Postherpetic itch (PHI), like postherpetic neuralgia (PHN), occurs in shingles patients who have persistent nerve damage after the rash subsides. The neuropathic itch condition is caused when the herpes zoster (HZ) virus kills or damages the neurons that mediate normal itch and pain sensations. PHI gets less attention than PHN, but it is just as disabling, and is a challenge to treat.
Many cases of shingles are preceded or accompanied by itch or pain. In older or more severely affected patients with the most severe nerve damage, mild or severe pain and itching can last for months or years.
PHI is most common after shingles affecting the head or neck. PHI and PHN, which can occur together, are more prevalent and longer-lasting in the elderly because they are less able to regenerate the neurons damaged by shingles.
PHI is believed to be the result of isolated remaining nerve fibers firing when they shouldn't, giving false sensations that fool the central nervous system. Patients often report the sensation of insects on their skin when there aren't any.
Neurologist Anne Louise Oaklander, MD, director of the Nerve Unit at Massachusetts General Hospital and Harvard Medical School, notes that while itch and pain are distinct sensations, they are closely related and are the two major nocifensive sensations. “The job of itch and pain is to protect us from harmful stimuli,” she says.
When itching develops in normal skin, a person will only scratch until they trigger pain that causes them to stop. With PHI, the skin sometimes has diminished sensation due to the shingles-induced nerve damage. If the scratching isn't painful, patients can continue scratching for too long, sometimes resulting self-injury.
Oaklander says that much of the neuropathic itching occurs while the patient is asleep and unable to control themselves. In 2002, she published an extreme example of intractable postherpetic itch in a 39-year-old woman with nerve damage and skin desensitization after having shingles above one eye.
The patient’s painless scratching of her continually itchy forehead opened up a wound through her skull and into her brain despite attempts to stop her. Oaklander finally calmed the itching and scratching by administering bupivacaine through a catheter directly into her forehead several times a day. This treatment allowed the wound to close.
PHI and PHN often respond to the same medications. “The notable exception is opioids,” explains Oaklander. “People who have both postherpetic itch and postherpetic pain may want to avoid opioids because opioids can make itch worse.”
Treatment of PHI is focused on dampening neuronal firing. Typically this involves the use of injectable or topical local anesthetics, including lidocaine transdermal patches and capsaicin, that numb the afflicted skin. Sodium channel blockers such as tricyclics are also prescribed. Antihistamines and topical steroids that are often used to alleviate symptomatic itching are generally ineffective for neuropathic itch.