Investigations for the Diagnosis of Painful Neuropathies

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Martin Koltzenburg
UCL Institute of Neurology and UCL Institute of Child Health
London, UK

Neuropathic pains affect a large group of individuals and estimates of life-time prevalence are as high as 10%. The International Association for the Study of Pain currently defines neuropathic pain as pain caused by a primary lesion or dysfunction in the nervous system. The usefulness of this wide ranging and imprecise classification has come recently under dispute and a more useful definition could be pain caused by a disease of the somatosensory nervous system. The term deafferentation syndrome is often used for those conditions in which there is extensive and complete disconnection of peripheral nerves from their peripheral target such as in amputations and plexus lesions. In these situations the pain is generally ongoing and stimulus-independent. In contrast, when the connections in the peripheral nervous system are partially retained or at the borders of a completely deafferentated zone the pain has often stimulus-independent and stimulus-induced components (hyperalgesias).
Neuropathic pain is never a disease in its own right, but is a symptom of many disorders affecting the peripheral nervous system. The table provides a list of condition that might be considered in the differential diagnosis (Scadding and Koltzenburg, 2005). In other words, if one assumes that a pain in neuropathic in type a neurological diagnosis needs to be sought.

It is now recognized that – perhaps paradoxically – it is the lesion of peripheral nociceptive neurons that lead to neuropathic pain. Investigations of neuropathic pain states therefore need to firstly determine the location and pattern of the underlying neuropathy and secondly the etiology of the peripheral nerve disease. Although standard clinical neurophysiological test are useful in determining the presence of a peripheral nerve lesion these techniques assess only large myelinated sensory or motor fibres and a normal result does not exclude the presence of a small fibre neuropathy. Sensory small fibres can be directly investigated using microneurography, but the invasive nature limit this useful technique to the research setting. Therefore indirect functional tests of thin myelinated or unmyelinated afferents are used clinically including thermal threshold testing, measurement of flare response and laser or contact heat evoke potentials. A number of autonomic function tests can also be used to diagnoses the involvement of unmyelinated axons in a neuropathy. Measurement of epidermal nerve fibre density is currently considered to be the gold standard in the clinical diagnosis of small fibre neuropthies (Lauria et al., 2005).

Knowledge about the history, findings on the neurological examination and outcome of investigations will guide the etiological differential diagnosis. Sometimes the differential diagnosis is straightforward as in the case of postherpetic neuralgia, but often there are nociceptive and neuropathic ocmponents of the pain as in the case of a traumatic brachial plexus injury or in many forms of low back pain. The cause for many idiopathic small fibre polyneuropathies often remains obscure although recent evidence shows that impaired glucose tolerance or an occult diabetes is a frequent cause (Benatar, 2007).

Benatar M (2007) Neuromuscular Disease: Evidence and Analysis in Clinical Neurology. Totowa: Humana Press.

Lauria G, Cornblath DR, Johansson O, McArthur JC, Mellgren SI, Nolano M, Rosenberg N, Sommer C (2005) EFNS guidelines on the use of skin biopsy in the diagnosis of peripheral neuropathy. Eur J Neurol 12:747-758.

Scadding JW, Koltzenburg M (2005) Painful peripheral neuropathies. In: Wall and Melzack's Textbook of Pain (McMahon SB, Koltzenburg M, eds), pp 973-999. Philadelphia: Elsevier.

Time of Presentation
1530