Pruritus

Question 1

What is pruritus?

Answer 1

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

Question 2

Where on the the body do we experience pruritus?

Answer 2

Predominantly the skin but also conjunctiva and/or mucous membranes

Question 3

How does the sensation of itch occur?

Answer 3

Unmyelinated C fibres subserving the itch sensation originate in the skin > sensations are conveyed into the lateral spinothalamic tract > sensation is processed in parts of the forebrain and hypothalamus

Question 4

What are the chemical mediators of itch in the skin?

Answer 4

Histamine, PGE2, acetylcholine, serotonin, IL-2, substance P, tryptase, kallikrein, etc

Question 5

The same nerve fibres which transmit itch also transmit pain and temperature. T/F

Answer 5

False - different fibres transmit pain

Question 6

What are the CNS mediators of itch?

Answer 6

Endogenous and exogenous opiates

Question 7

List some stimuli which cause the degranulation of mast cells

Answer 7

Allergens, anti-IgE, substance P, stem cell factor, codeine, C5a (complement), etc

Question 8

Mast cell degranulation causes the release of chemicals which cause itch. T/F

Answer 8

True

Question 9

What are the causes of itch?

Answer 9

Pruritoceptive, neuropathic, neurogenic, psychogenic

Question 10

What is pruritoceptive itch?

Answer 10

Something within the skin causes itch (dryness, inflammation, etc)

Question 11

What is neuropathic itch?

Answer 11

Damage to central or peripheral nerves causing itch

Question 12

What is neurogenic itch?

Answer 12

Itch caused by stimulation of CNS nerves with no evident damage to said nerves (e.g opiates)

Question 13

What is psychogenic itch?

Answer 13

Psychological causes without (detectable) CNS damage

Question 14

What is dermographic urticaria? What type of itch does this cause?

Answer 14

Urticarial rash occurs where the skin is stroked/scratched due to mast cell degranulation. Pruritoceptive

Question 15

A patient with itch due to liver disease has which type of itch?

Answer 15

Neurogenic

Question 16

What is xerotic/asteatotic eczema? What type of itch does this cause and how?

Answer 16

Eczema caused by dry skin. Pruritoceptive. Unmyelinated C fibres are directly exposed (in the bases of skin microfissures) to inflammatory mediators released in the skin

Question 17

What type of itch is caused by insect bite reactions?

Answer 17

Pruritoceptive

Question 18

What is lichen planus? What type of itch does it cause?

Answer 18

A lichen-like disease of the skin/mucous membranes with an unknown aetiology. Pruritoceptive

Question 19

What other itchy condition is lichen planus associated with? What type of itch does this cause?

Answer 19

Hep C infection. Neurogenic

Question 20

Which systemic diseases are associated with itch? What type of itch is most commonly associated with these?

Answer 20

Haematological, paraneoplastic, liver & bile duct, kidney disease, thyroid disease. Neurogenic

Question 21

How can itch be managed?

Answer 21

Treat underlying cause, sedative anti-histamines, emollients (with menthol or cooled), antidepressants (doxipin & SSRIs), phototherapy, opiate antagonists, ondansetron (serotonin antagonist), anti-epileptics (specific neurogenic itch)