Postherpetic neuralgia
Encyclopedia
Postherpetic neuralgia (PHN) is a neuralgia
caused by the varicella zoster virus
. Typically, the neuralgia is confined to a dermatomic area
of the skin and follows an outbreak of herpes zoster
(HZ, commonly known as shingles) in that same dermatomic area. The neuralgia typically begins when the HZ vesicles have crusted over and begun to heal, but it can begin in the absence of HZ, in which case zoster sine herpete is presumed (see Herpes zoster
).
Treatment options for PHN include antidepressant
s, anticonvulsants (such as gabapentin
or pregabalin
) and topical agents such as lidocaine patches or capsaicin
lotion. Opioid analgesics may also be appropriate in many situations. There are some sporadically successful experimental treatments, such as rhizotomy (severing or damaging the affected nerve to relieve pain) and TENS (a type of electrical pulse therapy).
GlobalData’s analysis suggests that the global Postherpetic Neuralgia (PHN) market was worth $648.5m in 2010. It is forecast to decline at a Compound Annual Growth Rate(CAGR) of 2% for the next seven years, to reach $563.3m by 2017.
of the skin to send abnormal electrical signals to the brain. These signals may convey excruciating pain
, and may persist or recur for months, years or until death.
A key factor in the neural plasticity underlying neuropathic pain is altered gene expression in sensory dorsal root ganglia (DRG) neurons. Injury to sensory nerves induces neurochemical, physiological and anatomical modifications to afferent and central neurons, such as afferent terminal sprouting and inhibitory interneuron loss . Following nerve damage, NaC channel accumulation causes hyperexcitability, downregulation of the TTX-resistant Nav1.8 (sensory neuron specific, SNS1) channel and upregulation of TTX-sensitive Nav1.3 (brain type III) channels. These changes contribute to increased NMDA glutamate receptor-dependent excitability of spinal dorsal horn neurons and are restricted to the ipsilateral (injured) side. A combination of these factors could contribute to the neuropathic pain state of PHN.
Less than 10 percent of people younger than 60 develop postherpetic neuralgia after a bout of HZ, while about 40 percent of people older than 60 do.
Signs:
may reduce the risk of developing PHN.
If patients do develop postherpetic neuralgia, they are also advised to see their doctor immediately. They may have to work with their doctor and sometimes other specialists such as neurologists to try a variety of treatments before they find something that helps.
Imaging Studies:
In some cases, treatment of postherpetic neuralgia brings complete pain relief. But most people still experience some pain, and a few don't receive any relief. Although some people must live with postherpetic neuralgia the rest of their lives, most people can expect the condition to gradually disappear on its own within five years.
High-Concentration Capsaicin Patch Granted Orphan Drug Designation for PHN
from:http://www.medscape.com/viewarticle/704117?sssdmh=dm1.489879&src=ddd On June 9, 2009, The FDA approved orphan drug designation for a high-concentration capsaicin dermal patch (Qutenza [formerly NGX-4010], NeurogesX, Inc) for the treatment of pain associated with postherpetic neuralgia (PHN). Relief of pain is possible up to three months with no to minimal side effects. Qutenza has been recently approved by the FDA for general use in PHN. Distribution is planned for the first half of 2010. See NeutrogesX for distribution plans.
(FDA) approved the vaccine
to prevent chickenpox. Its effect on PHN is still unknown. The vaccine — made from a weakened form of the varicella-zoster virus — may keep chickenpox from occurring in nonimmune children and adults, or at least lessen the risk of the chickenpox virus lying dormant in the body and reactivating later as shingles. If shingles could be prevented, postherpetic neuralgia could be completely avoided.
Recently, Merck has tested a new vaccine (Zostavax
) against shingles. This vaccine is a more potent version of the chickenpox vaccine
. Evidence indicates that the vaccine reduced the incidence of shingles by 51 percent. Additionally, the vaccine reduced the incidence of PHN by two-thirds compared to placebo. However, the vaccine's protective effects diminished over the three years that most patients were followed. In December 2005, an FDA advisory committee unanimously agreed that the vaccine is safe and effective for persons over 60 years old. This was followed on 26 May 2006 by the FDA formally approving the use of the vaccine for that same age group. Further studies may demonstrate if there is benefit in patients 50–59 years old and if a booster dose is recommended.
Neuralgia
Neuralgia is pain in one or more nerves that occurs without stimulation of pain receptor cells. Neuralgia pain is produced by a change in neurological structure or function rather than by the excitation of pain receptors that causes nociceptive pain. Neuralgia falls into two categories: central...
caused by the varicella zoster virus
Varicella zoster virus
Varicella zoster virus is one of eight herpes viruses known to infect humans . It commonly causes chicken-pox in children and Herpes zoster in adults and rarely in children.-Nomenclature:...
. Typically, the neuralgia is confined to a dermatomic area
Dermatomic area
A dermatome is an area of skin that is mainly supplied by a single spinal nerve.There are eight cervical nerves ,twelve thoracic nerves,five lumbar nerves and five sacral nerves....
of the skin and follows an outbreak of herpes zoster
Herpes zoster
Herpes zoster , commonly known as shingles and also known as zona, is a viral disease characterized by a painful skin rash with blisters in a limited area on one side of the body, often in a stripe...
(HZ, commonly known as shingles) in that same dermatomic area. The neuralgia typically begins when the HZ vesicles have crusted over and begun to heal, but it can begin in the absence of HZ, in which case zoster sine herpete is presumed (see Herpes zoster
Herpes zoster
Herpes zoster , commonly known as shingles and also known as zona, is a viral disease characterized by a painful skin rash with blisters in a limited area on one side of the body, often in a stripe...
).
Treatment options for PHN include antidepressant
Antidepressant
An antidepressant is a psychiatric medication used to alleviate mood disorders, such as major depression and dysthymia and anxiety disorders such as social anxiety disorder. According to Gelder, Mayou &*Geddes people with a depressive illness will experience a therapeutic effect to their mood;...
s, anticonvulsants (such as gabapentin
Gabapentin
Gabapentin is a pharmaceutical drug, specifically a GABA analogue. It was originally developed for the treatment of epilepsy, and currently is also used to relieve neuropathic pain...
or pregabalin
Pregabalin
Pregabalin is an anticonvulsant drug used for neuropathic pain and as an adjunct therapy for partial seizures with or without secondary generalization in adults. It has also been found effective for generalized anxiety disorder and is approved for this use in the European Union. It was designed...
) and topical agents such as lidocaine patches or capsaicin
Capsaicin
Capsaicin 2CHCH=CH4CONHCH2C6H3-4--3- ) is the active component of chili peppers, which are plants belonging to the genus Capsicum. It is an irritant for mammals, including humans, and produces a sensation of burning in any tissue with which it comes into contact...
lotion. Opioid analgesics may also be appropriate in many situations. There are some sporadically successful experimental treatments, such as rhizotomy (severing or damaging the affected nerve to relieve pain) and TENS (a type of electrical pulse therapy).
GlobalData’s analysis suggests that the global Postherpetic Neuralgia (PHN) market was worth $648.5m in 2010. It is forecast to decline at a Compound Annual Growth Rate(CAGR) of 2% for the next seven years, to reach $563.3m by 2017.
Pathophysiology
Postherpetic neuralgia is thought to be nerve damage caused by herpes zoster. The damage causes nerves in the affected dermatomic areaDermatomic area
A dermatome is an area of skin that is mainly supplied by a single spinal nerve.There are eight cervical nerves ,twelve thoracic nerves,five lumbar nerves and five sacral nerves....
of the skin to send abnormal electrical signals to the brain. These signals may convey excruciating pain
Pain
Pain is an unpleasant sensation often caused by intense or damaging stimuli such as stubbing a toe, burning a finger, putting iodine on a cut, and bumping the "funny bone."...
, and may persist or recur for months, years or until death.
A key factor in the neural plasticity underlying neuropathic pain is altered gene expression in sensory dorsal root ganglia (DRG) neurons. Injury to sensory nerves induces neurochemical, physiological and anatomical modifications to afferent and central neurons, such as afferent terminal sprouting and inhibitory interneuron loss . Following nerve damage, NaC channel accumulation causes hyperexcitability, downregulation of the TTX-resistant Nav1.8 (sensory neuron specific, SNS1) channel and upregulation of TTX-sensitive Nav1.3 (brain type III) channels. These changes contribute to increased NMDA glutamate receptor-dependent excitability of spinal dorsal horn neurons and are restricted to the ipsilateral (injured) side. A combination of these factors could contribute to the neuropathic pain state of PHN.
Frequency
In the United States each year approximately 1,000,000 individuals develop herpes zoster. Of those individuals approximately 20%, or 200,000 individuals, develop postherpetic neuralgia.Less than 10 percent of people younger than 60 develop postherpetic neuralgia after a bout of HZ, while about 40 percent of people older than 60 do.
Predisposing factors
- Race: It may influence susceptibility to herpes zoster. African Americans are one fourth as likely as Caucasians to develop this condition.
- Often an older, debilitated or immune compromised population.
Signs and symptoms
Symptoms:- With resolution of the HZ eruption, pain that continues for 3 months or more is defined as PHN.
- Pain is variable from discomfort to very severe and may be described as burning, stabbing, or gnawing.
Signs:
- Area of previous HZ may show evidence of cutaneous scarring.
- Sensation may be altered over involved areas, in the form of either hypersensitivity or decreased sensation.
- In rare cases, the patient might also experience muscle weakness, tremor or paralysis — if the nerves involved also control muscle movement.
When to seek medical advice
It is strongly recommended by professionals that patients see a doctor at the first sign of shingles. Treating shingles early — within three days of developing the rash — and aggressively with oral antiviral drugs may reduce the length and severity of postherpetic neuralgia. In addition, amitriptylineAmitriptyline
Amitriptyline is a tricyclic antidepressant . It is the most widely used TCA and has at least equal efficacy against depression as the newer class of SSRIs...
may reduce the risk of developing PHN.
If patients do develop postherpetic neuralgia, they are also advised to see their doctor immediately. They may have to work with their doctor and sometimes other specialists such as neurologists to try a variety of treatments before they find something that helps.
Lab and imaging studies
Lab Studies:- No laboratory work is usually necessary.
- Results of cerebrospinal fluidCerebrospinal fluidCerebrospinal fluid , Liquor cerebrospinalis, is a clear, colorless, bodily fluid, that occupies the subarachnoid space and the ventricular system around and inside the brain and spinal cord...
(CSF) evaluation are abnormal in 61%.- Pleocytosis is observed in 46%, elevated protein in 26%, and VZV DNA in 22%.
- These findings are not predictive of the PHN clinical course.
- Viral culture or immunofluorescenceImmunofluorescenceImmunofluorescence is a technique used for light microscopy with a fluorescence microscope and is used primarily on biological samples. This technique uses the specificity of antibodies to their antigen to target fluorescent dyes to specific biomolecule targets within a cell, and therefore allows...
staining may be used to differentiate herpes simplex from herpes zoster in cases that are difficult to distinguish clinically. - Antibodies to herpes zoster can be measured. A 4-fold increase has been used to support the diagnosis of subclinical herpes zoster (zoster sine herpete). However, a rising titer secondary to viral exposure rather than reactivation cannot be ruled out.
Imaging Studies:
- Magnetic resonance imagingMagnetic resonance imagingMagnetic resonance imaging , nuclear magnetic resonance imaging , or magnetic resonance tomography is a medical imaging technique used in radiology to visualize detailed internal structures...
(MRI) lesions attributable to HZ were seen in the brain stem and cervical cord in 56% (9/16) of patients. - At 3 months after onset of HZ, 56% (5/9) of patients with an abnormal MRI had developed PHN.
- Of the 7 patients who had no HZ-related lesions on MRI, none had residual pain.
Treatment
Treatment for postherpetic neuralgia depends on the type and characteristics of pain experienced by the patient. Pain control is essential to quality patient care; it ensures patient comfort. Possible options include:- AntiviralAntiviral drugAntiviral drugs are a class of medication used specifically for treating viral infections. Like antibiotics for bacteria, specific antivirals are used for specific viruses...
agents, such as famciclovirFamciclovirFamciclovir is a guanine analogue antiviral drug used for the treatment of various herpesvirus infections, most commonly for herpes zoster . It is a prodrug form of penciclovir with improved oral bioavailability...
, are given at the onset of attacks of herpes zoster to shorten the clinical course and to help prevent complications such as postherpetic neuralgia. However they have no role to play following the acute attack if postherpetic neuralgia has become established. - Analgesics
- Locally applied topicalTopicalIn medicine, a topical medication is applied to body surfaces such as the skin or mucous membranes such as the vagina, anus, throat, eyes and ears.Many topical medications are epicutaneous, meaning that they are applied directly to the skin...
agents- Aspirin mixed into an appropriate solvent such as diethyl etherDiethyl etherDiethyl ether, also known as ethyl ether, simply ether, or ethoxyethane, is an organic compound in the ether class with the formula . It is a colorless, highly volatile flammable liquid with a characteristic odor...
may reduce pain. - LidocaineLidocaineLidocaine , Xylocaine, or lignocaine is a common local anesthetic and antiarrhythmic drug. Lidocaine is used topically to relieve itching, burning and pain from skin inflammations, injected as a dental anesthetic or as a local anesthetic for minor surgery.- History :Lidocaine, the first amino...
skin patches. These are small, bandage-like patches that contain the topical, pain-relieving medication lidocaine. The patches, available by prescription, must be applied directly to painful skin to deliver relief for four to 12 hours. Patches containing lidocaine can also be used on the face, taking care to avoid mucus membranes e.g. eyes, nose and mouth.
- Aspirin mixed into an appropriate solvent such as diethyl ether
- Systemically delivered
- Non-opiates such as paracetamolParacetamolParacetamol INN , or acetaminophen USAN , is a widely used over-the-counter analgesic and antipyretic . It is commonly used for the relief of headaches and other minor aches and pains and is a major ingredient in numerous cold and flu remedies...
or the non-steroidal anti-inflammatory drugNon-steroidal anti-inflammatory drugNonsteroidal anti-inflammatory drugs, usually abbreviated to NSAIDs or NAIDs, but also referred to as nonsteroidal anti-inflammatory agents/analgesics or nonsteroidal Anti-inflammatory medicines , are drugs with analgesic and antipyretic effects and which have, in higher doses, anti-inflammatory...
s. - OpioidOpioidAn opioid is a psychoactive chemical that works by binding to opioid receptors, which are found principally in the central and peripheral nervous system and the gastrointestinal tract...
s provide more potent pain control and the weaker members such as codeineCodeineCodeine or 3-methylmorphine is an opiate used for its analgesic, antitussive, and antidiarrheal properties...
may be available over the counter in combination with paracetamol (co-codamolCo-codamolCo-codamol is a non-proprietary name used to denote a compound analgesic, a combination of codeine phosphate and paracetamol . Co-codamol Co-codamol (BAN) is a non-proprietary name used to denote a compound analgesic, a combination of codeine phosphate and paracetamol (acetaminophen). Co-codamol...
). Other opioids are prescription-only and include higher dosages of codeine, tramadolTramadolTramadol hydrochloride is a centrally acting synthetic opioid analgesic used in treating moderate pain. The drug has a wide range of applications, including treatment for restless legs syndrome and fibromyalgia...
, morphineMorphineMorphine is a potent opiate analgesic medication and is considered to be the prototypical opioid. It was first isolated in 1804 by Friedrich Sertürner, first distributed by same in 1817, and first commercially sold by Merck in 1827, which at the time was a single small chemists' shop. It was more...
or fentanyl. Most opioids have sedating properties, which are beneficial for patients who experience pain.
- Non-opiates such as paracetamol
- Locally applied topical
- Pain modification therapy
- AntidepressantAntidepressantAn antidepressant is a psychiatric medication used to alleviate mood disorders, such as major depression and dysthymia and anxiety disorders such as social anxiety disorder. According to Gelder, Mayou &*Geddes people with a depressive illness will experience a therapeutic effect to their mood;...
s. These drugs affect key brain chemicals, including serotoninSerotoninSerotonin or 5-hydroxytryptamine is a monoamine neurotransmitter. Biochemically derived from tryptophan, serotonin is primarily found in the gastrointestinal tract, platelets, and in the central nervous system of animals including humans...
and norepinephrineNorepinephrineNorepinephrine is the US name for noradrenaline , a catecholamine with multiple roles including as a hormone and a neurotransmitter...
, that play a role in both depression and how your body interprets pain. Doctors typically prescribe antidepressants for postherpetic neuralgia in smaller doses than they do for depression. Low dosages of tricyclic antidepressantTricyclic antidepressantTricyclic antidepressants are heterocyclic chemical compounds used primarily as antidepressants. The TCAs were first discovered in the early 1950s and were subsequently introduced later in the decade; they are named after their chemical structure, which contains three rings of atoms...
s, including amitriptylineAmitriptylineAmitriptyline is a tricyclic antidepressant . It is the most widely used TCA and has at least equal efficacy against depression as the newer class of SSRIs...
, seem to work best for deep, aching pain. They don't eliminate the pain, but they may make it easier to tolerate. Other prescription antidepressants (e.g. venlafaxineVenlafaxineVenlafaxine is an antidepressant of the serotonin-norepinephrine reuptake inhibitor class. First introduced by Wyeth in 1993, now marketed by Pfizer, it is licensed for the treatment of major depressive disorder , as a treatment for generalized anxiety disorder, and comorbid indications in...
, bupropionBupropionBupropion is an atypical antidepressant and smoking cessation aid. The drug is a non-tricyclic antidepressant and differs from most commonly prescribed antidepressants such as SSRIs, as its primary pharmacological action is thought to be norepinephrine-dopamine reuptake inhibition...
and selective serotonin reuptake inhibitorSelective serotonin reuptake inhibitorSelective serotonin re-uptake inhibitors or serotonin-specific reuptake inhibitor are a class of compounds typically used as antidepressants in the treatment of depression, anxiety disorders, and some personality disorders. The efficacy of SSRIs is disputed...
s) may be off-label useOff-label useOff-label use is the practice of prescribing pharmaceuticals for an unapproved indication or in an unapproved age group, unapproved dose or unapproved form of administration...
d in postherpetic neuralgia and generally prove less effective, although they may be better tolerated than the tricyclics. - AnticonvulsantAnticonvulsantThe anticonvulsants are a diverse group of pharmaceuticals used in the treatment of epileptic seizures. Anticonvulsants are also increasingly being used in the treatment of bipolar disorder, since many seem to act as mood stabilizers, and in the treatment of neuropathic pain. The goal of an...
s. These agents are used to manage severe muscle spasms and provide sedation in neuralgia. They have central effects on pain modulation. Medications such as phenytoinPhenytoinPhenytoin sodium is a commonly used antiepileptic. Phenytoin acts to suppress the abnormal brain activity seen in seizure by reducing electrical conductance among brain cells by stabilizing the inactive state of voltage-gated sodium channels...
(Dilantin, Phenytek), used to treat seizures, also can lessen the pain associated with postherpetic neuralgia. The medications stabilize abnormal electrical activity in the nervous system caused by injured nerves. Doctors often prescribe another anticonvulsant called carbamazepineCarbamazepineCarbamazepine is an anticonvulsant and mood-stabilizing drug used primarily in the treatment of epilepsy and bipolar disorder, as well as trigeminal neuralgia...
(Carbatrol, Tegretol) for sharp, jabbing pain. Newer anticonvulsants, such as gabapentin (Neurontin) and lamotrigineLamotrigineLamotrigine, marketed in the US and most of Europe as Lamictal by GlaxoSmithKline, is an anticonvulsant drug used in the treatment of epilepsy and bipolar disorder. It is also used as an adjunct in treating depression, though this is considered off-label usage...
(Lamictal), are generally tolerated better and can help control burning and pain.
- Antidepressant
- CorticosteroidCorticosteroidCorticosteroids are a class of steroid hormones that are produced in the adrenal cortex. Corticosteroids are involved in a wide range of physiologic systems such as stress response, immune response and regulation of inflammation, carbohydrate metabolism, protein catabolism, blood electrolyte...
s are commonly prescribed but a Cochrane Review found limited evidence and no benefit.
- Other non-pharmacological treatments for postherpetic neuralgia include the following:
- Relaxation techniques. These can include breathing exercises, visualization and distraction.
- Heat therapyHeat therapyHeat therapy, also called thermotherapy, is the application of heat to the body for pain relief and health. It can take the form of a hot cloth, hot water, ultrasound, heating pad, hydrocollator packs, whirlpool baths, cordless FIR heat therapy wrap, and many others. It can be beneficial to those...
. - Cold therapy. Cold packs can be used.
- Transcutaneous Electrical Nerve StimulationTranscutaneous electrical nerve stimulationTranscutaneous electrical nerve stimulation is the use of electric current produced by a device to stimulate the nerves for therapeutic purposes...
(TENS). This involves the stimulation of peripheral nerve endings by the delivery of electrical energy through the surface of the skin. - Spinal cord stimulatorSpinal Cord StimulatorA spinal cord stimulator is a device used to exert pulsed electrical signals to the spinal cord to control chronic pain. Spinal cord stimulation , in the simplest form, consists of stimulating electrodes, implanted in the epidural space, an electrical pulse generator, implanted in the lower...
. The electrical stimulation of the posterior spinal cord works by activating supraspinal and spinal inhibitory pain mechanisms.
In some cases, treatment of postherpetic neuralgia brings complete pain relief. But most people still experience some pain, and a few don't receive any relief. Although some people must live with postherpetic neuralgia the rest of their lives, most people can expect the condition to gradually disappear on its own within five years.
High-Concentration Capsaicin Patch Granted Orphan Drug Designation for PHN
from:http://www.medscape.com/viewarticle/704117?sssdmh=dm1.489879&src=ddd On June 9, 2009, The FDA approved orphan drug designation for a high-concentration capsaicin dermal patch (Qutenza [formerly NGX-4010], NeurogesX, Inc) for the treatment of pain associated with postherpetic neuralgia (PHN). Relief of pain is possible up to three months with no to minimal side effects. Qutenza has been recently approved by the FDA for general use in PHN. Distribution is planned for the first half of 2010. See NeutrogesX for distribution plans.
Prognosis
- The natural history of PHN involves slow resolution of the pain syndrome.
- In those patients who develop PHN, most will respond to agents such as the tricyclic antidepressants.
- A subgroup of patients may develop severe, long-lasting pain that does not respond to medical therapy. Continued research for new agents is necessary.
Primary prevention
In 1995, the Food and Drug AdministrationFood and Drug Administration
The Food and Drug Administration is an agency of the United States Department of Health and Human Services, one of the United States federal executive departments...
(FDA) approved the vaccine
Vaccine
A vaccine is a biological preparation that improves immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing microorganism, and is often made from weakened or killed forms of the microbe or its toxins...
to prevent chickenpox. Its effect on PHN is still unknown. The vaccine — made from a weakened form of the varicella-zoster virus — may keep chickenpox from occurring in nonimmune children and adults, or at least lessen the risk of the chickenpox virus lying dormant in the body and reactivating later as shingles. If shingles could be prevented, postherpetic neuralgia could be completely avoided.
Recently, Merck has tested a new vaccine (Zostavax
Zostavax
Zostavax is a live vaccine developed by Merck & Co. which has been shown to reduce the incidence of herpes zoster by 51.3% in a study of 38,000 adults aged 60 and older who received the vaccine. The vaccine also reduced by 66.5% the number of cases of postherpetic neuralgia and reduced the...
) against shingles. This vaccine is a more potent version of the chickenpox vaccine
Varicella vaccine
The varicella vaccine is a live virus that protects against the viral disease commonly known as chickenpox caused by Varicella Zoster Virus . Varicella vaccine is marketed as Varivax in the U.S. by Merck...
. Evidence indicates that the vaccine reduced the incidence of shingles by 51 percent. Additionally, the vaccine reduced the incidence of PHN by two-thirds compared to placebo. However, the vaccine's protective effects diminished over the three years that most patients were followed. In December 2005, an FDA advisory committee unanimously agreed that the vaccine is safe and effective for persons over 60 years old. This was followed on 26 May 2006 by the FDA formally approving the use of the vaccine for that same age group. Further studies may demonstrate if there is benefit in patients 50–59 years old and if a booster dose is recommended.
Secondary prevention
- A meta-analysisMeta-analysisIn statistics, a meta-analysis combines the results of several studies that address a set of related research hypotheses. In its simplest form, this is normally by identification of a common measure of effect size, for which a weighted average might be the output of a meta-analyses. Here the...
reported that treating zoster at the time of rash with antiviral agents such as acyclovir can reduce the chance of postherpetic neuralgia. - A randomized controlled trialRandomized controlled trialA randomized controlled trial is a type of scientific experiment - a form of clinical trial - most commonly used in testing the safety and efficacy or effectiveness of healthcare services or health technologies A randomized controlled trial (RCT) is a type of scientific experiment - a form of...
found that amitryptyline 25 mg per night for 90 days starting within two days of onset of rash can reduce the incidence of postherpetic neuralgia from 35% to 16% (number needed to treatNumber needed to treatThe number needed to treat is an epidemiological measure used in assessing the effectiveness of a health-care intervention, typically a treatment with medication. The NNT is the average number of patients who need to be treated to prevent one additional bad outcome...
is 6).