What causes Postherpetic Neuralgia (PHN)?
Herpes Zoster infection (Shingles) is caused by the reactivation of the virus which causes chickenpox. If one has had chickenpox before, the virus that causes it remains inside the body in an inactive state till it gets an opportunity to spread once again. The reactivation may happen as one ages or as a result of reduced immunity. The virus spreads along a nerve producing the typical rash of Herpes Zoster in the area supplied by the nerve. Rash generally heals in 2-4 weeks and is accompanied by pain, numbness, itching and altered sensitivity in the area.
One out of five patients with Herpes Zoster may still have persisting pain after 120 days (90 days as per WHO) of rash onset and this condition is referred to as Postherpetic Neuralgia (PHN). In PHN the nerve cells can get damaged leading to increased excitability and persisting pain. Approximately 50% of patients recover within a year and the course is variable in the remaining. In one study it was observed that the proportion of patients with spontaneous resolution of pain decreased with increasing time since the onset of herpes zoster.
Who are at risk for developing persisting pain/Postherpetic Neuralgia after the Herpes Zoster (Shingles) infection?
The risk factors can be viewed as those relating to the
- Risk of developing Herpes Zoster infection– such as increasing age and low immunity. Low immunity may be secondary to diseases such as cancer or medications used to suppress immunity such as those used in organ transplant, treatment of inflammatory arthritis and chemotherapy.
- Risk of persisting / non resolving pain– Pain preceding the onset of rash, severe pain at the time of rash onset, widespread rash and older age have been associated as risk factors for persisting pain (PHN).
Early use of antiviral agents (within 72 hours of rash onset) & corticosteroids (if indicated), early aggressive pain control may offer some protection, reducing the risk of developing PHN after Herpes Zoster infection.
What are the symptoms of Postherpetic Neuralgia?
PHN generally presents as burning, shooting, throbbing or electric shock like pain occurring spontaneously or in response to stimuli such as touch. Commonly involved areas include the chest wall (thoracic dermatomes) and around the eye (ophthalmic branch of the trigeminal nerve). Pain may be more severe at night time and during periods of stress. The affected area may
- Be hypersensitive with daily activities such as taking bath, wearing clothes becoming difficult as the touch of clothes or water is painful. This is known as allodynia where non painful stimuli become painful.
- Feel itchy
- Feel weak (muscle weakness)
- Develop skin pigmentation and scarring in the area of rash
How is Postherpetic Neuralgia diagnosed?
PHN is rare in the age group below 50 years and incidence increases after the age of 60 years. Generally the diagnosis can be made clinically as most patients would give the history of Herpes Zoster Infection/ typical painful rash in the affected area. In many cases the persisting skin colour changes and scarring also aid the diagnosis. The rash generally involves only one side and does not cross the midline.
Which medications can help to reduce the pain in Postherpetic Neuralgia?
Different types of medication are used to control pain in PHN and these include:
PHN pain does not respond well to the usual painkiller and special types of painkillers known as the neuropathic painkillers may be required. This includes medications such as
These are well known painkillers commonly used for nerve pain. The choice of drugs best suited for an individual is made after a thorough assessment and taking into account a number of factors such as age of patient, pain severity, other medical problems, general health etc. The dose of the selected medications is increased gradually giving the body a chance to adapt and reducing the side effects. These medications may take a few weeks to produce their full effects and hence it is important that you stick to the schedule recommended by your pain physician. Often a combination of different agents is utilised to maximise your gains and to keep the individual drug doses low so that the side effects are minimised. These medications not only help with the pain but can also help in improving sleep, mood, and quality of life.
This class includes morphine like drugs which are often used for severe pain, in combination with the above mentioned medications. Opioids can be classified as weak opioids (like tramadol) and strong opioids (fentanyl patches, morphine, oxycodone etc). Some of these are available in long acting and short acting formulations. These drugs are best prescribed by a specialist who is well versed with the available options and the limitations /advantages of each of them. It is important that you adhere to the schedule as prescribed by your pain specialist. Whilst deciding on medications the side effects should be kept in mind and discussed with the patients. Sometimes the side effects are used as an advantage like sedation with night time medications to improve sleep.
These have the advantage of fewer side effects and are sometimes used preferentially in the elderly/ frail patients and when side effects of oral medications limit their usage. They include various types of gels, creams and patches. Some contain local anaesthetic and soothing agents. Local anaesthetic patches may be especially useful in cases of increased sensitivity to stimuli such as touch. Unfortunately some of the topical options such as 8% capsaicin patch are not available in India as yet. A one off 60 minute application of Capsaicin 8% patch can provide significant pain reduction for a few months.
What other treatment options are available for pain control in Postherpetic Neuralgia?
A number of options are available to control PHN pain apart from the medications and these include:
Drug infusions- These are commonly used in cases with severe pain, especially when the other measures have not been successful. They work by reducing the sensitivity of the nerves transmitting the pain signals. These infusions are generally administered in day care setting which means that you do not have to stay in the hospital overnight. These are low risk options with the potential of significantly reducing the pain for weeks/ months. They can help reduce the requirement of other regular strong medications thereby reducing the side effects.
Nerve blocks, Dorsal Root Ganglion Block and Epidural injections can be considered based on the pain severity and the site involved. These procedures involve injection of local anaesthetic and steroids to reduce the pain and sensitivity of the nerves. They are generally performed as a day case using x-ray or ultrasound guidance with no requirement for overnight hospital stay. In severe acute cases however hospital stay may be required if continuous epidural drugs are administered to control the pain.
Sympathetic nerve blocks including stellate ganglion block are used for severe uncontrolled pain. Initially a diagnostic block is performed to assess if these special nerves (called the sympathetic nerves) are involved in transmitting the pain signals to the brain. These nerves do not carry pain signals normally but can get involved in chronic pain conditions. If the response to the diagnostic block is positive then other ways of prolonging the effect can be considered. Depending on the duration of the effect, these blocks may need to be repeated.
Pulsed radiofrequency lesioning (PRF) is a safe, non-destructive modality that can help in reducing the PHN pain. This treatment is performed as a day case, using special equipment and requires x-ray or ultrasound guidance. The treatment involves modulating the way pain signals are transmitted and processed and the resulting pain relief can last for weeks/ months.
Neuromodulation including Peripheral Nerve Stimulation, Peripheral Nerve Field Stimulation ― Peripheral Nerve Stimulation (PNS) in a non invasive treatment that is performed by placing the stimulating electrode in close proximity to the involved peripheral nerve. The nerve can be easily located using ultrasound in most cases, with the stimulation performed transcutaneously. More invasive methods may involve placing electrodes through the skin or in the spine (Spinal cord Stimulators). In Peripheral Nerve Field Stimulation (PNFS), stimulating electrodes are placed below the skin (percutaneously) close to the painful area and are removed once the treatment has been performed.