Dr Gordon Plant
National Hospital for Neurology and Neurosurgery

My role is as a neurologist concerned with disorders of vision and I work closely with ophthalmologists, because patients with visual problems tend to report first of all to the vision doctors. At Moorfields Eye Hospital, which has an extremely busy eye casualty department, we see about 25 new cases of pituitary tumours per year, and these are patients who are presenting with purely visual problems.

Early detection is of fundamental importance in patients with visual symptoms. Mostly we see the non-secreting tumours and prolactinomas, because patients whose tumours have associated endocrine problems tend to present first with those rather than disorders of vision. There is no doubt whatsoever that, in terms of recovery of vision, the eventual outcome depends upon how early in the story the patient has treatment. We know that vision can return entirely to normal if treatment is started early enough. I continue to see patients in whom the diagnosis has been delayed, who have had visual symptoms for some time before the penny drops. It is very important to look at the reason for that and why pituitary tumours produce symptoms which can be misinterpreted.

The main reason for this, is that the standard test of vision is the eye chart, and patients who have pituitary tumours develop what is known as a bitemporal hemionopia, where the loss of vision is to the outer side in each eye. Typically in those patients, visual acuity (the ability to read to the bottom of the eye chart) is affected very late. Therefore, if they are seen with visual symptoms and the only test they have is the measurement of visual acuity with the standard eye chart, then it may not be appreciated that they have a problem at all.

The basic anatomy that gives you the answer to why the visual symptoms are peculiar is the fact that the two optic nerves join at the back behind the eyes, just above the Pituitary gland. That is known as the optic chiasm, which is the Greek word for a cross. As the gland enlarges, it lifts the chiasm and stretches it. This is what causes the visual problems. Now if we were fish, I would not be needed, because animals who do not have binocular vision do not have this arrangement; one nerve goes to one side of the brain, the other nerve goes to the other side of the brain. The chiasm exists in humans so that we can see things in 3D.

In patients with a complete bitemporal hemionopia, with the left eye they do not see anything to the left of where they are looking and with the right eye they do not see anything to the right of where they are looking. With both eyes open, such a person has, in fact, a reasonably good field of vision, and I have known patients reach this sort of level of visual loss quite commonly without realising what was going on. What sort of odd symptoms do patients with bitemporal hemionopia have? Well one is that, because they have now effectively only one field of vision with each eye, the surviving fields of vision in each eye do not overlap, so very strange things may happen. One is that they may actually see the world apparently split along the vertical midline, so that, for instance, text overlaps and jumbles up. Patients often complain of double vision when reading; they say they see multiple letters and the words become jumbled. These are symptoms that are difficult for non-specialists to interpret.

I've actually seen recorded in ophthalmic notes by nurses doing visual tests 'normal vision' but that the patient 'can only see the letters on the left-hand side of the chart'. This was ignored by the doctors who subsequently saw the patient. Also, the letters may be legible, but actually appear fainter or of lower contrast and, for this reason, some centres have also developed low-contrast letter charts, in which the contrast goes down rather than the size of the letter.

Colour vision is also very important in picking up pituitary visual problems. Development of colour blindness in people who previously had normal colour vision is suggestive of optic nerve and chiasmal disease.

To conclude, there are a number of reasons why the visual problems associated with pituitary disease can be difficult to detect. However, provided they are detected early enough, then surgery or other therapy can usually relieve the pressure on the chiasm and the optic nerve and return vision to normal.