Histoplasmosis is a disease caused by Histoplasma capsulatum (histo), a fungus or mold. The disease causes trouble when the airborne spores of the fungus enter the lungs through inhalation. The fungus is microscopic and found in soil or river valleys where bat and bird droppings collect. Plowing the field, digging holes or sweeping chicken coops can release the fungus into the air.
Histoplasmosis is mild with no apparent symptoms. If symptoms occur, they may resemble those of the common cold or flu. Histoplasmosis presents a problem because it can move up the bloodstream into the eye eventually causing ocular histoplasmosis syndrome (OHS).
OHS is a serious disease and the leading cause of vision loss in individuals between the ages of 20 and 40. Sometimes, people with histoplasmosis have no idea they have it. Those who have lived in areas where histoplasmosis is prevalent should be checked by an ophthalmologist to rule out the condition.
Also Known As
- Ocular Histoplasmosis
- Ocular histoplasmosis syndrome (OHS)
- Presumed ocular histoplasmosis syndrome (POHS)
Causes and Risk Factors
Scientists believe that Histoplasma capsulatum spores cause OHS when they spread from the lungs to the eye. The spores settle in the choroid (blood vessels that nourish the retina with nutrients and blood). Abnormal blood vessels which grow under the retina cause choroidal neovascularization (CNV).
If CNV is not treated, the growth can turn to scar tissue and replace the normal tissue in the macula. The macula provides central vision which allows individuals to do routine activities like driving and reading. Visual messages from the retina to the brain are affected leading to loss of vision when the scar tissue develops.
Also, the abnormal blood vessels can leak fluid and blood into the macula. If the vessels grow in the direction of the center of the macula, they may damage the fovea and the cones in the retina. Damage to the cones and fovea can destroy straight-ahead vision. OHS does not involve peripheral vision, therefore, does not cause total blindness.
Those at risk of developing OHS include:
- Those who have previously been exposed to histoplasmosis
- Those who dwell in “Histo Belt”, America, where 90% of the adults have had histoplasmosis. These areas include Kansas, Iowa, Oklahoma, Texas, Maryland, Kentucky, Arkansas, Virginia, Alabama, Missouri, Louisiana, Nebraska, Mississippi, Ohio, Illinois, Indiana, West Virginia and Tennessee
Signs & Symptoms
OHS presents typically no symptoms in the early stages. The infection often goes away without treatment with the only evidence being tiny scars called ‘histo spots.’ These histo spots remain at the infection site and do not generally affect vision. However, for reasons not yet known, histo spots can result in complications even decades after the original eye infection. Abnormal blood vessels beneath the retina are associated with these histo spots.
In later stages, the following OHS symptoms may appear signifying a change in vision:
- Straight lines that appear wavy or crooked
- A blank spot in the field of vision especially the central vision
- Objects seem to have a different size for each eye
- Flickering or central light flashes
- Colors look different for each eye. The colors also lose their brightness
The eye care professional will do a careful eye examination that may include:
- A dilated eye examination - The presence of two conditions may help with diagnosis. These conditions include the presence of histo spots and swelling of the retina (indicate growth of abnormal blood vessels).
- Fluorescein angiography - Here, the professional will inject dye into an individual’s arm. The dye travels to the retina allowing a better view of the CNV lesion. Photographs are also taken to show the location and spread of OHS.
- Optical coherence tomography (OCT) - Determines how thick the retina is and also tries to find abnormal blood swellings.
OHS treatment aims to destroy the abnormal blood vessels underneath the retina.
The abnormal growth of blood vessels in the retina is caused by a chemical called vascular endothelial growth factor (VEGF). Anti-VEGF medications can block VEGF and reduce the growth and leakage of blood vessels. The drugs can slow vision loss and improve vision in some cases.
The eye doctor may also inject a patient with steroids to reduce swelling caused by histoplasmosis.
Photocoagulation can be used to treat OHS. It is a form of laser whose beam of light destroys the abnormal blood vessels and the affected overlying tissue in the retina. Destroying retinal tissue can cause vision loss. However, it protects the fovea and helps to preserve the vision the fovea provides. Photocoagulation is most effective when the CNV has not developed in the centre of the fovea. It is also effective if the surgeon can identify the entire affected area and destroy it. This prevents further leakage, bleeding and growth.
Patients with OHS will need to use an Amsler grid daily to check if the macula has been damaged. Both eyes should be checked because OHS that affects one eye is likely to affect the other.
They can use low vision aids which help patients with severe vision impairment.
Photocoagulation does not restore vision. Instead, it reduces the possibility of further growth of CNV. This means it halts any further vision loss. OHS remains a threat to sight for life.
OHS can recur even in individuals who have experienced one case of abnormal blood vessel lesion. Additional laser treatment is necessary.
After laser treatment, a patient may experience blurry vision but that should improve within a few weeks. Laser may result in a scar which a patient can see in their field of vision.
It may be challenging to control fungus. However, the following may help:
- Spray contaminated surfaces before digging or working in an area
- Use an effective respirator face mask. Different masks cater to varying levels of exposure
- Avoid exposure to projects and activities that might predispose one to fungus. Such activities include raising chicken or pigeons.