Medical Fun Facts Podcast

MFF0063: Eye infections

It’s Monday 8 May 2017.

I’m still in the diagnostic series of Medical Fun Facts. Tonight, I’m talking about eye infections. Eye infections give me the heebie-jeebies. Just the thought of anything going wrong with my eyes fills me with anxiety. As I grow older and my sight gets longer I’m acutely aware of just how important our eyes are. Infections of the eye can have devastating consequences.

It’s probably worth describing some ocular anatomy first so we can orient ourselves.

The conjunctiva is the tissue which overlies the sclera or the white part of our eyes, and continues to cover the inside part of our upper and lower eyelids.

The sclera accounts for about 5/6 of the eye’s out coat and is a protective layer. It’s the white part that you can see.

Aqueous fluid is found in the anterior third of the eye and circulates in the anterior chamber. The back end of the anterior chamber is the lens and the front end is the back surface of the cornea. The aqueous fluid is about 1/3 of the eye volume.

Vitreous is like a jelly which thins with age. It lies behind the aqueous.

The posterior chamber has vitreous gel as its back wall and the lens as its front wall.

The cornea is the clear part of the eye we can see when we look at someone’s eyes. It’s most important function is to refract light into the eye through the lens. The cornea has no blood vessels and is very delicate.

The lens is biconvex in shape and it focuses the light which entered through the cornea onto the retina in the back of the eye.

The retina is a layer of sensory tissue on the back of the eye. It contains two types of light receptors known as rods and cones. These light receptors send signals to other nerve cells to transmit information through the optic nerves to the brain.

Infections of the eye can occur in any of the anatomical sites and can have many causes. Conjunctivitis is common and is often caused by a viral infection.

If you scratch the surface of your eye it can become infected and ophthalmologists may take what is known as a corneal scraping. It sounds worse than what it is. The ophthalmologist will use a sharp needle to scrape the infected part of your cornea and then if he or she works in a good hospital, the microbiology staff will have delivered an eye kit consisting of sterile microscope slides, agar plates and enrichment broth. On one of the agar plates a microbiology scientist may have also coated the surface with some live bacteria if the infection may be due to Acanthamœba.  I also suggest if you have a serious problem with your eye see an eye doctor and not an optometrist. Optometrists are great when you want your eyes checked for reading glasses but for diseases of your eyes don’t muck around see a proper doctor trained to treat your eye problems.

When I was a trainee, the hospital was home to a world leader in the front of the eye. Like orthopædic surgeons who may specialise in hips or knees or hands, there are eye doctors who specialise in various parts of the eye. Through this ophthalmological surgeon, I learnt a lot of ocular microbiology. As I said, his niche was the front of the eye and he had an interest in infections caused by Acanthamœba. I recall a couple of patients who went swimming with their contact lenses in. That is a bad move, in fact it’s a dumb move. They ended up with Acanthamœba keratitis and very nearly lost their eyesight. The diagnostic element of this story was really very cool. Acanthamœba are free living amœbæ that live in fresh water. To make a diagnosis we need something for the Acanthamœba to feed on so we used a lawn of bacteria growing on an agar plate and then examine the plate under a stereo microscope looking for the amœbæ. In those days, we used what used to be known as Xanthomonas maltophilia but which is now known as Stenotrophomonas maltophilia as food. You could also use Escherichia coli.

You can also get infections on the aqueous and vitreous fluids. They only good way to get a specimen is for an eye surgeon to aspirate the fluid from the eye. This is where my heebie-jeebies go into overdrive. Can you imagine having someone inserting a needle into your eye and sucking the fluid out of it. When that happens we still like plates inoculated at the “bedside” to optimise the diagnostic potential.

These days we also do a lot of PCR on specimens collected from eyes, especially for Herpes simplex viral eye infections.

One last story to end this show. Did you know that you can get gonococcal conjunctival infection as an adult? It can certainly happen with babies born through an infected birth canal and fortunately, we don’t see it that often. Of course, if you get shot in the eye with infected semen you can get an ocular infection, but no I’m not talking about spread caused by sexual activity, I’m talking about fly-borne gonorrhoea. I mentioned it in episode 4 when I described the gonococcus. If you’re interested, go back and listen to episode 4.

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