Ocular histoplasmosis is a condition that occurs years after a histoplasmosis infection. Histoplasmosis infection is caused by a fungus Histoplasma capsulatum and commonly affects people living in the 'Histo belt', which includes Arkansas, Kentucky, Missouri, Tennessee and West Virginia.
Histoplasmosis primarily affects the lungs and manifests as a short bout of cold or flu infection. Most persons may be carriers or may have a mild infection without being aware. Since the infection is mild, many do not seek or need treatment.
However, years later there may be development of ocular histoplasmosis where there is formation of abnormal and fragile blood vessels within the eyes. This leads to an abnormal growth under the retina called the choroidal neovascularization (CNV).
This CNV lesion, as it heals, turns into a scar that damages a significant area of the retina causing loss of vision at that area in later years. Sometimes the abnormal blood vessels may bleed or leak leading to loss of vision.
CNV and retinal scarring or bleeding may be particularly damaging to the vision if the macula or the fovea is affected. Thus, early detection of the histo spots and eye lesions as well as treatment is vital to prevent vision loss. Ocular histoplasmosis is the leading cause of vision loss in America among those between ages 20 and 40 years.
Treatment of ocular histoplasmosis involves laser photocoagulation, lenses and so forth. 1-8
The best known therapy for this condition is the use of laser surgery called photocoagulation. In this therapy a small but powerful beam of light is focussed onto the back of the eyes over the retina. It is used to destroy the CNV and the fragile, abnormal blood vessels along with some of the overlying retinal tissue.
Destruction of the retinal tissue may lead to some loss of vision but it reduces the risk of affecting the fovea and the macula (most sensitive areas of the retina) by nearly 50%.
Laser photocoagulation is most effective when the CNV has not grown into the center of the fovea, where it can affect vision and the physician can identify and destroy the entire area of CNV.
There are two types of laser therapy - Photodynamic Therapy (PDT) and thermal laser.
PDT involves infusion of a medication Visudyne followed by an application of laser in the eye for 83 seconds. This collapses the CNV both from within as well as by outside laser therapy.
Thermal laser uses heat to cauterize the CNV from the outside of the blood vessel.
Laser photocoagulation, however, does not restore lost vision but reduces the risk of further CNV growth and any resulting vision loss. Laser photocoagulation also does not cure ocular histoplasmosis. The risk of vision loss due to ocular histoplasmosis remains for lifetime.
There may be recurrence of CNVs that may require additional laser therapy. Each time the damage should be detected as early as possible to prevent vision loss. Eyes should be regularly checked using an Amsler Grid to monitor central vision and recurrence of CNV and histospots.
For those who have lost significant vision special visual aids like special lenses or electronics to create enlarged visual images may be prescribed.
Sometimes steroid medicines are used to reduce inflammation. In patients with an identified active infection with the fungus, antifungal agents may be used.
Newer agents like Avastin have been tried in ocular histoplasmosis. However, this is not yet approved by the US food and Drugs Administration (FDA) for use in ocular histoplasmosis.
These agents help by reducing the formation of the abnormal blood vessels within the retina and prevent CNV formation. They may be given as injections into the eyes several times. Other similar agents that are being tried are Macugen and Lucentis. These have also been tried in combination with photodynamic therapy.