Don’t say “Scabies” (horribly overdiagnosed) without reading this first. And NEVER diagnose allergy to common household soaps & detergents [a seemingly reflex recommendation to avoid or change such products when the clinician doesn’t know what else to do).
We call this topic “diffuse” instead of “generalized” itching, because we mean itching that occurs in various parts of the body, though not necessarily all over. We won’t be discussing localized itching, such as contact dermatitis from a necklace, or Tinea Pedis.
First step is to determine if there’s a rash / primary skin lesions, or whether there’s simply itching without skin involvement (except excoriations from scratching, which can appear prominent or become infected, thus confusing the picture).
The differential diagnosis of dermatological (vs. systemic) disorders below is clinical. We’ll go through them condition by condition, then offer a summary approach at the end.
Diffuse Pruritis — Differential Diagnosis
Skin Lesions Noted
No Rash or Lesions
URTICARIA — a.k.a. Hives. An IgE mediated systemic reaction, to something ingested or injected (rarely touched or inhaled). Diagnosis is easy, because hives have a typical appearance. The “Triple Response of Lewis” begins with 1) focal erythema, with 2) subsequent flare, and finally 3) a raised wheal with (classically) central pallor.
The key to Hives is that they’re both transient and migratory. They appear, disappear, & pop up somewhere else. Even if you can’t find any lesions, if a patient gives such history, believe them. If you only see red macules (flare), diagnose urticaria if they come & go, and itch.
Itching is the key.
If you see one or two hives, they could be bug bites. The difference is that bites remain permanently, until they resolve, whereas hives vanish & migrate. Ask the patient.
Diagnosing Urticaria is easy; identifying the cause is hard. It might be something they ate a few weeks ago. Inquire about obvious possibilities; if you find something, counsel avoidance. If nothing’s suggestive, treat with a non-sedating antihistamine & hope it never comes back (usually doesn’t). Occasional patients require referral.
Infections (viral & bacterial) are common causes in children. Certain parasites can cause urticaria in returning travelers (don’t order stool O&P otherwise, & then, only if there’s eosinophilia on CBC).
The most common ingested allergens include:
- Medications, esp. antibiotics & NSAIDs
- In children: also milk, eggs, soy, wheat, tomatoes, strawberries
- Narcotics, esp. codeine (no hives; since not an IgE-mediated allergy)
An association is convincing if the reaction occurred within 2 hours of food ingestion. Suspect a recent medication no matter when in the course it happened.
Excessive Latex use (e.g. gloves all day) should prompt investigation (patch testing), lest a subsequent reaction occur during parenteral injection.
Then there are the rare physical urticarias: hives provoked by cold, heat, exercise, or sunlight. Cold urticaria is the most important, since fatalities have occurred upon diving into cold water (3rd-spacing, as the entire skin surface becomes a hive).
ATOPIC DERMATITIS (ECZEMA) — Usually local, not particularly generalized, but may appear in various parts of the body. Onset is predominantly in childhood. Key clues are:
- dry itchy skin, often erythema with vesicles or scales
- bilateral symmetry
- flexor surfaces, esp. antecubital fossa, popliteal fossa, volar wrist, neck
- cheeks in children
Main confounding conditions are:
- psoriasis (extensor surfaces, not so itchy)
- contact derm (focal; not bilaterally symmetrical)
- seborrheic dermatitis (eyebrows, forehead, bridge of nose)
- photodermatitis (acute onset, sun-exposed areas only)
TINEA CORPORIS (RINGWORM) — Not usually all over the body, but can be somewhat dispersed. The key components of diagnosis:
- Discrete, flat, annular lesions, sometimes scaly, with leading edge (prominent borders)
- Topical treatment works rapidly
A topical antifungal TID-QID starts to help in a couple of days. If not, question diagnosis. I’ve seen atypical extensive T. corporis covering virtually the entire low abdomen; treatment was proof. Give an antifungal alone, without combination steroid, or you won’t know what did it.
SCABIES — Be real strict in your diagnostics, or you’ll be throwing repeated permethrin courses at everyone (including, unnecessarily, family), & helping nobody. You won’t even realize your failures, since patients will shop elsewhere when they didn’t improve. A word about the mechanics of scabies first.
The mite Sarcoptes scabiei is transmitted by close skin-to-skin contact. In adults, that invariably means sexual; not shaking hands, a touchy-feely hug with clothes on, or sitting on a couch. Competitive wrestling or other such atypical activities would do it. Families with young children transmit among each other given their more constant contact.
The incubation period is 3-6 weeks from moment of infestation, the time it takes the body to develop delayed type-4 hypersensitivity to mites eggs & feces. If a person had already had scabies, the reaction can start right away. So if person X becomes infested, & transmits to person Y a few days later, both may develop symptoms at the same time in a few weeks.
Scabetic lesions can look like almost anything: usually unimpressive papules, but also pustules, vesicles, nodules, and wheals, commonly excoriated, sometimes with secondary impetigo or furuncles. They contain mite products. The pathognomonic burrow, the mite’s actual current trail, is a faint thin gray or brownish line with maybe a barely-perceivable dot at an end (the mite). But burrows (see pictures) are rarely noted, because they’re scratched away.
Distribution is typically on & between fingers, volar wrists, axilla, periumbilical, waist, & buttocks. It’s almost always present near breast areola in women, and on the penis or scrotum in men. It’s virtually never on the back or head (except infants).
Every source I’ve read says the itching is worse at night. But every source for every other itchy condition says the same thing. Everything itches worse at night.
The easiest (& maybe best) diagnostic technique available is to press nice-and-sticky transparent packing tape hard against a lesion, pull off rapidly, & examine under low power microscopy. You might see a mite [bad pun]; look for mite eggs & feces (see last picture). Need lots of specimens. Positive clinches it, negative doesn’t rule it out.
“Crusted scabies” (aka “Norwegian scabies,” an adjective poorly-received in Oslo) occurs in immunocompromised or debilitated patients. Diffuse scales & crusts, millions of mites, very contagious, very hard to treat. Often doesn’t itch (patients are too debilitated). May need a KOH prep to dissolve all the junk on top of the mites.
How do I diagnose scabies? If 2 or more members of a family, or patient & sex partner, have the same itchy rash, it’s easier (though multiple occupants can get bug bites). If it’s just one patient with a diffuse, very itchy rash, I think carefully about:
- Significant exposure 3-6 weeks before onset (more recently if prior history)
- Typical distribution of lesions (especially the “almost always” & “never” areas)
- NONE of the other causes of diffuse itchy rash, especially the next 4 on our list
Final Note: Treatment kills the mites immediately, but itching persists another few weeks (it’s a hypersensitivity reaction to mite products). So don’t worry about treatment failure.
BODY LICE — Among homeless persons who can’t change clothes, think of this before scabies. Itchy excoriated papules can appear any- / everywhere on the body. But you won’t see bugs, unless you examine the clothing. Spread the seams, & they’ll swarm with lice & nits. See some pictures.
Treatment is 100% effective — new clothes. Nothing else works.
PUBIC LICE — Pubic, or “crab” lice, are primarily local to the genital area. But on a hirsute person (usually men), they can crawl wherever body hair is contiguous with pubic hair. I’ve seen them go up the belly, back, & down the thighs. Worst case are eyelashes (from close face-to-affected-hair contact), since pediculocides are too toxic for eyes. Not the head, however, because scalp hairs are too far apart; Phthirus pubis is shorter & stubbier than cousin Pediculus humanus capitis, the head louse.
Incubation period from sex to infestation can be 2-3 weeks, for the 1 or 2 acquired lice to lay eggs (nits), & the nits to hatch. Transmission can also occur by other close skin-to-skin contact, or by occupying a bed where an infested person had slept within the past 24 hours.
Diagnosis requires noticing a scurrying louse, or seeing tiny white nits firmly attached to hairs. Distinguish them from lint, etc., since nits can’t be flaked off. You may need a magnifying glass, since they’re much smaller than the nits of head lice. The louse itself is often too fleeting to notice.
A pathognomonic sign, if present, are “maculae cerulae” — small bluish macules due to enzymes injected from louse saliva.
Treatment failures commonly result when only one dose of pediculocide is given, since none are ovicidal. Nits hatch in 10 days. Must apply a second, exactly 11-12 days later, to kill any new nymphs before they mature (in >12 days) to lay new eggs.
Unfortunately, I couldn’t find any useful pictures of tiny nits or maculae cerulae [sorry].
BUG BITES — Biopsies of scabies & lice lesions look just like any other bug bite. Most bugs can produce similar findings. Bedbugs classically bite 3 in a row. Other bugs can cause persistent and new lesions if exposure continues.
Pets can harbor fleas, as can mice residing inside walls. Spider nests can produce lots of hungry babies that bite at night. Occupational or avocational exposures (e.g. gardening) may provide clues. Mosquitos, midges, and sandflies emerge in specific seasons & environs.
Strongly suspect bug bites, & not scabies, if lesions are only on exposed areas. But don’t forget to ask how the patient sleeps (naked, pajamas, just underwear?).
FOLLICULITIS — Macules, papules, even pustules & occasional furuncles (boils) or carbuncles (coalescence of furuncles), which initiate in & emanate from hair follicles. Caused by Staph aureus (often MRSA, depending on local resistance patterns).
Look carefully at primary lesions, & you’ll see the telltale circular erythema with a hair in the middle. Sometimes the hair may not have grown, or may have been shaved. Shaving of course spreads the Staph. Distribution is usually localized but may be diffuse, especially if the patient scratches & auto-inoculates. Click summary page for picture.
Folliculitis is much more common than scabies. You can’t imagine how often I’ve seen patients who’ve failed multiple courses of Elimite, only to really need a week of antibiotics.
A patient presents with diffuse itching, and has a rash.1. If the rash comes & goes, it’ll be Urticaria (even if you don’t see it at the visit)
- If lesions are present, you see wheals (hives)
- Maybe you just see erythematous flares
- Bilateral (often symmetric) scales or vesicles → Atopic Dermatitis (Eczema)
- Annular macules / patches with leading edge → T. corporis (Ringworm)
- Yellowish-tan / Reddish-tan plaques with extreme recalcitrant symptoms, consider cutaneous Mastocytosis (Rare!!! Send to Derm to confirm)
- Spreading contiguously from pubis → Pubic Lice (seek nits & maculae ceruleae)
- Homeless? Check seams of clothes for Body Lice
- Exposed areas only? Probably Bug Bites (papules-in-a-row = Bedbugs)
- Follicular pattern (inflamed hair follicles) → Folliculitis (common!!!)
- Sex / close contact 3 weeks prior to onset, or others in house infested
- Lesions on fingers & in webs, flexor creases of wrists
- Burrows are pathognomonic (but often absent)
- Virtually always on genitals, buttocks, near areolas (women)
- NOT on back or face
- Consider transparent tape microscopy (see last picture)
- Can offer therapeutic trial
NEXT TIME — Itching without rash.