Ocular Histoplasmosis

Definition:
Ocular histoplasmosis is  an eye disease that is a leading cause of vision loss, due to the spread of spores of the fungus Histoplasma capsulatum (histo) from the lungs to the eye where they lodge in the choroid (a layer of blood vessels that provides blood and nutrients to the retina).

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There the spores cause fragile, abnormal blood vessels to grow underneath the retina. These abnormal blood vessels form a lesion known as choroidal neovascularization (CNV). If left untreated, the CNV can turn into scar tissue and replace the normal retinal tissue in the macula (the central part of the retina that provides sharp central vision. If these abnormal blood vessels grow toward the center of the macula, they may affect a tiny depression called the fovea (the region of the retina with the highest concentration of special retinal nerve cells, called cones, that produce sharp, daytime vision). Damage to the fovea and the cones can severely impair, and even destroy, straight-ahead vision. Since the syndrome rarely affects side, or peripheral vision, the disease does not cause total blindness.

It’s a common problem in the USA, particularly in a region now known as the ‘Histo belt’, which includes Arkansas, Kentucky, Missouri, Tennessee and West Virginia, where as many as 90 per cent of the population have had the infection.

Symptoms:
Many patients with histo spots in their eyes have no symptoms. Others may experience the following:

 

*Distorted vision…....CLICK & SEE
*Blind spots
*Scars in the retina, ranging in severity

Causes:
Histoplasmosis is caused by a fungus commonly found in the dust and soil of the Mississippi-Ohio River Valley region.  Approximately 62% of the adult population living in this region are carriers. It affects men and women equally.

Histoplasmosis is contracted by inhaling dust that carries the fungal spores. Its effect on the body can vary widely in severity from one person to another.  Many carriers have no symptoms at all, but those with mild exposure may experience flu-like symptoms and mild respiratory infections. Histoplasmosis is more likely to become a serious problem in people who already have a weakened immune system.

The fungus may affect the eye by causing small areas of inflammation and scarring of the retina. These are called “histo spots” and may be found in both eyes. Their affect on vision depends on the location of the scars.  Scarring in the peripheral area of the retina may have little or no impact on vision, while a central scar affecting the macula may cause a prominent blind spot.

Most people with histo spots in the retina are totally unaware of their presence unless the central vision is affected. Studies indicate that only about 5% of those with histo spots are at risk of losing vision. Scientists have been unable find a link between the patients with minor histo spots and those who develop a severe loss of their central vision.

The syndrome is thought to be linked to hypersensitivity to Histoplasma capsulatum, rather than a direct exposure of the eyes to the micro-organism, but some experts have found DNA or genetic material from the fungus in a layer of the eyeball known as the choroid, and suspect fungal spores may lodge here and cause problems.

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Risk Factors:
Some people go on to develop symptoms – usually of lung disease, although the fungus may spread to other organs – known as disseminated histoplasmosis and this can be fatal.

Very rarely the organism can spread to the eye to cause acute ocular histoplasmosis, which needs urgent treatment with antifungal medicine.

Ocular histoplasmosis can cause blindness, although it mostly affects central vision and rarely involves peripheral vision so total blindness is rare. Anyone who has lived in an area where they may have been exposed to histoplasmosis and develops eye problems must be checked for the condition.The loss of vision in POHS is caused by choroidal neovascularization.

Diagnosis:
Ocular histoplasmosis is detected with a dilated examination of the retina using ophthalmoscopy. It is usually diagnosed based on its distinctive appearance and characteristics. Fluorescein angiography is required for diagnosis and follow-up of patients with POHS.

Treatment:
Treatment requires careful consideration of FA findings and few cases may respond to corticosteroids and laser photocoagulation. A vitreo-retinal specialist should be consulted for proper management of the case.

Presumed ocular histoplasmosis syndrome and age-related macular degeneration (AMD) have been successfully treated by the drug Bevacizumab (trade name Avastin, Genentech/Roche). Ophthalmologists are using Avastin “off-label” to treat AMD and similar conditions since research indicates that vascular endothelial growth factor (VEGF) is one of the causes for the growth of the abnormal vessels that cause these conditions. Some patients treated with Avastin had less fluid and more normal-appearing maculas, and their vision improved. Avastin injections into the affected eye have been used by retina specialists since early 2005. Thus there is no long term 10 to 15 year follow up data for possible late complications. Early treatment is critical to maintaining vision.

Other treatments include Ranibizumab (trade name Lucentis) which is approved by the FDA for intraocular use. Lucentis uses a smaller molecule compared to Avastin and according to GenenTech, the smaller molecule helps lower the systemic toxicity of the drug, thereby lowering the overall risks compared to Avastin. However, Lucentis costs approximately $1,600 per injection compared to less than $100 per injection for Avastin. Research has shown  that Avastin and Lucentis to be equally effective in the treatment of POHS and AMD.

To get the best effect, the whole area affected by ocular histoplasmosis has to be treated.

Once a person has ocular histoplasmosis, they have it for life.

Regular eye exams and routine use of an Amsler Grid to monitor central vision is recommended for anyone with histo spots.

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Disclaimer: This information is not meant to be a substitute for professional medical advise or help. It is always best to consult with a Physician about serious health concerns. This information is in no way intended to diagnose or prescribe remedies.This is purely for educational purpose

Resources:
http://en.wikipedia.org/wiki/Presumed_ocular_histoplasmosis_syndrome
http://www.medterms.com/script/main/art.asp?articlekey=24114
http://www.stlukeseye.com/Conditions/histoplasmosis.html
http://www.bbc.co.uk/health/physical_health/conditions/ocularhistoplasmosis.shtml

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