When 53–year–old Subhash felt the vision in his left eye deteriorate, he consulted an ophthalmologist and underwent a battery of tests. The diagnosis of his eye condition left him stumped. He had ocular tuberculosis, or tuberculosis of the eye. A rare medical condition, his was the second case in the last few months that doctors at the city–based National Institute of Ophthalmology (NIO) have come across.

In both the cases, it took long for the doctors to establish the condition as the two patients were not suffering from systemic (general) tuberculosis. It was only when the fluid from their eye was tested with a high–end DNA test method of polymerase chain reaction, that the diagnosis could be done.

In Subhash's case, doctors noticed a pus–like formation in the left eye, turbidity in the gel–like substance (vitreous) in the back part of the eye and also a lesion on the uvea (middle layer of the eye). "All this gave a very strong suspicion of tuberculosis," said eye surgeon Aditya Kelkar of National Institute of Ophthalmology (NIO).

The initial investigation including chest X–ray did not reveal any feature of tuberculosis. Mantoux testing or tuberculin skin test, which is routinely conducted for diagnosing tuberculosis also reported negative. Two chest X–rays, blood cultures, and early–morning urine samples were also negative for TB. Hence there was no systemic illness pertaining to the disease.

"We later obtained aqueous tap (fluid in the front part of the eye) and samples were sent for culture and PCR testing for TB, toxoplasma (which is caused by parasite), herpes simplex (which is caused by viral infection) and toxocara species (which is also caused by parasite) – all of them are causative agents for uvities. But they were ruled out. Microscopy and cultures failed to isolate any organism, but PCR testing identified M tuberculosis," Kelkar said.

In the second case, a 32–year–old woman consulted the doctor complaining of a five–day history of floaters in the left eye. Notably, she had good vision in both the eyes. The front part of the eye was found to be completely normal.

"We noticed inflammation on the retina and a few new blood vessels on it which were likely to herald complications. Mantoux testing failed to provoke a reaction. Two chest x–ray films, blood cultures, and early–morning urine samples were negative for TB. Cultures were negative but M tuberculosis was identified by PCR testing. The patient is responding well to the treatment now," Kelkar said.

In both the patients, sputum, urine, and blood cultures failed to isolate the causative organism. And in both cases, the chest x–ray films were not suggestive of TB.

"That simply means that tuberculosis may manifest itself in the eye without obvious involvement of other commonly affected organs or evidence of a systemic illness. Our experience suggests that investigations conventionally used to detect TB are sometimes ineffective for diagnosing ocular infection," Kelkar said.

Doctors said the cost–intensive PCR test conducted on the aqueous sample could make the diagnosis of intraocular TB possible. "We would advocate its (PCR testing) early use in any instance where there is suspicion of ocular TB and conventional tests have been unhelpful. The consequences of a misdiagnosis or even a late diagnosis may be disastrous," he added.

Elaborating, eye surgeon Parikshit Gogate said, "A high index of suspicion helps diagnose ocular tuberculosis. It forms part of the differential diagnosis of any chronic or recurrent uveitis, especially in an at–risk patient."

"Earlier when high–end diagnostic facility like PCR testing was not available, the diagnosis and management of ocular tuberculosis mainly depended upon the type of infection and also as a first choice of treatment in management of chronic infections of uvea," said eye surgeon Col Madan Deshpande, president Vision 2020 Right to Sight India, a global initiative to reduce avoidable blindness in India by 2020.

Deshpande said that there may be potential increase in ocular tuberculosis because there is a substantial rise in medical conditions that reduce immunity against all sorts of infection including TB.

"Presence of ocular tuberculosis does not mean that a person diagnosed with the disease has active TB and would spread to his/her near relations and associates unless proved otherwise," Deshpande said.

• Tuberculosis (TB) is an infection caused by M. tuberculosis that can cause disease in many organs, including the eye
• Ocular TB can involve any part of the eye and can occur with or without evidence of systemic TB
• Ocular TB can affect any vascular (having blood supply) part of the eye
• The predominant route by which tubercle bacilli reaches the eye is through the bloodstream, after infecting the lungs
• The most common manifestation of ocular tuberculosis in patients with pulmonary tuberculosis is choroiditis (vascular pigmented spongy layer behind the retina that protects it)