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
Psoriasis pathogenesis
Multi factorial
1. Genetic (certain HLA-alleles)
2. environmental:
— stress
— cold, trauma
— infections (Eg: strep, staph, HIV)
— alcohol & drugs (eg: iodides, steroid withdrawal, aspirin, lithium, beta-blockers, botulinum A, antimalarials)
3. Immunologic
Inflammatory pathway involved in Psoriasis
Type 1 inflammation - Th1 disease
1. DC
2. IL 23
3. Th17
4. IL-17A
5. Keratinocytes proliferate
(Back to 1. = inflammation neutrophilic)
Fungal skin infections
Dermatophytes
Non-Dermatophytes
- yeast-like fungus = Candida albicans
- molds = eg: malassezia furfur
KOH
Postassiez hydroxide scrape (skin scrape)
Dermatophyte
- septate hyphae
Candida
- round / oval cells
- pseudohyphae
- septate hyphae
Malassezia
- yeasts & hyphae = “spaghetti & meatballs”
Dermatophyte infection
- ringworm
- tinea
- only grow in keratin
(Stratum corneum, hair, nail)
Organism is most common depends on area to area - Microsporum = canis
- trichophyton = violaceum
- Epidermophyton = floccosum
Types of Dermatophytes based on mode of transmission
Anthrophilic = human to human = mild, inflammatory, chronic (mostly - urbanization)
Zoophilic = animal to human = intense inflammation (pustules & vesicles possible), acute
Geophilic = soil to human/ animal = moderate inflammation
Papular urticaria
- very common = children
- hypersensitivity response to antigens in arthropods’ saliva
- immunological tolerance develops = less common in adults
— arthropods - fleas
(Dogs, cats, birds = can be aggressive) - bed bugs
- mosquitoes
- mites
Often recurs when exposed to new insects at new venues
Insect bites
- vectors for major infections eg: Black Death/ tick bite fever/ malaria
Lice - pediculosis: head lice (pediculus capitis)
- clothing lice (pediculus humanus)
- crab lice (pthirus pubis)
Fleas
Bed bugs (cimex lectularius)
Animal mites
Ticks
Paraneoplastic pruritus
- precede, follow or occur concurrently
- improves when malignancy cleared
- often recurs during remission
- skin appear normal, except for secondary changes (eg: excoriations)
- mechanism not fully understood
Any malig. But commonest - haemtological malignancies (leukemia & lymphoma)
- cholangiocardinoma
Advanced age
Dry skin
- inherited factors
- increasing age = decreased production of ceramides in stratum corneum
- mostly legs, widespread
- cool weather, windy/ low humidity
- air conditioning, central heating
- excessive bathing, showering/ swimming, strong chlorination
- soap, detergents & solvents
