Over the years, a variety of different operations have been tried to treat the pain of Post-Herpetic Neuralgia (PHN). The literature has been clouded by different investigators using different outcome scales so that it has been difficult, if not impossible, to compare one study with another. I believe a second difficulty has arisen because there are (at least) two different types of pain associated with PHN and one operation may be effective at one type of pain and not the other. With this in mind, our approach to PHN begins with trying to understand which component of pain is disabling the patient.
Touch-induced electrical pain can be reduced by eliminating the ability of the face to feel the touch. Percutaneous trigeminal rhizotomy can create numbness in the region of the face that was triggering the electrical pain. This is analogous to trigeminal rhizotomy for trigeminal neuralgia. It is important to be aware that the rhizotomy will not be expected to reduce the constant burning pain (and may even increase this aspect of the pain). The operation, however, is relatively easy to perform and we have gained an experience with over 1000 cases in the treatment of trigeminal neuralgia. One warning is especially warranted- the pain of PHN is often in and around the eye. A rhizotomy in this location will leave the covering of the eye (cornea) numb. A numb eye has an increased risk of corneal abrasion because you can not feel anything scratch your eye. If your eye becomes red or inflamed, you must seek medical attention even if it does not hurt – you may have scratched your cornea and just not feel it. Ignoring this risks blindness if the cornea gets infected. Most patients check their eye each day (while brushing their teeth) and if it is white there is no problem. On the rare occasion that it is red, they should get it checked by their family doctor of ophthalmologist.
The burning pain is more difficult to treat. We believe that this pain is due to inappropriate over activity in the brain following the nerve damage. This type of pain is called ‘neuropathic’ pain because it is due to injury of the nervous system. Sensation from the face is relayed to the brain along the trigeminal nerve. This nerve carries all the information received from the face including touch, movement of the jaw as well as pain and temperature. Once the trigeminal nerve enters the brainstem, however, the information is relayed to different areas. The pain and temperature area is different from the touch area and that is the key to our surgical attack on this pain. We can destroy the area of the brain responsible for receiving and transmitting facial pain (and temperature) and leave the region responsible for touch alone.
The spinal dorsal root entry zone (DREZ) lesion is effective for PHN in the body. It therefore makes sense that a similar operation would be effective for PHN in the face. The operation of trigeminal caudalis nucleotractotomy is the analogous operation. The name of this operation varies because both the fibres leading to the trigeminal nucleus and the nucleus itself are very close together and a lesion in this area destroys both the fibres (tractotomy) and the nucleus (nucleotomy). The trigeminal nucleus is separated into three parts. One deals with proprioception (monitoring movements of the jaw), one deals with touch and one deals with pain and temperature. It is therefore possible to destroy the nucleus dealing with pain and temperature and leave the nucleus dealing with touch sensations uninjured. This anatomical separation allows us to destroy the pain centre (nucleus trigeminal caudalis) without injuring the touch centre. Following the operation, therefore, the patient is expected to lose the ability to feel pain in the affected area of the face (and thus no longer feel the burning pain of PHN) but will still be able to feel light touch in the same area. Although most patients do not notice, they will also lose the ability to feel temperature differences (i.e. hot or cold) in the same area of pain loss. Interestingly, there is also some touch fibres which descend into the caudal is nucleus therefore the patient will notice some ‘numbness’ in the face but this sensation usually fades and the eye remains protected with a blink reflex.
We consider this operation a major undertaking. It is performed under general anesthetic and involves placing a few very small lesions (each the size of an apple seed) in the upper spinal cord where the trigeminal nucleus caudalis is located. There are very few centres in the world performing this operation. The risks of this operation are related to placing the lesions in the right location. If they are misplaced the patient could suffer incoordination of their arm or weakness of their arm and leg. Additional potential risks must be discussed directly with your neurosurgeon.
We would not recommend this operation for everyone with PHN in the face. Many patients will get adequate pain relief with appropriate medications or will learn to adapt to the pain after several years. For those with incapacitating pain there is hope. This operation is an option that needs to be discussed with those who have medically intractable pain. Patients need to understand the potential benefits (dramatic pain relief) and the potential risks (a small chance of arm/leg weakness or arm incoordination) before making a decision about surgery.