Acanthamoeba keratitis: the contact lens eye infection

Acanthamoeba keratitis, often shortened to AK, is an infection of the cornea. Moorfields Eye Hospital describes the cause as a microscopic organism found in water, soil and air. People meet it all the time without harm, and it rarely invades the eye. The CDC makes the same point, adding that the organism usually stays confined to the eye in healthy people and that the infection can cause permanent vision loss.

The same organism family can cause other, far rarer, illnesses elsewhere in the body. According to the CDC these include a brain infection called granulomatous amoebic encephalitis, which is almost always fatal, and skin and sinus infections in people with impaired immunity. These are separate problems from the eye infection discussed here.

Moorfields reports that about 85 per cent of cases involve contact lens wearers. The risk factors it lists are swimming or showering while wearing lenses, rinsing or storing lenses in water, and poor lens and case hygiene. People who do not wear lenses can still develop AK after a corneal injury, but this is less common.

The CDC treats lenses as medical devices and says that failing to wear, clean and store them as directed raises the risk of microbial keratitis, including Acanthamoeba and fungal infection. The principle is that tap water, pool water, hot tub water and sea water are not sterile, and a lens can carry organisms from the water against the surface of the eye.

• Do not wear lenses when swimming or showering, or remove them before water exposure.

• Never rinse or store lenses or cases in tap water; use the solution recommended by your optometrist.

• Wash and dry hands before touching lenses, and clean and replace the lens case regularly.

• Follow the replacement schedule for your lenses and do not sleep in them unless they are designed for it.

Moorfields lists severe eye pain, though its intensity varies, together with sensitivity to light, excessive watering, redness and blurred or worsening vision. Early on the picture can resemble a more common bacterial or viral eye infection, which is one reason diagnosis is sometimes delayed.

A lens wearer whose eye becomes painful and red, particularly after water exposure, should remove the lens, keep it and its case for testing if possible, and seek urgent assessment instead of waiting for an ordinary red eye to clear. The CDC likewise tells people with symptoms to obtain medical attention without delay.

Urgent eye clinics and hospital eye departments can examine the cornea under a slit lamp and take corneal scrapes for culture and microscopy. Some units use specialised scanning of the corneal surface or molecular tests. The sampling is typically done after numbing drops are given.

Because treatment differs from that for bacterial keratitis, laboratory confirmation and clinical experience both matter. A referral from an optometrist, GP or emergency department to an ophthalmologist is the normal route, and contact lenses should not be put back in the eye in the meantime.

Treatment is directed by a specialist and usually involves specific antiamoebic eye drops used frequently over a prolonged period, along with pain control. Details vary between units and are not set out here. Moorfields notes that vision often improves during treatment as inflammation reduces.

The outlook depends strongly on how early treatment starts. Moorfields reports that about a quarter of its cases have needed a corneal transplant and that about a quarter of cases end with permanent loss of vision. These figures describe a specialist referral centre, where more advanced cases are sent, but they show why prevention and prompt review are emphasised.

Prevention is almost entirely about hygiene and water avoidance, as listed above. Moorfields also encourages reporting of cases through regulatory schemes so that product problems can be identified. A lens wearer who has had AK should discuss with the eye team whether and how to return to lens wear, since many specialists advise caution.