Diagnostic and Therapeutic Issues of Neuropathic Pain
There are two types of pain: nociceptive and neuropathic. The former is of a mechanical nature and relates to nerve pain through direct nerve irritation, such as an arthritic joint. The latter implies a specific injury or insult to nerves causing pain and discomfort. Based on my clinical observations in treating patients with neuropathic pain for almost 20 years, it is my belief that neuropathic pain and, in particular, radicular pain has been under-diagnosed and under-treated for a variety of reasons. Partly it’s our reliance on electrodiagnostic testing in making the diagnosis of radicular pain and the difficulty in obtaining an accurate history from the patient, and an absence of other diagnostic testing to make this diagnosis. Also, health care professionals often overlook the neuropathic component of pain and focus on other causes of chronic pain. Further, the difficulty in the treatment of neuropathic pain presents a clinical challenge.
Neuropathic pain includes, but is not limited to diabetic neuropathy, all peripheral neuropathies, radiculopathy, complex regional pain syndrome, sympathetically mediated pain, fibromyalgia, and interstitial cystitis. A variety of medical conditions that cause peripheral neuropathy include inherited diseases, alcoholism, nutritional vitamin deficiencies, cancers, autoimmune reactions, medications, kidney and thyroid disease and infections such as Lyme disease, shingles or AIDS. Peripheral neuropathy of idiopathic etiology, diabetic peripheral neuropathy, radiculopathy and mono-neuropathies make up the majority of all the causes of neuropathic pain.
Although the diagnosis of a specific type of neuropathic pain can be complicated, distinguishing between neuropathic pain and nociceptive pain is not as complicated. It is important to distinguish pain patterns in making the diagnosis of neuropathic pain. Practitioners should evaluate the patients as to whether the pain involves an extremity and, if so, is it radiating from the spine or is it a separate pain of a joint or a separate part of the extremity.
Pain location is important and the patient needs to elaborate specifically where the pain occurs. It is helpful to use a pain diagram to show where their pain is located. The practitioner should then go over the diagram with the patient and complete an examination and have the patient show them physically on their body where the pain occurs. If the pain involves an extremity, is it sock-like pain or is it linear in fashion? Are there neurological changes on examination and does the patient have other neurological symptoms or complaints such as numb-ness or tingling?
For the purpose of this article, I examined 30 patients in my clinic with chronic back pain and confirmed compressive pathology in patients who also had extremity pain. Of those 30 patients, only two were able to adequately describe a radicular symptomatology that could be considered as anatomically accurate. Less than half the patients had EMG studies and, of those, about two thirds were reported as negative. In our practice, we have found that the treatment of radicular pain has frequently been denied specifically based on these two issues: that there is a lack of a clear dermatomal pain pattern, and that there is a normal EMG report.
A review of the literature seems to indicate that there is some information which substantiates these concerns. A study from the Journal of Neurology indicated, “The subsequent clinical course of patients examined in an EMG laboratory has not been adequately studied.”1 The purpose of this study, in addition to their observations of EMG results, was to review the existing literature and determine the treatment outcomes of patients based on those EMG findings. What they found in this retrospective review of patients was that those patients who had had a pathological EMG had significantly better clinical outcomes in the three year post period than those patients who did not. This indicates that practitioners were treating patients for neuropathy based on the EMG results and denying treatment on those patients who did not have positive results.1
A separate study, completed in 2002, attempted to determine specific protocols for the use of EMG. It recognized deficiencies in the diagnosis. The studies stated specifically, “The assessment of the spontaneous activity in extremity muscles alone minimizes the influence of error-causing factors on the EMG result. Nevertheless, lack of assessment of motor units potentials makes electromyographic investigation of chronic radiculopathy impossible. “Sensitivity and specificity of the method have to be determined before the clinical use of the assessment of motor unit potential, MUP, and chronic radiculopathy, especially in patients with motor weakness. This indicates shortcomings in needle EMG in diagnostic radiculopathy.”2
Further difficulties with paraspinal EMG and lumbar radiculopathies were outlined in a 2000 article regarding the use of a 0–4+ scoring scale in determining paraspinal muscle involvement in radiculopathies. They found deficiencies in the scoring system when the electromyographer suspected that the patient had radiculopathy. It was a more common use of equivocal paraspinal scoring and underscores the inconsistencies and subjectivities that may occur regarding EMG diagnosis.3 This seems to indicate that electrodiagnostic studies are not extremely accurate in some neuropathic pain and there seems to be inconsistencies regarding its use in determining the diagnosis of radicular pain. Since practitioners have frequently treated radiculopathy based on EMG findings and have neglected to treat as aggressively those patients who did not have positive EMG findings, I believe additional studies need to be done to clarify these issues.
Clinical Diagnosis of Neuropathic Pain
The diagnosis of neuropathic pain is also based on clinical examination and clinical history as well as laboratory studies and imaging studies. As far as examination and diagnosis of neuropathic pain, observations in our clinic have revealed that very few patients adequately describe what would be considered an anatomical dermatomal pain pattern for radicular pain. This was emphasized during my informal evaluation of a group of patients in our clinic. The difficulties patients have in describing neuropathic pain occur for a variety of reasons and was highlighted in a 2005 article: “Sensory examination could be challenging and confusing because responses are indirect and represent a patient’s interpretation of the test and questions. It is obviously easier to determine in patients if there is a stocking glove-type dermatomal or radiating type pattern, but as far as following a dermatomal pattern this difficulty remains.”4