TB of the eye

Ocular TB broadly refers to inflammation in the eye (uveitis) that can be related to infection with the tuberculosis bacilli, Mycobacterium tuberculosis. The ocular inflammation can form various patterns such as retinal vasculitis or serpiginous-like choroiditis (that progresses like a serpent) depending on the eye tissue primarily affected by the infection. Such patterns are also seen in various other forms of infectious and non-infectious diseases. So, how do we link them to tuberculosis infection?

The key is to rule out all those other possibilities through meticulous examination of the eye and the body, and also lab tests, if required. Then we focus on TB infection. Was the person recently exposed to TB infection by family, friends or at work? That could be our first clue. Then there are some tests that tell you if the person had ever been infected by the TB bacillus. These include a skin test called the tuberculin test (or Mantoux test) and a newer blood test (called Interferon Gamma Release Assay).

Unfortunately, one third of the world’s population and a much greater proportion in India is infected with the TB bacillus. But all of them do not have eye diseases, let alone eye TB. So any of these two tests being positive doesn’t confirm that the eye disease is due to TB infection. We can also get an X-ray or CT-scan of the chest to look for signs of healed or active TB disease, but this too is not always helpful, since lung TB often heals without any telltale signs.

So, how do we confirm if there is TB infection in the eye? It is almost impossible to find bacteria in eye fluids. Therefore, we resort to molecular biology and do a test called PCR (polymerase chain reaction) to detect the TB DNA in the eye. Once we have zeroed in on the diagnosis of eye TB, treatment is done with anti-TB therapy and often, steroids, to bring down the inflammation.

Eye TB has been a focus of active research, especially in India. We now know that the TB bacillus can escape from the lungs (the original site of infection) and reach the eye to cause disease, just as in TB for other parts of the body.

Researchers are trying to find out how the TB bacillus causes the various patterns of diseases in the eye. Soon, we should be able to achieve greater accuracy in defining and detecting eye TB. This will lead to early diagnosis of this condition and prevention of vision loss.

(The author is head, retina-vitreous and uveitis services, L V Prasad Eye Institute, Bhubaneswar)