What is erythromelalgia?

Erythromelalgia shows symptoms of redness and a sensation of burning pain in the extremities. The skin may also feel abnormally warm, and sometimes swelling and itchiness can also be present, though the severity of symptoms varies from patient to patient and can also be triggered or exacerbated by heat, stress, alcohol, and warm temperatures.¹

There are different types of erythromelalgia, split into two categories, primary and secondary Erythromelalgia, of which primary is not related to other conditions, whereas secondary is. Primary erythromelalgia is further split into idiopathic erythromelalgia (arising for unknown reasons) and inherited erythromelalgia (arising due to genetic mutations). Secondary erythromelalgia develops due to underlying health conditions such as autoimmune diseases, Raynaud’s disease, and thrombocytopenia.²

Diagnosis of this condition is particularly challenging due to the episodic nature of this disease. Flare-ups may occur outside of a physical exam, and so it can be hard to properly diagnose in a clinical setting. The symptoms of this disease are also shared by other diseases, such as Vasculitis, Complex Regional Pain Syndrome, and Peripheral Neuropathy, adding further difficulty to make an accurate diagnosis.³

Diagnostic imaging is crucial to properly define subjective symptoms such as pain and warmth. As these are not physical symptoms that are easy to detect, tools need to be used to verify that flare-ups coincide with a measurable change and monitor this over time. This can allow Erythromelalgia to be distinguished from similar diseases and assess how well the treatment is working, and advise any changes that should be made. Without diagnostic imaging, it can be easy to make a misdiagnosis, which may not lead to the most effective treatment plan.⁴

Traditional diagnostic approach

The first step of diagnosis is a physical examination of symptoms alongside a review of the patient’s clinical history. The key characteristics of pain, redness, and warmth are observed in patients with this condition, leading to a clear indication of erythromelalgia as a diagnosis, particularly if symptoms occur episodically and are exacerbated by heat, exercise, alcohol, and other triggers.⁵

Once an initial assessment is made, if erythromelalgia is suspected, laboratory investigations can be carried out to rule out underlying causes that demonstrate the same symptoms. Some common tests that can be done are:

• Blood count tests - particularly looking at elevated platelet levels, elevated hematocrit levels, and elevated white blood cells. This can be used to diagnose conditions such as thrombocytosis and polycythemia much more accurately, which can be an underlying cause of Erythromelalgia¹
• Antibody tests - different tests for specific antibodies can be carried out in order to determine if an autoimmune disorder is present¹
• Infection disease screening - infectious diseases such as HIV can be a secondary cause⁵
• Metabolic testing - blood sugar tests can diagnose diabetes, and tests for uric acid can diagnose gout, both of which can possible underlying causes¹
• Genetic testing - if a hereditary cause is suspected, then a genetic test can confirm if this is the underlying cause of symptoms⁴

The initial process of reporting subjective symptoms provides a limited view of the condition and can make an objective diagnosis difficult to define, reducing the efficacy of this traditional diagnostic approach. Symptoms may not be present during a physical examination if this time does not coincide with a flare-up, symptoms vary between patients, and it relies on the memory of patients to remember the timing of flare-ups and triggers that caused these.⁶

Imaging modalities in erythromelalgia

Thermography

Thermography is an imaging method that measures the temperature of the body by recording how much infrared radiation is emitted. This can be used to map the temperature of different body parts and can show if the temperature of extremities has increased.⁷

During this test, the patient will acclimatise to the room temperature to ensure results are as accurate as possible (this often takes around 10 to 20 minutes), and a special camera is used once it has been calibrated. If the camera picks up localised areas of high temperature in the same body area where the patient has reported pain, this indicates that erythromelalgia is a likely diagnosis. Similarly, asymmetries in heat mapping (for example, if one hand is much warmer than the other) are also commonly found with this disease.⁸

This form of imaging is a fast, easily accessible, and non-invasive method of diagnosing this condition that is highly informative. As well as being used in diagnosis, it can also be used to monitor the efficacy of treatment, which can inform any changes that need to be made. On the other hand, this method is subject to different conditions and can provide misleading results, particularly if the subject being imaged has not fully acclimatised to the room temperatures.⁷

Laser doppler flowmetry

Laser Doppler Flowmetry is a method of measuring blood flow on a microscopic level. This is achieved by measuring the Doppler shift (measuring sound frequency to determine the motion of an object or fluid) of blood flow at a specific point in the body. The results of this investigation will usually show increased areas of microvascular blood flow in areas of reported pain if erythromelalgia is present.⁹

This is a helpful additional measurement to give a more in-depth picture of what is happening inside the body, but it can miss some changes and probably should not be relied upon in isolation. This is also a much less readily available technique, so it is not routinely done when erythromelalgia is suspected.¹⁰

MRI/MR angiography

These tests can be used to further investigate abnormalities in blood flow. These are not used very often for erythromelalgia, though, as they show little extra insight but can rule out the possibility of issues with large blood vessels as the cause of symptoms. This is only usually carried out if other tests suggest there is a lesion somewhere within the vascular system or some neurological condition is suspected¹¹. 

High-resolution ultrasound

Ultrasound can be used to measure blood flow; this can show areas with highly directed blood flow, which can be used to inform an erythromelalgia diagnosis. This is particularly useful to rule out other arterial diseases and can also flag soft-tissue or neuropathic changes, which can help define the details of the patient’s condition. Overall, this is a useful technique for excluding other issues, but it is not the most accurate method to actually diagnose erythromelalgia.¹²

Comparative value of thermography vs. other tools

The crucial benefits of thermography are that characteristics of the condition can be objectively measured, as opposed to relying on reported subjective symptoms. This can also more accurately determine temperature asymmetry and clearly show which areas of the body are most affected. These characteristics can also help to assess treatments and inform any changes that should be made.⁷

This can be complemented through the use of laser Doppler flowmetry to provide an extra insight by mapping blood flow, which, alongside thermography, should give a fairly clear idea of the patient’s condition. These results can be used to inspire confidence in a diagnosis if they agree, and imply that there may be some other issues if they do not.⁹

Thermography is much more useful in the diagnosis and treatment of this disorder than MRI and Ultrasound methods. Whilst these other methods can provide additional details and help to confirm results, they are not strongly indicative of any particular issue and are mostly used to rule out other health problems that could be related to similar symptoms.¹¹

Summary

Erythromelalgia is an uncomfortable condition characterised by the reported symptoms of burning pain, high temperature, and redness of the extremities. The biggest challenge when tackling this condition is that it is difficult to diagnose and assess, as most of the symptoms are subjective and episodic. Diagnostic imaging is, therefore, crucial in order to accurately assess the patient’s condition and monitor the efficacy of treatment plans. Thermography is currently our most accurate way of viewing these symptoms objectively by mapping the temperature of the body. This must be done taking into account the patient's reported symptoms and ideally confirmed by further imaging, such as laser Doppler flowmetry, to establish abnormal blood flow. Together, these methods can be used to more precisely determine the extent of the condition and inform the best course of action to tackle symptoms.