Itch

Question 1

what is it?

Answer 1

pruritus - an unpleasant poorly localised, non-adapting sensation which provokes the desire to scratch

Question 2

how does it present?

Answer 2

predominantly a skin symptom

can also occur in the conjunctiva or mucous membranes

Question 3

how is itch processed?

Answer 3

either localised or generalised
pruritus tends to be used to refer to generalised itch without cause, primarily within the skin
the sensation of itch is processed in parts of the forebrain and hypothalamus

Question 4

how is an itch mediated?

Answer 4

chemical mediators in the skin e.g. histamine, serotonin etc.
nerve transmission - unmyelinated C fibres, the ones that transmit itch are different from the ones that transmit pain
CNS mediators - exogenous and indigenous (internal and external causes) e.g. opiates

Question 5

what causes it?

Answer 5

1. Pruritoceptive: something in skin that triggers rash, usually associated with inflammation/dryness
2. Neuropathic: damage to central/peripheral nerves causing itch (no evident damage in CNS)
3. Neurogenic: itch caused by e.g. opiate, as it affects CNS receptors
4. Psychogenic: psychological causes (no CNS damage) e.g. itch in delusions of infestation
   you can have combos

Question 6

what systemic diseases are associated with mostly neurogenic itch?

Answer 6

haematological
paraneoplastic (syndrome which is a consequence of cancer in the body)
liver and bile duct 
psychogenic 
kidney disease 
thyroid disease 
*often multifactorial

Question 7

How is itch managed?

Answer 7

find cause - pruritoceptive usually easiest because it’s due to inflammatory skin diseases
anti-itch treatments

Question 8

Anti-itch treatments

Answer 8

• Sedative anti-histamines [non-sedative only work if excess histamine is part of the mechanism]
• Emollients (with menthol/cooled in fridge [counter-irritant])
• Antidepressants e.g. doxepin, SSRIs
• Phototherapy
• Opiate antagonist, ondansetron (serotonin antagonist) etc.
• Anti-epileptics can be used for some types of itch