Erythromelalgia

Erythromelalgia, or Mitchell's disease (after Silas Weir Mitchell), is a rare vascular peripheral pain disorder in which blood vessels, usually in the lower extremities or hands, are episodically blocked (frequently on and off daily), then become hyperemic and inflamed. There is severe burning pain (in the small fiber sensory nerves) and skin redness. The attacks are periodic and are commonly triggered by heat, pressure, mild activity, exertion, insomnia or stress. Erythromelalgia may occur either as a primary or secondary disorder (i.e. a disorder in and of itself or a symptom of another condition). Secondary erythromelalgia can result from small fiber peripheral neuropathy of any cause, polycythemia vera, essential thrombocythemia, hypercholesterolemia, mushroom or mercury poisoning, and some autoimmune disorders. Primary erythromelalgia is caused by mutation of the voltage-gated sodium channel α-subunit gene SCN9A.

In 2004 erythromelalgia became the first human disorder in which it has been possible to associate an ion channel mutation with chronic neuropathic pain, when its link to the SCN9A gene was initially published in the Journal of Medical Genetics. One year later, in an article in Brain, Dib-Hajj et al., demonstrated that NaV1.7 mutants channels, from families with inherited erythromelalgia (IEM), make dorsal root ganglion (DRG, peripheral and sensory), neurons hyper excitable, thereby demonstrating the mechanistic link between these mutations and pain, thereby firmly establishing NaV1.7 gain-of-function mutations as the molecular basis for IEM. Conversely, in December 2006 a University of Cambridge team reported an SCN9A mutation that resulted in a complete lack of pain sensation in a Pakistani street performer and some of his family members. He felt no pain, walked on hot coals and stabbed himself to entertain crowds. By 2013, nearly a dozen gain-of-function mutations of NaV1.7 had been linked to IEM. The multi-decades search which identified gene SCN9A as the cause of inherited erythromelalgia is documented in a book by Stephen Waxman, Chasing Men on Fire: The Story of the Search for a Pain Gene.

Classification

Primary erythromelalgia may be classified as either familial or sporadic, with the familial form inherited in an autosomal dominant manner. Both of these may be further classified as either juvenile or adult onset. The juvenile onset form occurs prior to age 20 and frequently prior to age 10. While the genetic cause of the juvenile and sporadic adult onset forms is often known, this is not the case for the adult onset familial form.

In rural areas of southern China, outbreaks of erythromelalgia have occurred during winter and spring at 3–5 year intervals among secondary school students. This epidemic form of erythromelalgia has been viewed as a different form of non-inherited primary erythromelalgia and affects mainly teenage girls in middle schools. The disease is characterized by burning pain in the toes and soles of the feet, accompanied by foot redness, congestion, and edema; a few patients may have fever, palpitations, headache, and joint pain. In the 1987 epidemic in Hubei, 60.6% of patients had a common cold before the onset of erythromelalgia and 91.2% had pharyngitis. Symptoms may present gradually and incrementally, sometimes taking years to become intense enough for patients to seek medical care. In other cases symptoms emerge full blown with onset.

Epidemic erythromelalgia is characterized by burning pain in the toes and soles of the feet, accompanied by foot redness, congestion, and edema; a few patients may have fever, palpitations, headache, and joint pain. In the 1987 epidemic in Hubei, 60.6% of patients had a common cold before the onset of erythromelalgia and 91.2% had pharyngitis.

Side effect of medication

Several medications, including verapamil and nifedipine, as well as ergot derivatives such as bromocriptine and pergolide, have been associated with medication-induced erythromelalgia.

Mushroom poisoning

The consumption of two species of related fungi, Clitocybe acromelalga from Japan, and Clitocybe amoenolens from France, has led to several cases of mushroom-induced erythromelalgia which lasted from 8 days to 5 months.

Possible infectious cause

An epidemic form of this syndrome occurs in secondary school students in rural areas of China. A large epidemic erythromelalgia was occurred in Hubei province of China in 1987 and the disease was characterized by burning pain in the toes and soles of the feet, accompanied by foot redness, congestion, and edema; a few patients had fever, palpitations, headache, and joint pain. 60.6% of patients had a common cold before the onset of erythromelalgia and 91.2% had pharyngitis. Serological characterization can easily distinguish human ERPV from ectromelia virus and vaccinia virus by cross-neutralization and plaque reduction assays

Pathophysiology

There are 10 known mutations in the voltage-gated sodium channel α-subunit NaV1.7 encoding gene, SCN9A. This channel is expressed primarily in nociceptors of the dorsal root ganglion and the sympathetic ganglion neurons. Nine of these mutations have received further study and they have all shown to result in similar biophysical alterations, Table 1. As can be seen from table 1, the primary effect of erythromelalgia mutations is NaV1.7 channels that activate at more hyperpolarized potentials. NaV1.7 channels act largely as threshold sensors and initiate action potentials. Consequently, this shift in their activation profile results in channels that open closer to the resting membrane potential. In many mutations, this shift of activation is accompanied by shifts in the voltage sensitivity of fast and/or slow inactivation, often in the depolarized direction. These gain-of-function mutations in NaV1.7 channels cause them to open more easily and stay open longer, resulting in hyperexcitability of the dorsal root ganglion (DRG) nociceptive neurons, which significantly lowers the pain threshold and produces the burning pain.

Some of these mutant channels have been expressed in dorsal root ganglion (DRG) or sympathetic neurons. In DRG neurons expressing the F1449V mutation, a lower threshold is required for action potential creation (93.1 ± 12.0 pA) than those expressing wild-type channels (124.1 ± 7.4 pA). Furthermore, while DRG neurons expressing wild-type channels only respond with a few action potentials, those expressing F1449V channels respond with a high-frequency train of action potentials. There is a similar effect in DRG neurons expressing the L858H and A863P mutants. Here, there is also a notable change in resting membrane potential, being depolarized by 4-7 mV versus wild-type channel expressing cells. The situation is different, however, in sympathetic neurons expressing the L858H mutation. While L858H expressing sympathetic ganglion are depolarized ~5mV relative to wild-type expressing neurons, their threshold for action potential initiation is notably higher. Furthermore, while current injection of 40pA for 950ms provokes an average of 6 action potentials in sympathetic neurons expressing wild-type channels this stimulation evokes only approximately 2 action potentials with reduced overshoots in sympathetic neurons expressing L858H mutant channels. Further investigation has demonstrated that the differences in response between DRG and sympathetic neurons is due to expression of NaV1.8 in the former. Consequently, expression of NaV1.8 channels in sympathetic neurons also expressing L858H mutant NaV1.7 results in neurons with a depolarized resting membrane potential that nevertheless have a normal action potential threshold and overshoot.

{| class="wikitable" style="text-align:center"

|+Table 1. Summary of mutations NaV1.7 associated with primary erythromelalgia

|- valign="bottom"

! Mutation

! Region

! Shift of activation V½

! Shift of inactivation (fast and/or slow) V½

! Other effects

! References

! I136V

| D1S1

! F216S

| D1S4

| Hyperpolarized

| Faster entry into fast-inactivation

! S241T

| D1S4-5

| Hyperpolarized

! N395K

| D1S6

| Hyperpolarized

| Depolarized

| Creation of a large window current, decreased lidocaine sensitivity

! L858F

| D2S4-5

| Hyperpolarized

| Depolarized

| Slowed deactivation, faster recovery from inactivation, cooling depolarizes activation and hyperpolarizes inactivation V½

! L858H

| D2S4-5

| Hyperpolarized

| Slowed deactivation, enhanced slow inactivation,

| colspan="6" | Region nomenclature: DA-B, linker between domains A and B; DASB, transmembrane segment B in domain A; and DASB-C, the linker between transmembrane segments B and C in domain A.

Diagnosis

Erythromelalgia is a difficult condition to diagnose as there are no specific tests available. However, reduced capillary density has been observed microscopically during flaring; and reduced capillary perfusion is noted in the patient. Another test that can be done is to have the patient elevate their legs, and note the reversal (from red to pale) in skin color. Tests done at universities include quantitative sensory nerve testing, laser evoked potentials, sweat testing and epidermal sensory nerve fiber density test (which is an objective test for small fiber sensory neuropathy). Due to the aforementioned factors, patients may face delays in diagnosis.

Once it has been established that it is not secondary erythromelalgia — see below — a programme of management can be put in place. Some diseases present with symptoms similar to erythromelalgia. Complex regional pain syndrome (CRPS), for instance, presents with severe burning pain and redness except these symptoms are often unilateral (versus symmetric) and may be proximal instead of purely or primarily distal. Furthermore, attacks triggered by heat and resolved by cooling are less common with CRPS.

Erythromelalgia is sometimes caused by other disorders. A partial list of diseases known to precipitate erythromelalgia is below. Feedback from some EM patients has led to reduction in usage as they believe it is only effective for short periods. Living with erythromelalgia can result in a deterioration in quality of life resulting in the inability to function in a work place, lack of mobility, depression, and is socially alienating; much greater education of medical practitioners is needed. As with many rare diseases, many people with EM end up taking years to get a diagnosis and to receive appropriate treatment. Research into the genetic mutations continues but there is a paucity of clinical studies focusing on living with erythromelalgia. There is much urgency within pharmaceutical companies to provide a solution to those who suffer with pain such as that with erythromelalgia.

Pain relief

Patients find relief by cooling the skin. All patients must be notified to not apply ice directly on to the skin, since this can cause maceration of the skin, nonhealing ulcers, infection, necrosis, and even amputation in severe cases.

Mild sufferers may find sufficient pain relief with tramadol or amitriptyline. Sufferers of more severe and widespread EM symptoms, however, may obtain relief only from opioid drugs. Opana ER has been found to be effective for many in the US, whilst in the UK slow-release morphine has proved to be effective. These powerful and potentially-addictive drugs may be prescribed to patients only after they have tried almost every other type of analgesia to no avail. (This delay in appropriate pain management can be a result of insurer-mandated or legally-required step therapy, or merely overly-cautious prescribing on the part of sufferers' doctors.)

The combination of Cymbalta (duloxetine) and Lyrica (pregabalin) has also proven to be useful in controlling pain, but many EM patients have found this combination has side effects that they are unable to tolerate.

Epidemiology

Only a small number of studies that have investigated the prevalence of EM, with four studies conducted to date.

The mean of all the studies combined results in an EM estimation incidence of 4.7/100,000 with a mean of 1 : 3.7 of the male to female ratio, respectively. In 2009 there was a population-based study of EM in the USA (Olmsted County, Minnesota), that reported that the annual incidence was 1.3/100,000, with a 1 : 5.6 male to female ratio in this study population, respectively.

The incidence in this study of primary and secondary EM was 1.1 : 0.2 per 100 000 people per year, respectively.

In New Zealand (Dunedin) a study estimated that in 2013 the incidence of EM is 15/100,000, with a 1 : 3 male to female ratio in this study

population, respectively. This last study has an estimation that is at least ten times higher than the prevalence previously reported. This study recruited individuals based on self-identification of symptoms (after self-identification, patients were invited for an assessment of an EM diagnosis), instead of participants that are identified through secondary and tertiary referrals as in the other studies.

Some confusion was introduced when Smith and Allen suggested changing the name to erythermalgia in order to emphasise the symptoms of painful inflammation and warmth. They established three categories: erythromelalgia (platelet-mediated and aspirin-sensitive), primary erythermalgia, and secondary erythermalgia. The primary/idiopathic form of erythromelalgia is not associated with any other disease process and can be either early onset (in children) or adult onset.

| Gerhardt's disease || Carl Gerhardt in 1892

| Erythralgia || Thomas Lewis in 1933

| Erythermalgia || L.A. Smith and F.N. Allen in 1938

| Acromelalgia || J. Huizinga in 1957

Amputation

Because of the severity of the pain in erythromelalgia, and the lack of good pain medication then, there have been reports dating back to 1903 of amputation of the affected limb. In 1903 H. Batty Shaw reported that in three cases the pain was so severe, and that the affected extremities so useless, that amputation was performed.

Differences with Raynaud's disease

thumb|140 px|Sir Thomas Barlow

Back in 1899 Thomas Barlow had already summarized with great detail the contrast between erythromelalgia and Raynaud's disease as following: Dependence produces considerable increase of the dusky red or violaceous tint of the extremity affected; the arteries in this position of the limb may pulsate forcibly; pain is common, sometimes constant, and more especially when the limb is dependent or parts pressed upon; in wintry weather, or on the application of cold, the conditions are relieved; on the other hand, warmth and summer weather increases pain; there is no loss of sensation, but there may be increased sensitiveness; the local temperature of the affected parts may be raised or lowered; gangrene does not occur; the affection is asymmetrical; there is a certain amount of swelling, sometimes allowing pitting on pressure, sometimes not; incisions over such swelling, even down to the bone, have proved useless; excessive pain on pressure upon the nerves supplying the parts affected is not found; muscular wasting is found, but explainable by the disuse of the limb, and is not at all as severe as in cases of disease of the peripheral nerves; a reaction of degeneration in the nerves of the affected parts has not been found; the deep reflexes, with few exceptions, are not reduced.