Presumed Ocular Histoplasmosis Syndrome

(POHS) causes atrophy around the optic nerve and multiple scars scattered throughout the retina. There is a high risk that scars within the macular may lead to new blood vessel growth (neovascularization). If this occurs in the centre of the macula the patient experiences vision loss, and distortion.

Causes

Histoplasmosis is a disease caused by Histoplasma capsulatum (H. capsulatum), a soil fungus prevalent in certain parts of the American continent, in particular the Ohio and Mississippi River valleys, and in Canada in the Niagara peninsula.

Consequently, it is thought that H. capsulatum enters the eye from the bloodstream, invading the choroidal vessels and ultimately causing scarring. However, it has never been directly proven that H. capsulatum causes POHS, hence the name presumed ocular histoplasmosis syndrome.

The vast majority of patients with POHS have no known history of systemic infection. One theory is that when childhood infection with H. capsulatum occurs, the fungus spreads through the bloodstream to the eye and choroid, where scars form and remain long after the systemic infection subsides.

The histoplasmosis infection usually causes only mild flu-like symptoms so most people never realize they have histoplasmosis scars in their retina. The scars can later lead to neovascularization in the macula resulting in visual loss.

Diagnostic testing

Ocular coherence tomography (OCT) and fluorescein angiography (FA) are performed to evaluate for neovascularization in patients who visit their doctor with symptoms of visual loss (Figure 2).

Treatment and prognosis

Treatment is generally required only when there is neovascularization. injections of medications that inhibit neovascularization are recommended. These medications include bevacizumab (Avastin® ), ranibizumab (Lucentis® ) and aflibercept (Eylea® ).

Return of vision depends on how large an area of the macula is damaged.