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WHY ARE SOME RASHES ACRAL?
I'm channeling my "Curious Clinician" today for this different type of #dermtwitter #tweetorial.
Let's start with a question. What does acral mean?
#MedEd #FOAMEd #medtwitter #medstudenttwitter
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If you answered, "hands, feet, and ears," you're correct! This is tricky, and it wasn’t until dermatology residency that I learned it’s not palms and soles!
If you haven't yet, take a look at my old #tweetorial on acral rashes as a primer.
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Let’s first cover the easy stuff. We use our hands for so many different things that by virtue of that fact, certain things happen more frequently.
✔️Contact dermatitis from hand washing
✔️Herpetic whitlow (especially in dentists)
✔️Actinic skin damage (UV on dorsal hands)
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But what about the knee-jerk differential we usually think about? Syphilis, RMSF, Hand foot mouth?
Well, a natural guess might be temperature. We know that T pallidum replicates best at lower temps👀👇. So maybe, the cooler distal extremities lead to the acral predilection?
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Not so fast!
The counterpoint I'd make is that despite the same ideal lower temperature for other spirochetes (eg: B. burgdorferi in Lyme), we don't see the same acral predilection in disseminated lyme!
pc: ncbi.nlm.nih.gov/pmc/articles/P…
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But what about viruses? Coxsackie leads to Hand-Foot-Mouth syndrome (1), and other viruses (like Parvovirus B19) can lead to something called Papular Purpuric Gloves and Socks Syndrome (2). Maybe temperature plays a role here?
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Well, let's start with Coxsackie. This paper from 1958 (!!) showed that virus replicates in mice much better a 4C than at 25/36! Perhaps this plays a role?
Unfortunately, nothing in humans, and our acral surfaces are certainly not THAT different in temperature.
PMID: 13514070
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What about Parvo? Well, this paper showed that porcine parvovirus replicates differently at different temps, but it’s not clear that it’s a standard preference for lower temps, nor do we have this in human models, so again – conjecture at best.
pc: ncbi.nlm.nih.gov/pmc/articles/P…
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So that’s a lot of tweets to say, “I don’t know.” What do we know?
Well, what are the anatomical differences between acral skin & other sites?
We know skin of the palms/soles (L) has a thicker/compact epidermis compared to other skin (R).
pc: education.med.nyu.edu/Histology/cour…
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We also know that our palms and soles don’t have hair! As glabrous (smooth) skin, there are no hair follicles.
Instead, there is an overrepresentation of eccrine (sweat) glands on the palms and soles.👇👀
Pc:
anndermatol.org/DOIx.php?id=10…
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A brief aside to highlight how eccrine glands are different from apocrine glands!
Eccrine – all over the body, high concentration on palms and soles. They stand alone.
Apocrine – associated with hair follicles, highest concentration in axillae.
pc: mayoclinic.org/diseases-condi…
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So this helps explain certain skin eruptions we see! Ever heard of Hand Foot Syndrome?
This is something that happens to patients who get certain chemotherapies (eg: cytarabine, capecitabine). These patients get red, inflamed, blistered hands.
pc2: ijdvl.com/hand-and-foot-…
12/ So knowing the differences, why might this happen?
The chemo is excreted out of eccrine glands. Since the eccrine gland concentration is so high on the hands & feet, this rash is from the direct toxicity the chemo has on that skin!
Knowing this, how might you treat it?
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We use ice packs! By asking the patient to hold ice packs during chemo infusion, we vasoconstrict decreasing blood flow (and therefore chemo) to the palms. This leads to decreased excretion in the eccrine glands, and improvement of the rash!
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Another example of how anatomical differences might matter is GVHD. The pathology of GVHD has on eccrine involvement). This might help explain why GVHD starts acrally and moves in (versus a drug rash that starts on the torso and moves out!).
pc: plasticsurgerykey.com/pediatric-graf…
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So while I don’t have all the answers, we at least have some take home points:
✔️Acral means whole hand/foot/ears
✔️Anatomical differences play a role in why certain rashes present with an acral predilection
✔️Temperature may also play a role, but it’s a little murkier.
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Thanks for joining today! If you liked what you read, check out a future episode of the @curiousclinicians! I had the pleasure to record as a guest with the amazing trio of @tony_breu, @AvrahamCooperMD, and @HannahRAbrams on this very topic!
Thanks, and stay safe!
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