Pruritus

Question 1

What is the common name for pruritus?

Answer 1

Itch

Question 2

What is an itch?

Answer 2

An itch is a usually unpleasant, poorly localised, non-adapting sensation that provokes the desire to scratch

Question 3

What percentage of the nerves in the human spinothalamic tract are associated with itching?

Answer 3

5% (Compared to 30% in dogs)

Question 4

What are the types of mediators involved in itch?

Answer 4

• Chemical mediators (Histamine, PGE2, Acetylcholine, Seretonin, IL-2)
• Nerve transmission (Unmyelinated C fibres - Different from those that transmit pain)
• Central nervous system mediators (Opiates)

Question 5

Where in the brain is itch processed?

Answer 5

Parts of the forebrain and hypothalamus

Question 6

What are some pre-formed mediators that are released by mast cells that can cause itching in allergy?

Answer 6

• Proteases (E.g. tryptase)
• Heparin
• Histamine
• Cytokines

Question 7

What are some newly-formed mediators of itch released by mast cells in response to allergy?

Answer 7

• Prostaglandin D2
• Leukotrienes
• Platelet activating factor
• Cytokines

Question 8

What are the 4 triggers of itching?

Answer 8

• Pruritoceptive
• Neuropathic
• Neurogenic
• Psychogenic

Question 9

What is meant by a pruritoceptive itch trigger?

Answer 9

Something (E.g. Inflammation, dryness) in the skin that triggers itch

Question 10

What is meant by a neuropathic itch trigger?

Answer 10

Damage to any sort of nerve causing itch

Question 11

What is meant by a psychogenic itch trigger?

Answer 11

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

Question 12

What is meant by a neurogenic itch trigger?

Answer 12

No evident CNS damage, but itch caused by effect on the CNS (E.g. opiate effect on CNS receptors)

Question 13

What is meant by dermatographic pruritus?

Answer 13

Scratching or stroking skin causes excessive mast cell degranulation and release of histamine and other in-skin mediators

Question 14

What are some systemic diseases associated with itching?

Answer 14

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

Question 15

What are some examples of anti-pruritic treatments?

Answer 15

• Sedative antihistamines
• Emollients
• Anti-depressants
• Anti-epileptics in some types of neuropathic itch
• Phototherapy
• Opiate antagonists, ondansetron (Serotonin antagonist)