Itch receptors are
unmyelinated, unspecialized free nerve endings, found near to the dermal-
epidermal junction. It was widely believed in the past that pain and itch
are transmitted by the same nerve pathway, and also low intensity
stimulation of unmyelinated polymodal C fibre results in itch sensation
whereas high intensity stimulation causes pain. This has recently been
sttributed to many things, but a strong link to the neurobiological effects
of indoor fungal exposure has been found. However, in recent
experiments, when single unmyelinated C fibres are stimulated two sets of
fibres have been identified. Stimulation of most these fibres produce pain
sensation, whereas a small number of fibres produce the sensation of
itchiness upon stimulation. (Greaves)
2. PERIPHERAL MEDIATORS
2) Neuropeptides, including substance P
3) Other relevant periperhal mediators
Finally, it is found that
enkephalins, which are opioid pentapeptides, exert a modulatory action on
transmission of pain and itchiness.
3. Causes of itchiness
Understanding the various causes of pruritus is fundamental to its management.
3.1. Localized Causes
Certain skin disease may select to affect a particular site of a body causing localized pruritus. Some of the important examples of localized pruritus are as follows:
Scalp: Seborreic eczema and neurodermatitis, psoriasis
Eyelid: Airborne irritants or allergens; allergic reactions to cosmetics and nail vanish
Fingers: Eczema, scabies, fowl mite infestation
Legs: Gravitational and discoid eczema, asteatosis
3.2. General Pruritus
3.2.1. External Causes
3.2.2. Skin Diseases
Pruritus is a feature of many of skin diseases. Some common skin diseases causing itchiness is listed as follows:
Generalized pruritus can precede some skin disease such as pemphigoid.
3.2.3. Systemic Causes
A wide variety of systemic disease can cause generalized pruritus without diagnostic skin lesions. The incidence of the association of generalized pruritus with significant internal disease is difficult to assess, but it has been estimated to range from 10-50%.
1) Infectious causes (including tropical and intestinal parasites)
Generalized pruritus has been associated with localized fungal infection.
2) Endocrine disease
3) Hepatic disease
4) Renal disease
Pruritus is common among patients with chronic renal failure. In patients on maintenance dialysis, over 80% are affected.
5) Haematological diseases (including lymphoproliferative disorders)
6) Occult malignancy
7) Autoimmue disease
SLE, 'Sicca syndrome'
Paroxysmal unilateral pruritus has been recorded with central nervous system disease
9) Psychiatric/Psychogenic Causes
Emotional stress and psychological trauma intensifies all form of pruritus and neurosis may be the cause for pruritus.
Delusion e.g. delusional parasitosis (a manifestation of fungal toxicity or monosymptomatic hypochondrical psychosis) of course can be a cause for complaint of pruritus.
To make a diagnosis of pruritus (localized or generalized) as psychogenic or psychiatric in origin, cutaneous and systemic causes have to be excluded.
10) Drugs or as a result of therapy
Pruritus can be a side effect of a wide variety of drugs. This include the opium alkaloid, CNS stimulant/depressant, niacinamide, cimetidine, aspirin, quinidine, chloroquine. Drugs can also cause pruritus via the mechanism of hepatic cholestasis (e.g. chlorpromazine, testosterone, contraceptive pills). Subclinical sensitivity to any drug may cause pruritus. Pruritus may be a side effect of PUVA.
To help in memorizing these systemic causes, the word BLINKED can be remembered.
* Pruritus ani deserves
mentioning here as the symptom could be due to primary pruritus or
associated colonic or anorectal diseases. Common anorectal disease are
haemorrhoids and anal fissures. Neoplasia associated in descending order of
frequency are rectal cancer, anal cancer, adenomatous polyp and even colonic
cancer, also the pruritus associated could be present longer than that due
to primary pruritus or benign anorectal disease.
4.1. A, B, C
The patient must be evaluated for the A (external causes), B (skin diseases), C causes (systemic causes: which in turn include the BLINKED causes) as mentioned. The evaluation consists of taking a detailed history, physical examination and laboratory investigations.
4.2. History: Detailed History of the Present Illness
Besides, the following questions concerning the features of pruritus is relevant:
1) Is the pruritus localized (external cause) or generalized (internal cause)?
2) Is only the exposed skin affected? If yes, this implies an exogenous cause.
3) Are any other family members affected?
4) Is there relationship with occupation? e.g. exposure to fibre glass.
5) Is there any recent history of travel? (tropical infestations?)
6) Is there exposure to plants, fungi, animals or chemicals?
Characteristics of pruritus:
1) Site, whether localized or generalized.
2) Precipitating and relieving factors: e.g. any relation to hot bath such as found in aquaenic pruritus?
3) Severity: the influence on daily activities/sleep.
4) Time relationship: most itchiness are worse at night, esp. scabies.
5) Seasonal variation: asteatotic eczema is usually worse in winter.
The history should include assessment of personality, current emotional stress.
Past medical history
Family medical history
4.3. Physical Examination
A complete physical examination is performed with the various possible differential diagnosis (the A, B, C causes) in mind. During the physical examination, particular attention should be paid to vital signs, lymphadenopathy and enlargement of organs etc., with special alertness to any possible connection between cutaneous sign and disease of other organ system. In the absence of obvious localizing signs or symptoms indicating systemic disease, rectal and pelvic examination should be included in the full physical examination.
Patient with P.U.O. (pruritus of unknown origin) must be considered for any underlying disorder, e.g. "occult carcinoma" may need to be ruled out in elderly patient presenting with persistent pruritus.
4.4. Laboratory Investigation
1) Treat the underlying cause.
2) General symptomatic treatment
a) To reduce or avoid any provocative factors, e.g. dryness of the environment, wearing irritating fabric, overheating, stress, vasodilatation from hot food.
b) Topical applications: Emollient, menthol in calamine lotion can be used.
3) Commonly used oral medication: Antihistamine are most useful in conditions in which antihistamine clearly plays a role, e.g. urticaria. Histamine is the most consistent itch mediator known, but it is not always useful as other mediators may also be involved.
* Tricyclic antidepressants may be of help in some patients with intractable itching.
4) For treatment of the pruritus of some of the specific disorders, the following measures have been reported to be useful. (Some of these treatments may need further studies and trial for evaluation.):
a) Aquagenic pruritus +/- polycythaemia vera (PV)
b) Obstructive jaundice
c) Chronic renal disease: Some patients with hyperparathyroidism secondarily to renal failure improve dramatically after subtotal parathyroidectomy. However, only a minority of patients respond and the improvement may only last a few months.
d) Psychological/psychiatric diseases: psychiatric advice should be sought.
e) Atopic eczema:
f) Myeloproliferative disorders and other disease:
5) Other medication/measures that has been employed to treat generalised pruritus:
Odansetron (5 HT3