Pruritus

Question 1

Define pruritus

Answer 1

‘Unpleasant sensation that elicits the desire or reflex to scratch (rub, lick, chew)’

Question 2

What are the two ways pruritus can be caused?

Answer 2

Pruriceptive pruritus – due to stimulation of peripheral receptors in skin (in presence of healthy nervous system). Usually due to skin disease

Neuropathic pruritus – generated in CNS in response to circulating pruritogens (eg cholestasis), pharmacological mediators or a CNS lesion.

Question 3

What do nociceptors detect?

Answer 3

Itch and pain

Question 4

What nerve fibres are associated with nociceptors?

Answer 4

Mainly unmyelinated slow-conducting C-fibres with some A-delta fibres

Question 5

Describe the neural pathway for itching

Answer 5

Sensory afferent fibres from the skin (C-neurones) send impulse to the dorsal nerve root where the C- fibre synapses in the dorsal horn.
Impulse crosses and ascends in the lateral spinothalamic tract.
Impulse reaches the thalamus and is directed to the sensory cortex.

Question 6

What is the neural pathway when the animal itches?

Answer 6

Scratch sensation travels via fast conducting A-beta neurons and this activates the inhibitory neuronal circuits

Question 7

What is the neural pathway when the animal is distracted?

Answer 7

Distraction causes increased activity in descending pathways from reticular formation, this causes activation of inhibitory pathways in the dorsal horn of the spinal cord. This diminishes the effect of the itch messages.

Question 8

What pro-inflammatory mediator has a direct pruritogenic effect?

Answer 8

IL-31

Question 9

What is a body-made pruritogen?

Answer 9

Histamine

Question 10

How does IL-31 cause pruritus?

Answer 10

Binds to receptors on surface of neurons in skin which leads to activation of Janus kinase (JAK) enzymes which stimulate pruritic nerve impulse to brain

Question 11

What therapy can be used against IL-31 (allergic skin disease) pruritus?

Answer 11

Drugs to block JAK1 pathway

New monoclonal antibody therapy against IL-31

Question 12

Do pruritic agents add up?

Answer 12

Effects summate and may take pruritus over the pruritic threshold even if the obvious agent is very small.

Question 13

What is hyperkineisis?

Answer 13

During chronic pruritus the animal can become hyposensitised where even a touch can set the animal off itching = hyperkineisis (marked itching beyond level of stimulation)

Question 14

How can we help treat pruritus?

Answer 14

Reduction of skin inflammation
Blockage of peripheral inflammatory mediators
Moisturisers – dry skin itches
Topical cooling preparations (act via transient afferent receptor nociceptor channels)