A recent issue of JAMA the Journal of the American Medical Association had a review article of this condition.    Chronic pruritis is defined as an itch that lasts 6 weeks or longer.  It occurs in 22% of people during their lifetime and it accounts for 1% of doctor visits in the US.  Chronic pruritis can be classified as either inflammatory or neuropathic.   

The inflammatory causes are myriad.  Believe it or not a very common cause of inflammatory itch is dry skin.  Dry skin can occur due to overzealous use of soap and due to aging.  When the skin is dry it releases an inflammatory molecule (cytokine) called interleukin 33.  In eczema other interleukins (IL-4, IL-13 and IL-31) are the inflammatory cytokines.   These various interleukins activate another mediator of inflammation:  Janus Kinase (JAK).   

In people with hives, it is the tissue release of histamine that is the main cause of pruritis.  Contact dermatitis (poison ivy, nickel allergy) and insect bites are also inflammatory causes of itching.  Neuropathic causes for itch are also myriad.  Two very specific examples are due to nerve impingement:  notalgia paresthetica which is a chronic itch on the shoulder blade and brachioradial pruritis which is a chronic itch on one arm.  Chronic itch can also occur due to nerve injury from a previous case of shingles.  The patient will have itching only in the area where he/she had shingles.   

Some neuropathic itch conditions are due to amplification of the itch/scratch cycle.  Scalp pruritis and scrotal pruritis (watch out baseball players) are common examples.  The seminal event that led to initial scratching is often lost to memory but repetitive scratching strengthens the neural circuit to produce a greater sense of itch and therefore more scratching.  An especially problematic condition in this genre is prurigo nodularis.  In this condition the itch/scratch cycle is so severe that patients develop calloused nodules from the repetitive scratching.  The nodules themselves strongly stimulate the cutaneous nerves leading to a greater sensation of itch.   

Sometimes the nerve irritation is generalized.  In its most severe form it causes formication, a sensation that bugs (formic refers to ants) are crawling under the skin.  Diabetic neuropathy can do this.  Narcotic medications are another common cause.  And amphetamine-based stimulants including medications used to treat ADHD are known causes.   

Sometimes chronic itch is due to serious underlying health conditions including hepatitis, renal failure, lymphoma or other cancers, hyper-thyroidism and polycythemia (too many red blood cells).   

Obviously, the treatment needs to be directed at the cause.  Moisturizing the skin is always a good first step.  There are a variety of anti-inflammatory therapies starting with OTC hydrocortisone on up to prescription drugs that target the interleukins or Janus Kinase.  For neuro pathic issues therapies that break the itch/scratch cycle are important and include topical anesthetics, capsaicin, menthol, nerve blocks, acupuncture and neuroleptics.  Neuroleptics are medications that reduce aberrant nerve transmission (for example gabapentin and SSRI’s).