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Chronic Pruritis 

Chronic Pruritis 

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).   

Chronic Pruritis

Chronic Pruritis

A recent issue of the New England Journal of Medicine contained an excellent review article on the condition known as chronic pruritis – defined as an itch (either localized or generalized) that lasts longer than six weeks. The problem is more common in women than in men and more common in Asians than other races.
The authors of the review article point out that chronic itch can be as debilitating as chronic pain,
leading to changes in sleep patterns and mood disturbances including anxiety and depression.
The sense of itch is carried on unmyelinated C fibers which overlap with the fibers carrying pain.  Over-stimulation of the brain areas receiving these nerves can lead the brain to overreact to noxious stimuli that would otherwise normally inhibit itch, such as scratching or heat/cold exposure. This brain overreaction can also lead to curious symptoms, such as having intense itching associated with undressing.
There are four primary sources of chronic pruritis:
1. Inflammatory skin conditions
2. Systemic diseases
3. Neuropathic disorders. Believe it or not, these can’t be distinguished by the presence or absence of rash because most individuals with chronic pruritis end up with a rash due to trauma to the skin from scratching.
4. Psychiatric conditions.
The most common skin conditions leading to chronic itch are eczema, psoriasis, dry skin, scabies, contact dermatitis, Lichen planus and insect bites. These are best diagnosed by the patient’s history, the appearance of the skin and perhaps, with a skin biopsy.
The most common systemic diseases to cause itch are chronic kidney disease, liver disease, Hodgkins disease, polycythemia, HIV infection and hyperthyroidism. These conditions are best identified by the patient’s history, examination and lab evaluation.
The most common neuropathic causes are brachioradial pruritis (previously discussed in this newsletter), notalgia parasthetica and post-herpetic itch. In all these cases there is a peripheral nerve that is injured or irritated, leading to the itch signal being sent to the brain.
The most common psychogenic causes are OCD (obsessive compulsive disorder), delusions of parasitosis and substance abuse.
Obviously, the treatment is going to vary greatly depending on the cause of the itch. However, even with an accurate diagnosis the itch may be resistant to treatment. In most cases, keeping the skin moist and hydrated is helpful.
Since the C-Fibers are histamine-activated nerves, the use of sedating antihistamines is also helpful. Because of the brain over-stimulation scenario, anticonvulsants and antidepressants have also proven useful.
Finally, because of the overlap of itch and pain fibers, mu-opiod antagonists have often been of help.