Presumed ocular histoplasmosis syndrome

Overview

Presumed Ocular Histoplasmosis Syndrome (POHS) is a rare eye condition believed to result from a previous infection with the fungus Histoplasma capsulatum. It primarily affects the choroid and retina, and is most commonly diagnosed in individuals living in areas where the fungus is endemic, particularly the Ohio and Mississippi River valleys in the United States. POHS is termed "presumed" because the link to Histoplasma infection is based on epidemiological association rather than direct evidence. The condition may remain asymptomatic for years but can progress to serious vision loss if choroidal neovascularization (CNV) develops.

Causes

POHS is thought to result from a prior exposure to Histoplasma capsulatum, a fungus commonly found in soil contaminated with bird or bat droppings. Inhalation of fungal spores can cause a mild or asymptomatic lung infection. Although the fungus does not directly invade the eye, it is believed that the immune response to the infection causes inflammation and scarring in ocular tissues. Key factors include:

• Environmental exposure: Living in or traveling to endemic areas where histoplasmosis is prevalent

• Immune response: An abnormal immune reaction to prior systemic histoplasmosis may affect the retina and choroid

• Genetic susceptibility: Some individuals may have a predisposition to develop ocular lesions following histoplasmosis exposure

Symptoms

POHS may be asymptomatic for years and is often detected during routine eye exams. Symptoms usually occur when complications like choroidal neovascularization arise. Common symptoms include:

• Blurry or distorted central vision (metamorphopsia)

• Sudden or gradual vision loss

• Scotomas (blind spots) in central vision

• Difficulty reading or recognizing faces

• Visual distortion of straight lines

In early stages, there may be no pain or redness, which can delay detection until vision is significantly affected.

Diagnosis

Diagnosis of POHS is based on characteristic clinical findings and imaging, not direct detection of the fungus. Key diagnostic features include:

• Fundus examination: Reveals distinctive signs such as:

  • Punched-out atrophic chorioretinal scars (histo spots)

  • Peripapillary atrophy (scarring around the optic disc)

  • Absence of vitreous inflammation (vitritis)

• Optical coherence tomography (OCT): Assesses retinal layers and detects fluid accumulation or neovascular membranes

• Fluorescein angiography (FA): Highlights areas of leakage and neovascularization in the retina

• Visual acuity testing: Evaluates the extent of central vision impairment

• Visual field testing: Helps detect scotomas and subtle vision loss

The diagnosis is clinical and based on exclusion of other causes of similar retinal findings.

Treatment

There is no cure for POHS, but treatment aims to control complications—particularly choroidal neovascularization—to preserve vision. Therapeutic strategies include:

Observation

• Asymptomatic patients or those without CNV are typically monitored with periodic eye exams and imaging

Anti-VEGF Therapy

• Intravitreal injections of anti-vascular endothelial growth factor (anti-VEGF) agents like bevacizumab, ranibizumab, or aflibercept are the mainstay of treatment for CNV

• These injections reduce leakage and slow progression of abnormal blood vessel growth

Laser Photocoagulation (Rarely Used)

• Previously used to destroy abnormal blood vessels, but now largely replaced by anti-VEGF therapy due to better outcomes and reduced damage to surrounding tissue

Photodynamic Therapy (PDT)

• May be considered in selected cases where anti-VEGF therapy is not effective or contraindicated

Lifestyle and Monitoring

• Use of an Amsler grid at home to monitor for early signs of visual distortion or CNV

• Prompt reporting of any visual changes to an ophthalmologist

Prognosis

The prognosis for patients with presumed ocular histoplasmosis syndrome varies depending on the extent of CNV and how quickly treatment is initiated. Without treatment, CNV can lead to permanent central vision loss. With timely anti-VEGF therapy, many patients experience stabilization or improvement of vision. The condition is typically unilateral but can affect both eyes over time. Regular follow-up with an eye care professional is critical to monitor for recurrence or progression. Overall, with modern treatment options, long-term visual outcomes have significantly improved.