Pruritus

Question 1

Define pruritus

Answer 1

A usually unpleasant, poorly localised, non-adapting sensation that provokes the desire to scratch (i.e. itch)

Tends to refer to a generalised itch rather than localised

Question 2

Describe the mechanism of pruritus

Answer 2

• Unmyelinated C fibres originating in the skin subserve (further/promote) Itch sensation and pain. Info on both itch and pain is then conveyed in two separate systems that both use the lateral spinothalamic tract.
• Itch sensation is then processed in parts of forebrain and hypothalamus

Question 3

What are the different mediators of itch?

i.e. what they travel along, their chemical and CNS mediators

Answer 3

• Chemical mediators causing itch - e.g. Histamine, PGE₂, acetylcholine, serotonin, kallikrein, IL-2, substance P tryptase, etc
• Nerve transmission: unmyelinated C fibres
• CNS mediators: opiates (endogenous and exogenous)

Question 4

Appreciate some of the stimuli of mast cell degranulation

Answer 4

Question 5

Appreciate some of the mediators released by mast cell degranulation

Answer 5

Question 6

Define what is meant by a pruritoceptive cause of pruritus (itch)

Answer 6

Pruritoceptive - something (usually associated with inflammation or dryness) in skin that triggers itch

Question 7

Define what is meant by a neuropathic cause of pruritus (itch)

Answer 7

Neuropathic - damage of any sort to central or peripheral nerves causing itch

Question 8

Define what is meant by a neurogenic cause of pruritus (itch)

Answer 8

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

Question 9

Define what is meant by a psychogenic cause of pruritus (itch)

Answer 9

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

Question 10

What type of nerve fibres does itch sensation travel across ?

Answer 10

Unmyelinated C fibres

Question 11

What tract is info on itch sensation conveyed in ?

Answer 11

Spinthalamic tract

Question 12

Give some examples of the systemic diseases associated with itch and what type of pruritus are they generally defined as ?

Answer 12

Most but not all neurogenic pruritus

Some of the systemic diseases associated with pruritus:

• Haematological
• Paraneoplastic
• Liver and bile duct e.g. primary billary cirrhosis
• Psychogenic
• Kidney disease
• Thyroid disease

Question 13

What are the main points of the management of pruritus

Answer 13

Determine and treat the cause if possible

Anti-itch treatments:

• Sedative anti-histamines (non-sedative only useful if excess histamine in the skin i.e. urticaria & insect bites)
• Antidepressants, e.g. doxepin
• Opiate antagonists, e.g. ondansetron (a serotonin antagonist)
• Phototherapy
• Anti-eplieptics