pruritus Flashcards
define pruritus
unpleasant sensation that elicits the desire or reflex to scratch, rub, lick, chew
how does pruritus commonly manifest in dogs
- scratch
- rub
- nibble
- lick
- temperament changes
how does pruritus often manifest in cats
- often secretive and not noticed by owner
- overgrooming (sometimes bilateral symmetrical alopecia not caused by endocriopathy)
- hair plucking (resulting in hair balls, tufts of hair around house and hair in feces)
what are the 3 classifications of pruritus
- pruriceptive pruritus
- neuropathic pruritus
- psychogenic (stress)
how does pruritus often manifest in horses
- rubbing
- stamping
- biting
- scratching
- bucking/kicking
- temperament changes
what is pruriceptive pruritus
due to stimulation of peripheral receptors in skin (in presence of a healthy nervous system) usually due to a skin disease
originates from the skin
what is neuropathic pruritus
generated in the CNS in response to:
- anatomical lesion of PNS or CNS
- circulating pruritogens
- pharmacological mediators
what receptors are responsible for sensation of itch
nociceptors
what nerve fibres conduct itch
- unmyelinated slow-conducting C fibres
- A-delta fibres
what other itch receptors are in the skin
- fine network of free nerve endings at dermo-epidermal junction and within epidermis contain itch-associated receptors
- epidermis with free C-neuron receptors act as main itch receptor in skin
what is the main itch receptor type
free C-neuron receptors within the epidermis
what chemical mediators are involved in cutaneous itch
a lot of different mediators
- various proteases/leukotrienes/neurotropins
- peptides
- prostaglandins
- cytokines
- histamine
where do chemical mediators involved in itch originate
- keratinocytes (leukotrine, cannabinoids, thymic stromal lymphopoietin)
- mast cells (histamine, LTB-4, IL-2, NGF)
- Skin leucocytes (T-cells produce IL-31 and eosinophils which produce NGF)
how do chemical mediators act to cause itch
- direct stimulation of intraepidermal nerve fibres
- stimulate mast cells
- stimulate other mediators/transduction cascades
why is IL-31 important
- produced by activated T-cells in allergic skin disease
- binds to receptors on surface neurons in skin and activates JAK enzymes to stimulate pruritic nerve impulse to brain
- important focus of therapy for allergic skin disease in dogs (apoquel and cytopoint)
describe the neural pathway for itch
- sensory afferent from skin (mainly via C-neurons which are slow conducting)
- dorsal nerve root to spinal cord
- dorsal horn and synapse with spinal interneurons (some inhibitory, some activatory)
- cross and ascend in lateral spinothalamic tract
- thalamus to internal capsule
- sensory cortex (left hemisphere dominance)
itch perception can be altered by emotiuonal factors and competing cutaneous sensations
sensory fibres from thehead carry itch sensations via which cranial nerves
- trigeminal (V)
- facial (VII)
- Glossopharyngeal (IX)
- Vagus (X)
discuss neuronal regulatory mechanisms
multiple factors on/in skin can also suppress/exacerbate pruritus
- physical factors (painful heat/cold supresses while moderate cold exacerbates)
- mechanical factors (rubbing/scratching causes pain) induces brief suppression of itch
- distraction
NB a truly pruritic animal (one that is severely itchy) cannot be distracted and will itch even while eating
how does scratching work to inhibit itchy sensations
scratch
1. fast conducting A-beta neurons stimulated
2. activation of inhibitory neuronal circuits
3. results in widespread surround inhibition of sensation
how does distraction work to prohibit sensations of itch
- increased activity in descending pathways from reticular formation
- activation of inhibitory circuits in dorsal horns of spinal cord
- closes gated mechanism
- diminishes afferent itch messages
discuss how sensitisation in chronic pruritus works
not well studied in animals but in man:
peripheral sensitisation: scratching leads to increased local inflammation, leading to production of pruritogens by inflammatory cells leading to increased C-fibre responsiveness
central sensitisation: chronic inflammation of skin leads to altered perception of gentle mechanical/other stimuli which is the perceived as pruritus (allokinesis)
list and discuss some therapeutic interventions for pruritus
- reduction of skin inflammation (steroids/ciclosporin)
- blocking of peripheral inflammatory mediators (cytopoint and antihistamines)
- moisturiser (dry skin is itchy)
- topical cooling preparations (calamine and menthol)
why are some treatments for pruritus preferred over others
pathogenesis of pruritus involves a variety of cells/mediators/neurological pathways which can vary between species and individuals
- a single drug is unlikely to uniformly be effective in all cases
- narrowly targeted treatments generally give less side effects than those with a broader action (steroids) but will be effective in a lower % of patients
- treatments that affect a target that is only present for a short time are less likely to be effective than those in which the target is long term
