What it is
Histoplasma capsulatum is a fungus that lives in soil enriched by bird and bat droppings. In parts of the United States a large share of the population inhales it at some point, usually with no illness or only a mild flu-like episode. Presumed ocular histoplasmosis syndrome, or POHS, is a characteristic set of findings in the retina believed to be a late consequence of that exposure. The word presumed is there deliberately: the organism itself is not found in the eye, and the link is inferred from the geography and the pattern of scarring.
The classic triad is small round pale scars scattered in the periphery of the retina (“histo spots”), scarring around the optic nerve, and the absence of any active inflammation in the eye. The eye is quiet and white; this is not a red, painful condition.
The problem arises when one of these scars sits at or near the macula, the center of vision. Scarred tissue there can allow choroidal neovascularization — abnormal new blood vessels growing up from beneath the retina, which leak fluid and blood. That is what threatens vision, and it is what treatment is aimed at.
Who gets it
POHS is most common in people who have lived in the Ohio and Mississippi River valleys. It is typically identified between the ages of twenty and fifty, often in both eyes, and it is not contagious, not inherited, and not related to how healthy your immune system is.
Symptoms
The scars themselves are silent. Symptoms appear only if neovascularization develops near the center:
- Straight lines looking bent, wavy, or interrupted
- A grey, dark, or blurred spot in the center of vision
- Objects appearing smaller or misshapen in one eye
- A drop in reading vision that new glasses do not fix
These changes can develop over days. They are the reason to be seen quickly rather than waiting for a routine appointment.
How we diagnose it
The scars are recognized on dilated examination and documented with retinal photography. When we suspect new vessels are growing, optical coherence tomography (OCT) shows fluid and the membrane itself, and fluorescein angiography or OCT angiography maps exactly where it is leaking. Blood or skin testing for histoplasmosis is not needed and does not change management.
Treatment
Inactive scars need no treatment — only observation and awareness.
When choroidal neovascularization develops, the treatment is intravitreal anti-VEGF injections, the same class of medication used for wet macular degeneration. They are given in the office, and they are effective: in POHS the membranes often respond quickly and, unlike in age-related macular degeneration, many patients need only a limited course of injections rather than indefinite treatment. Response is monitored with repeat OCT scans. Photodynamic therapy and thermal laser were used before anti-VEGF agents existed and still have a narrow role in selected cases.
Because treatment works best before the center of the macula is permanently scarred, catching a new membrane early has a direct effect on the vision you keep.
Monitoring at home
Patients with histo spots near the macula should check their vision at home, one eye at a time, using an Amsler grid. It takes about fifteen seconds a day, and it is the most reliable way to catch a new membrane while it is still small.
Call us at (315) 445-8166 promptly if lines on the grid become wavy, broken, or missing, or if you notice a new blurred or dark spot in the center of vision in either eye.