Itch

Question 1

What is an itch?

Answer 1

A usually unpleasant, poorly localised, non-adapting sensation that provokes the desire to scratch

Question 2

What chemical mediators of itch are present in the skin?

Answer 2

Chemical mediators in skin: histamine, PGE2, acetylcholine, serotonin, kallikrein, interleukin 2, substance P tryptase, etc

Question 3

How is the itch sensation transmitted through nerves?

Answer 3

Unmyelinated C fibres (different ones transmit itch and temperature from those that transmit pain). Then processed in parts of forebrain and hypothalamus

Question 4

What pre-formed and newly synthesised mediators are released in dermal mast cell degranulation?

Answer 4

Protease (tryptase), heparin, histamine, cytokines, prostaglandin D2, Leukotrienes C4,D4,E4, Platelet-activating factor

Question 5

What is the definition of pruritoceptive in relation to the causes of itch?

Answer 5

Something (usually associated with inflammation or dryness) in skin that triggers itch

Question 6

What is the definition of neuropathic in relation to the causes of itch?

Answer 6

Damage of any sort to central or peripheral nerves causing itch

Question 7

What is the definition of neurogenic in relation to the causes of itch?

Answer 7

No evident damage in CNS, but itch caused by, e.g., opiate effects on CNS receptors

Question 8

What is the definition of psychogenic in relation to the causes of itch?

Answer 8

Psychological causes with no (currently detectable) CNS damage, e.g. itch in delusions of infestation

Question 9

What kind of itch may be present in a patient with generalised itch due to central effect of endogenous opiods secondary to small bile duct obstruction (autoimmune disease – primary biliary cirrhosis)?

Answer 9

Pruritoceptive and neurogenic

Question 10

What causes the pruritoceptive itch in asteatotic eczema?

Answer 10

Direct exposure of itch-associated unmyelinated C fibres in bases of microfissures and inflammation with release of mediators in skin

Question 11

How to you manage itch?

Answer 11

Determine cause if possible, and treat. Anti-itch treatments : Sedative anti-histamines (non sedative useless for most itch except when excess histamine is part of mechanism), emollients, antidepressants e.g. doxepin, SSRIs), anti-epileptics (some neuropathic itches), phototherapy, opiate antagonists, ondansetron (serotonin antagonist)