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Diagnosis of ocular histoplasmosis is based on examination of the eye rather than the clinical features or symptoms. This is because most individuals fail to notice histoplasmosis infection that is usually mild and may not cause symptoms more severe than a common cold or a flu infection. Eyes are rarely affected after a generalized histoplasmosis infection that primarily affects the lungs.
However, in rare cases when there are eye problems, especially those living in the “histo belt” which includes Arkansas, Kentucky, Missouri, Tennessee and West Virginia, eyes must be evaluated for exclusion of ocular histoplasmosis. In these areas nearly 90% of the individuals have had a histoplasmosis infection and around 62% are carriers of the infection and are unaware.
In early stages the infection may not cause any symptoms. However, the person may develop eye problems years later. This is due to the fact that the infection leaves scars and leakages of blood vessels behind that damage the normal central vision of the retina.
Some speculate that this re-emergence of infection after years is more due to a hypersensitivity to the fungi Histoplasma capsulatum rather than the infection itself.
In all cases however “histo spots” may be detected in the eyes.
Diagnosis of the condition involves asking about the history of histoplasmosis infection, eye examination and so forth.
The patient is asked about history of a histoplasmosis infection. This may or may not be positive as many patients are unaware of a mild histoplasma infection years earlier.
Those with a suppressed immunity are at a greater risk of ocular histoplasmosis. This includes persons with HIV AIDS. Tests to exclude HIV infection thus are important.
The eyes are examined. Ocular histoplasmosis may affect both eyes and thus each eye must be examined separately despite the fact that patient might not have symptoms in both eyes.
The pupils of the eyes are dilated temporarily using special eye drops. This gives a better view of the back of the eyes or the retina. The eye specialist or the ophthalmologist uses a hand-held light device called the ophthalmoscope to look at the retina.
Examination usually reveals presence of histo spots, which indicate previous exposure to the histo fungus spores and also may find swelling of the retina, which signals the growth of new, abnormal blood vessels.
If abnormalities are detected on eye examination the ophthalmologist may prescribe another test called the fluorescein angiography. In this procedure a special dye is injected into the patient's arm, travels to the blood vessels of the retina.
The pupils are dilated and the retina is examined with the dye within the blood vessels. The dye allows a better view of the CNV lesion. This may be documented with photographs as well. This shows the location and extent to which it has spread.
This test allows physicians to see how much of the central macula has been affected and if the most sensitive area of the retina called the fovea has been affected. The spots nearest to the fovea are of most concern.
Another diagnostic test that may be performed is called the Optical Coherence Tomography (OCT). This test also helps in obtaining pictures of the retina for diagnosis. The OCT provides a cross-sectional or 3-dimensional view of the retina and the depth and extent of the affected blood vessels within the retina.
The technology is similar to ultrasonography but light is used here instead of ultrasound waves to generate images. OCT is also used to detect other eye conditions like diabetic retinopathy, macular degeneration, macular holes or macular pucker, central serous retinopathy etc.