Pruritus Flashcards
pruritus
sensation that elicits a desire to scratch
clinical signs of pruritus
- excessive scratching, rubbing, self-chewing, or overgrooming
- alopecia
- erythema
- excoriations
- lichenification
- hyperpigmentation
sequelae to pruritus
secondary infections
atopic dermatitis
inflammation of the skin leading to pruritus, erythema, lichenification, discoloration, hyperpigmentation
what is the most common sign of food allergies
face and head pruritus
pruritus vs inflammation
NOT the same thing
pruritus can be a sequelae or an inducer of inflammation
often seen concurrently
what is the main cell involved in the pruritic pathway
mast cells
- contain more receptors and mediators than any other cell
- required for wound healing and establishing skin barrier
what is unique about mast cell’s after degranulation
able to resynthesize mediators after degranulation to continue to function
histergenic itch
- antigenic stimulation
- antigen binds IgE on mast cell
- induces histamine release
- histamine interacts with sensory neurons
- induces itch sensation
non-histergenic itch
- antigenic stimulation
- antigen binds Mrgprb2 on mast cell
- induces tryptase release
- tryptase interacts with sensory neuron
- induces itch sensation
what is the most abundant mediator stored in mast cell granules
tryptase
dermatomes
areas of skin that send information to the brain via a spinal nerve
contain large number of sensory nerves that end in FREE NERVE ENDINGS
free nerve endings
transmit touch, temperature, pain, and pruritus
how do dermatomes transmit pruritus
- keratinocytes release inflammatory mediators
- binds to receptors on the sensory neurons
- carries the itch sensation from free nerve endings to the brain via non-myelinated, slow conducting C fibers
what senses do C fibers transmit
pain, pruritus, heat
what spinal tract carries pain and itch (histamine and non-histamine) sensations to the brain
contralateral spinothalamic tract
effect of pain on pruritus
pain inhibits pruritus
animals will scratch to the point of hurting themselves to get the itch to stop
scrapie
prion disease in sheep and goats
causes damage to the inhibitory gating mechanism for itch in the midbrain
what are the three theories of how pain and pruritus are transmitted differently
- specificity
- pattern
- central processing
specificity
different subsets of nociceptors transmit pain vs pruritus
(they each have their own peripheral and central neurons)
pattern
spatio-temporal pattern (location of the neural activity on the skin) permits distinction
central processing
the brain differentiates between pain and pruritus
why is it difficult to differentiate between pain and pruritus in animals
animals will often lick at painful sites as well as itchy sites
what are the mediators of pruritus
- histamine
- proteases
- interleukins
- substance P
- leukotrienes
- opioid peptides
histamine receptors
H1 and H4: activating; elicit itch
H3: inhibiting; blocks itch
effect of benadryl
H1 receptor blocker
not proven effective for canine pruritus
proteases & protease receptors
endogenous: tryptase
exogenous: mites, bacteria, fungi
- mama animals are allergic to proteolytic enzymes in insect saliva
receptors: PAR-2
interleukin pruritus mediators
IL-31: bind keratinocytes
IL-4 and 13: atopic dermatitis; stimulate IgE and IgE from B cells
IL-5: insect allergies; stimulate eosinophils
cytopoint vs apoquel
cytopoint: inhibits IL-31
apoquel: inhibits JAK-1 signaling (further down IL-31 pathway)
substance P
involved in CNS and sensory neurons
what medications affect substance P function
- capsaicin
- maropitant (cats)
leukotrienes
LTB4; associated with essential fatty acid metabolism
opioid peptides
Mu receptors
**do NOT give a pruritic animal opioids
what environmental factors worsen pruritus
heat and dryness
effect of gabapentin & pregabalin on pruritus
GABA analogs - used as a last resort for neurologic pruritus
effect of corticosteroids on pruritus
decrease neutrophil migration, circulation of eosinophils, basophils, monocytes, macrophages, and production of pro-inflammatory cytokines
affect many steps of the pruritic pathway
effect of cyclosporine on pruritus
inhibits T cell activation
threshold phenomenon
what happens IN the animal
a certain pruritic load may be tolerated without provoking clinical signs, but increasing that load may push the individual over their threshold and initiate clinical signs
summation of effect
what happens TO the animal
additive pruritic stimuli from different chemical mediators or coexistent skin diseases may raise an animal above its individual pruritic threshold
