What is Postherpetic Neuralgia?
Shingles is an infection caused by a virus called Varicella Zoster which also causes chickenpox. It usually affects the nerves of the chest and abdomen and causes pain and a rash over one side of the body. An episode of shingles will usually last for 2 to 4 weeks. Postherpetic Neuralgia (PHN) may occur as a complication of shingles when the pain persists for longer than this time or persists after the rash has healed. PHN may also be diagnosed if the pain returns three months after the initial episode of shingles. PHN refers to severe, constant nerve pain (‘neuralgia’ = severe pain coming from a nerve). Approximately 10-18% of people with shingles will develop PHN. The risk of PHN also increases with age.
What causes Postherpetic Neuralgia?
It is not known why some people develop PHN. After a bout of shingles, the affected nerves usually recover however sometimes the nerve damage caused by shingles leads to scar tissue forming next to the nerves. The scar tissue may press on the nerves and cause them to send out involuntary messages of pain to the brain.
Symptoms of Postherpetic Neuralgia
The pain of PHN is a severe nerve pain which is felt in the same area as the shingles rash. It is usually described as burning, throbbing, aching or shooting. It is often very tender to touch and any light touch such as clothing, draughts or a slight breeze can cause excruciating pain. This is known as ‘allodynia’ (i.e. pain from stimuli which is not normally painful). It is often also intensely itchy. PHN may interfere with the ability to carry out daily activities such as bathing and dressing. Some people with PHN have trouble sleeping and experience fatigue.
Diagnosis of Postherpetic Neuralgia
If a person has had shingles their doctor will be able to base a diagnosis of PHN on their symptoms and the length of time they have had them for. It is very important to recognise the symptoms of PHN as early as possible. If pain persists or returns within 3 months of the shingles attack a doctor must be informed immediately.
Treatment of Postherpetic Neuralgia
- Analgesia
There are several classes of medications which have been found to help with the pain of PHN. These drugs work in different ways and may be needed to be taken in combination with each other.The initial type of pain relief that a doctor will prescribe is based on the severity of the pain. For people with mild to moderate pain, over-the-counter drugs such as paracetamol may be recommended. If these pain medications are not strong enough then opioid analgesia may be prescribed however these drugs do cause side effects such as drowsiness and constipation and are usually not recommended for long term use.Medication which has a direct effect on the nervous system has been shown to be helpful for treating PHN. Tricyclic antidepressants such as Amitriptyline belong to this group. Tricyclic antidepressants may cause side effects such as a dry mouth, drowsiness, constipation or nausea.
- Lignocaine patch
If a person is unable to tolerate the medications or creams that are usually prescribed to treat PHN then a lignocaine patch may be recommended. This patch contains a local anaesthetic which has a numbing effect on the area to which it is applied.
Other types of medications include a group known as anticonvulsants (originally designed to treat epilepsy) is shown to be effective as a treatment for the nerve pain of PHN. Side effects include drowsiness and weight gain.
- Capsaicin cream
If over-the-counter medications are not helpful in relieving the pain and antidepressant or anticonvulsant medication is not suitable then a cream called capsaicin cream may be recommended. Capsaicin is the substance in red chilli peppers that makes them hot. This cream stings and burns when first applied and needs to be applied several times a day for a number of weeks before a benefit is received.
Other treatments
Injections (such as local injection, peripheral nerve block, neuraxial block and stellate ganglion block), nerve stimulation (such as peripheral nerve stimulation and pulsed radiofrequency), spinal cord stimulation.
