Itch 101
Chronic pruritus is defined as itch lasting at least this long.
6 weeks
The diagnosis of active disease cannot be made if there is no history of pruritus in this condition.
Atopic dermatitis
Cholestatic pruritus is classically worse at night and on these two body sites, along with skin constricted by clothing.
Palms and soles
Focal, intense itch over the medial scapular border with a hyperpigmented patch from chronic rubbing.
Notalgia paresthetica
This chili-pepper alkaloid depletes substance P, transiently destroys intraepidermal nerve fibers, and needs 3–6 daily applications for maximal effect.
Capsaicin
Sparing of the hard-to-reach mid upper back in a patient covered in scratch-induced lesions is called this sign.
Butterfly Sign
In a first-time scabies infestation, pruritus typically begins this long after exposure.
3–6 weeks (within days on reinfestation)
Aquagenic pruritus can precede this myeloproliferative neoplasm by several years and eventually affects 30–50% of patients with it.
Polycythemia vera
Itch or burning of the dorsolateral forearms that flares with UV exposure, improves with ice, and is linked to cervical spine degenerative disease in over half of patients.
Brachioradial pruritus
This anion-exchange resin is first-line for most forms of cholestatic pruritus and must be separated from other medications by 4 hours.
Cholestyramine
Generalized pruritus turns out to be a manifestation of an underlying systemic disease in this percentage range of affected patients.
15–50%
This cytokine is a putative mediator of CTCL itch, and its serum levels fall alongside pruritus after treatment with HDAC inhibitors or mogamulizumab.
IL-31
Localized pruritus of the perianal or vulvar region that improves with supplementation points to this deficiency
Iron deficiency
Trigeminal trophic syndrome ulcerates the nasal ala but classically spares this structure, supplied by the external nasal branch of the anterior ethmoidal nerve.
The nasal tip
In intrahepatic cholestasis of pregnancy, this is first-line therapy and lowers both itch and serum bile acids.
Ursodeoxycholic acid (UDCA)
Name four labs in the basic initial evaluation for pruritus of unknown etiology.
Any four of: CBC with differential, creatinine/BUN, liver transaminases/alk phos/bilirubin, LDH, TSH, fasting glucose, ferritin/iron/TIBC
Two biologics, one blocking IL-4/IL-13 signaling, the other the IL-31 receptor, have RCT evidence for reducing pruritus and lesions in prurigo nodularis.
Dupilumab and nemolizumab
Patients with this leukemia are classically prone to exaggerated reactions to insect bites, in addition to generalized pruritus.
Chronic lymphocytic leukemia (CLL)
Notalgia paresthetica with onset in childhood or adolescence should raise suspicion for this hereditary syndrome.
Multiple endocrine neoplasia type 2A
Highly effective for recalcitrant prurigo nodularis and used for HIV-associated pruritus, this drug is limited by teratogenicity and irreversible peripheral neuropathy.
Thalidomide
In pruritus of unknown etiology, direct immunofluorescence of perilesional skin screens for one disease, while DIF of normal-appearing skin screens for another. Name both.
Bullous pemphigoid (perilesional) and dermatitis herpetiformis (normal-appearing)
This historic entity in the differential of lichen simplex chronicus presents with rows of papules and cord-like fibrotic bands that may be both pruritic and painful.
Lichen ruber moniliformis
In pediatric patients, unexplained itch is most often associated with this category of malignancy.
Hematologic malignancies
Loss-of-function mutations in this sodium channel gene cause congenital insensitivity to pain, while gain-of-function mutations cause erythromelalgia.
SCN9A
This NK1 receptor antagonist relieved intractable CTCL itch in case reports, but failed to beat placebo in a randomized trial in Sézary syndrome.
Aprepitant