Ultrasound: Redefining neuropathic pain diagnosis

Neuropathic pain is a complex condition affecting millions worldwide that stems from damage or dysfunction of the nervous system. While diagnosis traditionally relies on clinical assessments and imaging modalities like MRI, a burgeoning field is emerging: diagnostic nerve sonography using ultrasound.

Here, Dr Andrzej Krol, renowned consultant in anaesthesia and pain medicine, delves into the revolutionary role of ultrasound in diagnosing neuropathic pain, exploring its benefits, advancements, and potential implications for future treatments.

High-resolution ultrasound in nerve assessment

High-resolution ultrasound offers a valuable tool for diagnosing peripheral nerve conditions. When examining superficial nerves, ultrasound provides exceptional high-resolution imaging that often exceeds the capabilities of an MRI scan. Its ease of use at the bedside and suitability for post-operative evaluations, even with implanted osteosynthesis materials, highlights its practical advantages.

It’s important to note that ultrasound can only show morphological and structural changes in nerves and doesn't provide information about their functional status. Nonetheless, by closely examining the muscles connected to the nerves using sonographic techniques, it’s possible to identify potential functional deficits in motor nerves.

Essential factors in sonographic nerve evaluation

A thorough and precise understanding of the normal anatomy, the course, and the anatomical variations of peripheral nerves is crucial for sonographic evaluation. Additionally, expertise and experience in this field are essential, making diagnostic nerve sonography highly dependent on the examiner’s proficiency. The ability to capture subtle structural changes in nerves also relies on high spatial resolution, which necessitates the use of high-end transducers with frequencies up to 24 MHz and advanced image post-processing software.

Application in neuropathic pain evaluation

High-resolution nerve ultrasound is vital in assessing peripheral nerve involvement in neuropathic pain as part of a comprehensive evaluation. This assessment begins with detailed history and clinical examination. Depending on the pathology and affected nerve, it also includes a functional assessment using neurophysiological measurements and is complemented by a structural examination of the entire course of the nerves using high-resolution ultrasound and MRI/MR neurography.

When taking a patient’s history, pain characterised by burning, electrifying, tingling, shooting sensations, and needle-like pricking strongly indicates a neuropathic origin. Sensory nerve involvement often includes changes in sensory perception, which may be diminished or, more commonly, increased. Damage to mixed and motor nerves can result in impaired motor function, such as muscle weakness and paralysis. Electrodiagnostic evaluation, which are nerve conduction studies combined with electromyographic examination of corresponding muscles, assesses the functional status of a nerve and can classify nerve injuries as neurapraxia or axonotmesis.

Generally, changes in the shape or structure of a peripheral nerve are particularly anticipated when the reported pain in the sensory region corresponding to the nerve's distribution has neuropathic characteristics and is accompanied by sensory hypersensitivity in the same area. A test called the Hoffmann-Tinel sign, which involves gently tapping the nerve at the site of damage, can provide additional information.

It's important to note that a normal nerve ultrasound doesn’t rule out nerve involvement, nor does a sonographically-visible structural change necessarily cause the symptoms. Correlating clinical and electrophysiological findings with imaging results is essential. A useful technique to confirm the nerve as a pain generator is an ultrasound-guided diagnostic nerve block proximal to the lesion, using a small amount of local anaesthetic (0.5–3 ml, depending on the nerve size).

Diagnostic imaging techniques and interpretation

Most large peripheral nerves can be evaluated from end to end using ultrasound. Scanning the nerve in a cross-sectional view (similar to a horizontal slice) is crucial for locating the nerve within the surrounding tissue and assessing its architecture.

In different conditions, the structure of the nerve can change, affecting its appearance under ultrasound. The arrangement of nerve fibres, called the fascicular structure, may remain intact, become fuzzy, or be disrupted altogether. When measuring the cross-sectional area of a nerve, it’s crucial to scan the nerve exactly perpendicular to its course (in a transverse view) to avoid mistakenly making it seem wider than it is. This distortion, for example, can be observed in conditions like nerve entrapment syndromes.

If suspicious regions are found, they should also be examined from a second angle (longitudinal view) to assess the length, integrity, or disruption of the nerve. In subtle cases, comparing nerves from both sides of the body can be helpful. Additionally, the structural assessment should be supplemented with Colour/Power Doppler ultrasound to visualise increased vascularisation, which can indicate certain nerve tumours or inflammatory processes.

In cases of post-operative or post-traumatic nerve lesions resulting in complete loss of nerve function, determining whether nerve continuity is intact is paramount. If nerve disruption is identified, immediate surgical revision should be considered.

An indirect sign of complete nerve discontinuity may be seen as a "wavy" pattern in the nerve near the injury site, caused by the nerve endings pulling back. While the nerve’s structure may not always be visibly altered, reduced mobility and gliding ability within scar tissue can lead to persistent nerve irritation, potentially explaining neuropathic symptoms. Additionally, the proximity of the nerve course to surgical access points, fractures, bone fragments, and foreign materials like screws, plates, and suture material, can further contribute to nerve irritation following trauma or surgical intervention.

Future perspectives and impact

Diagnostic nerve sonography has become a vital tool for neuropathic pain diagnosis and is now a standard part of routine clinical practice. The impact of diagnostic nerve sonography transcends the realm of pain medicine, extending to various disciplines including radiology and neurosurgery. Furthermore, I expect that diagnostic nerve sonography will also have a significant impact on surgical specialties like orthopaedics in the future.

If you would like to schedule an appointment with Dr Andrzej Krol, head on over to his Top Doctors profile today.