The visual impact of pituitary tumours
By Dr Denize Atan
Consultant in Neuro-ophthalmology, Bristol Eye Hospital
The pituitary gland is an important gland which sits in the middle of our brain. It is responsible for making a number of hormones that regulate our metabolic rate, responses to stress, growth, fertility, menstrual cycle and pregnancy hormones, and intake/output of fluids.
The most common problem which affects the pituitary gland is a benign growth of the gland, also known as an adenoma. Pituitary adenomas either produce excessive amounts of pituitary hormones themselves, or the growth actually interferes with the production of pituitary hormones.
Our bodies normally control the levels of these hormones very precisely, and too high or low levels lead to ill health. They do not have any direct influence on our vision. Why then do problems with the pituitary gland affect our vision?
The reason is simply this: pituitary adenomas can affect our vision because of where they are positioned in the brain, not because of what they are, or the hormones they produce.
When a pituitary adenoma reaches a certain size (>1cm) it is known as a macroadenoma and it can start to interfere with the function of the structures around it.
To understand why this is, it is important to know a little about the anatomy of the pituitary gland and what lies close to it in the brain. In other words, growth of the pituitary gland affects our vision indirectly, by exerting pressure on the structures that surround it.
A normal pituitary gland, or a microadenoma (<1cm in size) have no effect on vision at all.
Our eyes work like cameras that pick up patterns of light at different moments in time. However, the visual system includes not only our eyes, but the wiring that connects our eyes to visual centres in the brain which are responsible for interpreting visual information and forming visual images.
It is also important that our eyes are pointing in the correct direction to look at what we want to see, and so we have eye muscles that work like pulleys, to point our eyes in the right direction.
We even have muscles that work like shutters, to protect our eyes, our eyelids and others that regulate the amount of light that enters our eyes, our pupils. Messages to and from our brains are conveyed to these structures via nerves that are simply like the wires in a circuit board.
Figure 1: Anatomy of the pituitary gland and surrounding visual circuitry (coronal section) image: http://what-when-how.com/acp-medicine/pituitary-part-1/
How is this relevant to the anatomy of the pituitary gland?
By some accident of evolution, the pituitary gland sits in the middle of the circuitry that connects the brain to our eyes, eye muscles and eyelids. By simple expansion beyond the confines of the pituitary fossa, a pituitary macroadenoma can start to push and wrap around this wiring and interfere with the information flow between our eyes and our brains (Figure 1).
How might you notice a problem with your vision due to a pituitary macroadenoma?
Often, you might not be aware of a problem with your vision due to a pituitary macroadenoma. This is because a pituitary macroadenoma most commonly affects the optic nerves, chiasm or optic tracts before any other nerves of the visual system. These nerves lie directly above the pituitary fossa and are therefore the first structures affected by upward expansion of a macroadenoma (Figure 1).
The most common problem with vision to result from a pituitary macroadenoma is a bitemporal hemianopia when the optic chiasm is involved; in other words, an inability to notice things to either side of what you are directly looking at (Figure 2).
Because you can see everything directly in front of you, you may not realize that you have a problem noticing things to either side, meaning that you are more likely to bump into them. Often, people do not notice a problem with their peripheral vision for quite some time and only when it is quite advanced.
Frequently, it is a problem picked up by an optician at a routine check-up.
There are other patterns of visual impairment that can occur because of a pituitary macroadenoma, which depends on which part of the visual circuitry is affected (Figure 2). For example, when one of the optic nerves is involved, you might notice that the vision in the eye on that side is blurred or, at worst, absent.
Even though the eye itself is quite healthy, the information it would normally transmit to the brain via the optic nerve cannot get through.
Figure 2: Patterns of visual impairment caused by a pituitary macroadenoma. Adapted from: http://commons.wikimedia.org/wiki/File:1420_Optical_Fields.jpg
What other visual problems may be caused by a pituitary macroadenoma?
As mentioned earlier, normal vision also depends on your eye muscles pulling both eyes in the right direction to look at things simultaneously. If your eyes do not point in exactly in the same direction, you might see two images instead of one. This can be very disorientating and confusing.
The reason that a pituitary macroadenoma can cause double vision is that the nerves connecting your eye muscles to your brain are also positioned very close to the pituitary gland.
There are three nerves that control eye movements, called the oculomotor, trochlear and abducent nerves. They are also known as the third, fourth and sixth cranial nerves. If any one of these nerves is affected by a pituitary macroadenoma, it can result in double vision (Figure 1).
Why is it important to look for visual problems caused by a pituitary macroadenoma?
As mentioned earlier, many people may not be aware of a problem with their peripheral vision before it becomes quite advanced and so it is important that you are screened for any visual problems resulting from a pituitary macroadenoma. In addition, some or all of this visual impairment can be reversed with treatment of the pituitary macroadenoma – whether this is medication, surgery or radiotherapy to shrink the size of the adenoma.
As the adenoma gets smaller, the pressure it exerts on surrounding structures, including the visual circuitry, is relieved, leading to an improvement in vision.
Left untreated, some of this visual impairment may become permanent. This can sometimes affect your ability to drive.