinal scars that look like "punched out" lesions
  • Peripapillary atrophy (retinal pigment epithelium is affected around the optic nerve)
  • An absence of inflammation of the vitreous or the jelly inside the eyeball (vitritis)

Complications can be assessed with a dilated eye exam that might reveal bleeding, swelling, and scarring in the retina, indicating that neovascularization has occurred. The presence and severity of neovascularization can be confirmed with OCT and a fluorescein angiogram which assesses the circulation of dye injected via an IV in the retina.

Another diagnostic tool for POHS is optical coherence tomography (OCT). OCT is used to detect the presence and severity of neovascularization.

The doctor may also perform a diagnostic test called fluorescein angiography. In this procedure, a dye is injected via an IV into the retina. The dye travels to the blood vessels of the retina, assessing circulation.

Treatment

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

The main treatment for POHS cases that have progressed to neovascularization is medication injection into the eye (intravitreal injections), similar to age-related macular degeneration (AMD). Specifically, anti-vascular endothelial growth factor (anti-VEGF) medications like bevacizumab have been well-studied.

Coping

POHS is rare. Most people infected with the histo fungus will never 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% 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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Article Sources
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  1. Centers for Disease Control and Prevention. Histoplasmosis. Updated August 13, 2018.

  2. American Academy of Ophthalmology. What Is Histoplasmosis? Updated October 04, 2019.

  3. Centers for Disease Control and Prevention. Symptoms of Histoplasmosis. Updated August 13, 2018.

  4. Kim, JE. American Academy of Opthalmology. Eye Wiki: Presumed Ocular Histoplasmosis. Updated June 10, 2019.

  5. Merck Manual Professional Version. Histoplasmosis. Updated July 2019.

Additional Reading
  • Bakri SJ. Presumed Ocular Histoplasmosis Syndrome. American Society of Retinal Specialists (ASRS). 2016.