PAIN WITHOUT A CAUSE: TRIGEMINAL NEURALGIA

We will start with a fairly rare condition that causes severe pain which is so specific that one would think that there must be a local pathology, and yet none has been discovered. Tic douloureux, as trigeminal neuralgia is also known, is a terrible pain in the face with stabbing pain on one side often described as a strong electric shock. Between the attacks, the patient is pain-free and normal. The pain comes on abruptly and stops equally abruptly. The pain explodes if a gentle moving stimulus brushes a trigger point, usually in the area around the mouth. It occurs equally in men and women and is most common between the ages of 50 to 70 years but may occur at any age. If nothing is done, the pain may disappear for months or years only to reappear in the same area. The little trigger point that sets off the stab of pain may meander slowly from one part of the face to another.

It would seem entirely reasonable to patient and doctor that there must be some damage to the nerve fibres leading from the trigger point to the brain. However, this is not so reasonable when we consider the following facts. The nerves in the face have been examined in great detail and no abnormality has been found. The sensitivity of the skin of the trigger point is completely normal between attacks and yet, if nerves are damaged, one would expect abnormal sensation. Finally, as we have said, the trigger point meanders about and yet nerves do not meander as they remain in the same place for a lifetime.

The nerves from the face originate from cells in a ganglion in the base of the skull that is the equivalent of the dorsal root ganglia which lie on either side of the vertebrae and give rise to sensory nerves to the body. From the ganglion, the nerve fibres group into a root, which makes its way into the brainstem where the fibres connect with central nerve cells. The root runs free in the fluid around the brain. One theory proposes that the nerves are damaged by being pounded with pulses from nearby blood vessels. There may be some sign of minor damage but just such damage is present in many people who have no signs of the disease. In the brainstem, there are no signs of damage in most patients but some patients, who have multiple sclerosis in the brainstem area where the face nerve fibres arrive, do develop tic.

Fortunately for patients, two methods of treatment are used and they tell us a lot about the nature of the disorder. The French word tic means a twitch and is also used for an epileptic attack. With no more logic than the similar use of the word for both conditions, anti-epileptic drugs were given to patients with trigeminal neuralgia and found to be highly effective in the majority of patients. This is good for the patients but shifts the search for the cause of the disease. These drugs have no known effect on peripheral nerves but certainly affect abnormal firing of central cells, raising the possibility that the cause lies in the brain. Another quite different drug called tocainide also has no known peripheral action but affects the brain and stops trigeminal neuralgia. Unfortunately, some patients cannot tolerate these drugs because they cause nausea, dizziness, somnolence and confusion, and these patients are offered surgery.

There is no doubt that the trigger for the pain originates in the large, low-threshold nerve fibres in a small area of skin. These can be temporarily blocked by local anaesthetic, which temporarily interrupts the disease. For a longer-term effect, surgeons cut the nerves going to the trigger area, which stops the pain until nerves regenerate and the pain returns. In order to stop the regeneration, surgeons move centrally to attack the ganglia that contain the cell bodies of nerve fibres. Destruction here prevents regeneration because the cell bodies from which the nerve fibres grow have been killed. In the process of treating the ganglia, it was found that surprisingly gentle manipulations could also stop the pain on occasions.

We are left with two possible causes of the disease. One could be that there is some undiscovered and unknown process in the peripheral nerve which grossly amplifies the incoming signal. The other is that the brainstem cells that receive the sensory fibres have developed an extreme hyperexcitability so they explode in synchronous action when stimulated by a normally innocuous input. The beneficial effect of surgery for this central cause would come from blocking the input. The beneficial effect of the drugs would come from decreasing the excitability of the central cells. We know that cells can become hyper-excitable because that occurs in epilepsy, and we have described how pain-producing cells can become hyperexcitable after inflammation or nerve damage.

Trigeminal neuralgia is a disease without a known cause but the target area within which the cause should be sought is obvious. The cause might be in disordered peripheral nerves or in central cells.

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