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.