Pruritus

Question 1

Definition of pruritus?

Answer 1

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

Question 2

3 ways the itch is mediated?

Answer 2

Chemical mediators in skin - histamine, PGE2, acetylcholine, serotonin, IL-2, substance P, tryptase

Nerve transmission - unmyelinated C fibres (different ones transmit itch and temp from those that transmit pain)

CNS mediators - opiates (endogenous and exogenous)

Question 3

Mediators released by mast cell degranulation?

Answer 3

Proteases
Heparin
Histamine
Cytokines
Prostaglandin D2
Leukotrienes C/D/E4
Platelet-activating factor

Question 4

4 main categories of itching causes?

Answer 4

Pruritoceptive - something in skin, usually assoc. with inflammation or dryness, that triggers itch

Neuropathic - damage of any sort to CNS/PNS causing itch

Neurogenic - no evident damage in CNS but itch is caused by, e.g: opiate effects on CNS receptors

Psychogenic - psychological causes with no (currently detectable) CNS damage, e.g: itch in delusions of infestation

Question 5

Describe Primary Biliary Cirrhosis in terms of a rash

Answer 5

Pruritoceptive and neurogenic

Patient with generalised itch due to central effect of endogenous opioids secondary to small bile duct obstruction (autoimmune disease - PBC) and who is dermographic (most common type of urticaria, in which the skin becomes raised and inflamed when stroked, scratched, rubbed, and sometimes even slapped)

Question 6

Describe asteatotic eczema (due to very dry skin) in terms of a rash

Answer 6

A pruritoceptive rash due to direct exposure of itch-associated unmyelinated C fibres in bases of microfissures and inflammation with release of mediators in skin; almost always present on anterior shins

Question 7

Describe lichen planus in terms of a rash

Answer 7

Sometimes assoc. with chronic Hep C, which can, by itself, cause a neurogenic itch

Question 8

What is prurigo nodularis?

Answer 8

Skin condition characterised by very itchy firm lumps that can occur secondary to chronic itch

Question 9

Types of systemic diseases assoc. with itch?

Answer 9

Haematological
Paraneoplastic
Liver and bile duct disease
Psychogenic
Kidney disease
Thyroid disease

Question 10

Management of itch?

Answer 10

Determine cause if possible

Treat the cause (tends to be easier for pruritoceptive itch due to inflammatory skin diseases

Anti-itch treatments

Question 11

Types of anti-itch treatments?

Answer 11

Sedative anti-histamines (non-sedative antihistamines useless for most itch except where excess histamine in the skin is part of the mechanism)

Emollients (with menthol, or cooled in fridge – counter-irritant effects)

Antidepressants, e.g. doxepin and some of the newer ones including SSRIs

For some types of neuropathic itch, anti-epileptics

Phototherapy

Opiate antagonists, ondansetron (a serotonin antagonist), etc