Erythromelalgia

Primary erythromelalgia is a genetic pain condition that is caused by an autosomal dominant mutation in the SCN9A gene. It is a type of sodium channelopathy. Secondary erythromelalgia occurs in older people and is associated with certain conditions like myeloproliferative disorders.
== Classification ==
Key differences between primary and secondary erythromelalgia
Parameter 	Primary 	Secondary
Gene involved 	SCN9A gene 	None identified
Disease association	Not applicable 	Multiple disease associations
Distribution 	Symmetrical distribution more likely 	Asymmetrical distribution more likely
Age of onset 	Younger age of onset 	Older age of onset
Treatment 	Standard treatment plus targeted treatment (e.g. mexilitine, novel selective Nav1.7 modulators in trials)	Standard treatment plus diagnosis and treatment of associated disease
The primary form of the disease is related to the other two SCN9A neuropathic pain syndromes: small fibre neuropathy and paroxysmal extreme pain disorder.
== Aetiology ==
Primary erythromelalgia is caused by a gain of function mutation in the SCN9A gene
Secondary erythromelalgia has the following associated conditions
* Haematological disease: Polycythaemia vera, Essential thrombocythaemia, Idiopathic thrombocytopenia, Leukaemia, Systemic macrocytosis
* Medications: Iodine contrast, Calcium channel blockers (e.g. verapamil, nifedipine), Bromocriptine
* Rheumatological: Systemic lupus erythematosus, Rheumatoid arthritis, Sjogren syndrome, Vasculitis, Gout.
* Neoplasia: Primary colon or breast carcinoma, Paraneoplastic, Subcutaneous panniculitis-like T-cell lymphoma, Astrocytoma, Thymoma
* Toxins: mercury poisoning
== Clinical Features ==
Patients complain of red or purple, swollen, burning hands and feet that is aggravated by warmth and exercise. The feet are more commonly affected then hands. These episodes usually occur in the evening or at night. Affected individuals may fail to notice or mention the redness especially if it only affects the feet. The symptoms are typically bilateral. Manifestations of this disorder may vary significantly within a family.
SCN9A-EM usually begins in childhood or adolescence, but in some families, it has been identified in infants. While rare, later onset (age >20 years) has been reported in some individuals and families. Initially, the symptoms involve the soles of the feet and hands, but with time, the lower legs and arms may become affected. In advanced cases, symptoms may occur many times a day and persist for hours, particularly at night or become constant and unremitting.
Episodes are typically triggered by exposure to warmth and are relieved by cold. Other less consistent triggering factors include exercise, tight shoes, wearing socks, alcohol, spicy foods, and other vasodilating agents. Some patients have allodynia and hyperalgesia. This can affect sleep and normal activities, and even wearing shoes and socks.
Neurologic examination is typically normal, although reduced ankle reflexes and decreased distal sensation can be seen. Skin biopsy of individuals with erythromelalgia shows nonspecific thickening of blood vessel basement membrane, perivascular edema and mononuclear infiltrate, and reduced density of the autonomic nerve plexuses.
== Differential Diagnosis ==
* Neuropathies: Diabetes mellitus, alcoholism, HIV, neurofibromatosis, Multiple sclerosis, Small fibre neuropathies. Patients may have burning pain but are less likely to have redness, warmth, heat intolerance or relief with cooling.
* Complex Regional Pain Syndrome. The condition may be indistinguishable in the early stages however CRPS is more likely to be unilateral and occur following trauma.
* Peripheral Vascular Disease
* Raynaud's phenomenon
* Fabry disease
* Other genetic neuropathic pain conditions: TRPA1, SCN10A, and SCN11A familial episodic pain syndromes
== Treatment ==
Non pharmacological: Wear open toed shoes or have uncovered feet and stay in cool environments. Swimming for cooling of the limbs. Avoid triggers. Cool the extremities.
Medication: Mexiletine in primary disease. Topical capsaicin may help but can increase pain. SSRIs, anticonvulsants, calcium channel blockers, TCAs, gabapentin, carbamazepine.
For secondary erythromelalgia associated with essential thrombocythaemia, aspirin can relieve pain for up to several days. Other secondary causes and primary disease do not have the same response.
== See Also ==
GeneReviews - SCN9A Pain Syndromes
==Resources==
== References ==
Category:Polyneuropathies
Category:Genetic Disorders