The Itchy Skin Flashcards
What is itch?
- sensation (usually unpleasant) = desire to scratch
- NB protective mechanism = animal detect harmful substances
- scratching behavior, pleasant feeling = itch-scratch cycle
- cycle= serious skin damage & propagates itch
Itch-scratch cycle
- Scratch (automatic usually)
- Epidermal barrier/skin (damage)
- Production of Neuropeptides, tryptase
- Stimulate C nerve fibers (more itching, scratching etc)
- Itch
Pruritus
- most common complaint of patients with dermatologic disease
- arises from primary skin disorder
- manifestation of underlying systemic disease = 10% - 25% affected individuals
- occurs in absence of visible skin signs (have to be careful)
Classification of an itch
— mechanical - physical stimulus (wind blowing over skin/ tickle)
— chemical itch
- histamine-dependent
- histamine-independent
— ‘contagious itch’ (yawn)
- watching others scratching themselves/ talking about itchiness = desire to scratch, despite no chemical/ mechanical pruritic input
Itch pathway
Complex
- itch signal transmitted mainly through small, itch-selective C-fibers, histamine & non-histamine triggered neurons
- connects secondary neurons in dorsal root ganglion
- cross opposite side of spinothalamic tract
- ascend to parts of brain involved in sensation, emotion, reward & memory
Primary mediators of pruritus
- Histamine
- Tryptase
- Cathepsin S
- IL-31
Secondary mediators of pruritus
- Prostaglandins E1,2
- Substance P
- Opioid receptor agonists
- Nerve growth factor
- IL-2
Causes of itch = with skin rash (primary skin changes)
Skin disease with primary skin changes
- atopic eczema
- psoriasis
- fungal infection
- popular urticaria
- insect bites
- scabies
Causes of itch = no skin rash
- systemic disease
- underlying malignancy (paraneoplastic)
- medication
- advanced aging
- neurogenic/ neuropathic itch (stroke/ trigeminal neuralgia)
Causes of itch = with skin rash (secondary skin changes)
Secondary skin changes = scratching (itch-scratch cycle = local small fiber neuropathy)
- lichen simplex chronicus
(Habitual scratching eg: sitting in front of tv)
Atopic eczema/dermatitis
- chronically relapsing
- inflammatory
- non-contagious
- extremely pruritic skin disease
- families with atopic diseases (group of conditions inherited tog. = asthma, allergic rhinitis, conjunctivitis, food allergies)
Pathogenesis of AD
- complex genetic disease = gene-gene & gene-environment interactions
- genetic predisposition to very dry skin & overactive immune system
Epigenetics + genetics - polygenetic incl: FLG-mutation (well known but others)
Non-genetics - allergic incl: HDM (house dust mites) & food allergens
- non-allergic incl: Staph superAg + irritants (soap)
- dysbiosis
= barrier dysfunction + Th2-immunological response
(Type 2 inflammation contributes to skin barrier dysfunction in atopic dermatitis) - sub clinically, cytokine involvement = whole body affected
Skin barrier
- brick & motor model = stratum corneum (outermost layer) - prevents from evaporating
- brick = corneocyte
- motor = lipid lamellae
- steel rods = desmosomes keeping cells tog.
- lipid gets washed out = skin desquamantes earlier, desmosomes break down, infections get access to immune system
- increase in trans epidermal water loss
TH2 pathway of AD
- Dendricyte (APC)
- Stimulate IL-4 production
- Stimulate TH2 cell to produce IL4 & IL13 (NB cytokines)
- Triggers type 2 inflammation
(Back to 1.)
Psoriasis
- complex, chronic, multi factorial, immune-mediated, inflammatory disease = clearly defined, red & scaly plaques
- hyperproliferation of keratinocytes in epidermis, increase in epidermal cell turnover rate
- normal skin in between plaques, not like AD where there is whole body involvement
