What is it? When your eyes do not converge adequately in order to look at something close, or do not comfortably maintain this position.

What does it do? It may cause discomfort with close vision, frontal headache or double vision. Distance vision is not usually affected.

Cause: The brain-stem control centre for convergence eye movements does not function correctly. There is no known cause. It is not caused by overuse nor is it associated with other diseases. It may be exacerbated by tiredness and some drugs and sometimes follows head injury or Stroke.

Can it be treated with spectacles? Glasses may be required to correct an associated focusing error. Prisms in glasses sometimes help.

 Can it be treated with surgery? Occasionally surgery may be needed to reposition eye muscles but it is the last option considered.

 What should you do?

Adequate lighting from behind your shoulder helps near work.

Adequate rest and relaxation: tiredness worsens symptoms; take regular breaks when reading, etc, for prolonged periods.

Correct posture

• Exercises:

Convergence exercises usually overcome the symptoms but the underlying tendency remains. This means that exercises may take some weeks to show benefit and that "top up" exercises will be needed from time to time. The exercises are simple but must be performed conscientiously for good effect. The onus is on the sufferer to do them regularly.

Firstly, you must be able to recognise double vision. The end-point of the exercises is when you see double - not when the test object blurs.

Align two objects, one near (e.g. a pencil tip) and one further away (e.g. a light switch), and, with both eyes open, observe that the near object is double when you look at the far object and vice versa. Ignore the blurring. Do not proceed with the other steps until this "physiological double vision" is apparent to you.

Secondly, hold your target object (e.g. pencil tip, blade of grass, tiny picture) at arms length and steadily bring it towards the bridge of your nose until it doubles. It may blur before it doubles but ignore this. Wear reading or near work glasses if you have them. With time you will be able to bring the near point progressively closer. Repeat a dozen times.

Thirdly, while holding the object at the near point, switch your gaze to a distant object and then back to the near object. At first you will need to move the near object away for comfortable single vision but this will improve with repeated exercising. Repeat a dozen times.