Postherpetic Neuralgia: Causes, Risk Factors, Signs, Symptoms, Treatment, Investigations

Postherpetic neuralgia is a very common complication of shingles or herpes zoster. It can be described as pain that continues to remain for a certain period after herpes zoster lesions or rash has healed, usually about three months.

Shingles or herpes zoster is a viral infection of the nerves, which results in formation of a rash or lesions on the skin, which can be very painful. Occasionally, the nerves affected by shingles can continue to cause severe pain even after the shingles have cleared. Shingles appear as a patch, a rash or a line of painful blisters which form over the skin along a nerve in the shape of a band. This band forms in the distribution of the nerve where the virus has been residing before its spread to the skin.

Though postherpetic neuralgia pain does not affect everyone who has shingles, as many as 20% to 30% go on to develop it. It is estimated that about one in every five individuals affected with shingles are affected with postherpetic neuralgia. The older a person, the more likely he/she is to develop postherpetic neuralgia. People older than 60 years tend to get primarily affected. Older people who are infected with shingles have about 50% chances of developing postherpetic neuralgia.

Abnormal function of unmyelinated nociceptors and sensory loss leads to postherpetic neuralgia. Temperature detection system and pain get hypersensitive even with slight stimulation leading to acute pain. This pain may be related to formation of new connections that involve central pain transmission neurons. There can be instances where only severe sudden pain is experienced without allodynia, possibly secondary to increased spontaneous activity in de-afferented central neurons or reorganization of central connections. Imbalance involving loss of large inhibitory fibers and an intact or increased number of small excitatory fibers has been suggested. There is involvement of central as well as peripheral areas with pain production as per this input about abnormal dorsal horn containing de-afferented hypersensitive neurons.

The nerve damage that is caused by shingles or herpes zoster disrupts the proper functioning of the nerve. This faulty nerve gets confused and sends chaotic or uncontrolled pain signals to the brain on a random basis, which the person affected feels as a throbbing, burning pain along the nerve. It is believed that shingles result in scar tissue forming next to the nerves, which causes to press on them causing them to send inaccurate signals, most of which are pain signals, to the brain.

Postherpetic neuralgia is a result of changes in the nervous system, which are produced by the varicella virus. It is believed that nerve injury can change nerve cells in such a way that they are activated more easily, discharge spontaneously and show exaggerated responses to stimuli. In the process of regrowth after the nerve injury, nerve produces new nerve endings which also are prone to unprovoked discharges. The excessive activity is thought to lead to hyperexcitement of the portion of the spinal cord, which contains the cell bodies of the sensory neurons. This results in altered signal processing of the central nervous system and exaggerated central nervous system responses.

A number of factors have been identified that increase the chances of developing postherpetic neuralgia in people affected with shingles, hence having multiple risk factors tend to increase the chances of getting affected with postherpetic neuralgia.

• Being a woman.
• Old age.
• Symptoms being present before appearance of the rash such as pain, itching, numbness, or tingling.
• Severity of pain during initial stages of the illness.
• Severity of rash.
• Psychological to a certain extent.

Symptoms are basically limited to the area of the skin where the outbreak of shingles occurs first, most commonly a band around the trunk on one side of the body.

• Headaches, generally in cases where head or face is affected.
• Sharp shooting, jabbing, or burning pains or deep aching pains.
• Itching and numbness though less common.
• Sensitivity to temperature changes.
• Sensitivity to light touch, even to soft materials like clothing.
• Weakness or paralysis in very rare cases.

Treatment depends on the type of pain as well as the patient’s physical, neurological and mental status.

• Antidepressants: Antidepressants affect the key brain chemicals such as serotonin and norepinephrine, which influence not only depression, but also how the body interprets pain. Dosages given for postherpetic neuralgia will be lower than for depression unless the patient is actually undergoing treatment for depression. Tricyclic antidepressants such as desipramine, amitriptyline, nortriptyline and duloxetine are said to make the pain more bearable, but not get rid of the pain.
• Anticonvulsants: Anticonvulsants can effectively calm down nerve impulses and stabilize abnormal electrical activity in the nervous system caused by injured nerves, hence it helps in lessening the pain. Gabapentin and pregabalin are commonly prescribed anticonvulsants.
• Steroids: Corticosteroid injection into the area around the spinal cord are effective in treatment of chronic persistent long-term pain. However, these cannot be used until the rash has completely disappeared.
• Painkillers: Painkillers like tramadol or oxycodone can be used due to lesser risk of dependency.
• TENS: TENS stands for transcutaneous electrical nerve stimulation. Electrodes are placed over the areas where pain occurs and small electrical impulses are emitted to treat the pain. TENS unit can be turned on and off as required.
• Spinal Cord Or Peripheral Nerve Stimulation: This is similar to TENS, but the difference is that the devices are implanted subcutaneously or under the skin along the course of peripheral nerves. These devices are a safe, efficient, and effective in relieving not only postherpetic neuralgia, but many other types of neuropathic pains including trigeminal neuralgia. The device is surgically implanted, however, before implanting doctors do a trial using a thin wire electrode to make sure the patient responds well. The spinal cord stimulator is inserted into the epidural space through the skin over the spinal cord. The peripheral nerve stimulator is placed above the peripheral nerve under the skin.
• Lidocaine Patches: Lidocaine is a common local anesthetic and antiarrhythmic drug and can be used to relieve pain. Lidocaine can also be used topically i.e., applied onto the skin to relieve burning and itching sensation, and pain from skin inflammations. The patches are cut to fit the affected area, but they should not be used on the face.

Development of chronic pain at the site of the rash is an indication of postherpetic neuralgia.
Medical assessment should be aimed at evaluating the pain completely and ensuring that there are no other factors contributing to the pain such as physical dysfunction, mood, pain-coping strategies and lack of social support.