Neurology for the Rheumatologist
Julius Birnbaum, MD, MHS, associate director of the Johns Hopkins Jerome L. Greene Sjögren’s Syndrome Center, Baltimore, gave an overview of neurology for the rheumatologist. He began by describing how neurological examination can be a powerful tool in distinguishing between myelopathies, radiculopathies and neuropathies. He emphasized that distinguishing between a radiculopathy and a neuropathy does not require the memorization of a web of neuroanatomy. He encouraged rheumatologists to forget about most of the cross wiring between the spinal cord and peripheral nerves. Instead, he told the audience they should remember that radiculopathy has more diffuse sensory and motor features, while neuropathy has more focal sensory and motor features.
Dr. Birnbaum talked through an example of a patient who presented with wrist drop, explaining that all the healthcare provider has to do is check the number of affected muscle groups. If the patient has neuropathy, it’s likely only one or a few muscle groups are affected. In contrast, if the patient has weakness in three or more muscle groups, radiculopathy is more likely. Dr. Birnbaum explained the approach to diagnosing foot drop is similar. If the patient has an insidious onset of less severe, but diffuse sensory and motor features, then it’s likely they have L5 radiculopathy. In contrast, the patient who presents with acute severe localized symptoms likely has peroneal neuropathy.
Small fiber neuropathies can present as neuropathic pain in patients with systemic rheumatic diseases. They target thinly myelinated A-delta or unmyelinated C-fiber nerves, and the resulting pain is burning, paroxysmal and allodynic. Patients may even complain of pain when bed sheets graze their toes at night. They often have vivid and remarkable descriptions of neuropathic pain. “It is really amazing, the lyricism,” said Dr. Birnbaum.
For example, he discussed a patient who said, “I feel like my legs are being electrocuted at night.” Physical examination of patients with small fiber neuropathies often reveals small fiber deficits (i.e., decreased pinprick and cold sensation) even though electrodiagnostic studies are normal. Unfortunately, because small fiber neuropathy can be associated with a seronegative profile, it may be difficult for these patients to receive an accurate diagnosis. A delayed diagnosis can contribute to a more widespread and treatment-refractory, centrally sensitized pain state that results from central amplification.
The differential diagnosis of potential small fiber neuropathies includes vasculitis, infection and autoimmune/inflammatory disorders. Metabolic disorders are also important in the differential because impaired glucose tolerance can cause a reversible small fiber neuropathy. It is also important to exclude neoplastic disorders or structural mimics, such as syringomyelia and myeloradiculopathies. A punch skin biopsy will not only be diagnostic for small fiber neuropathy, but may also help identify which part of the peripheral nervous system is being targeted. Patients with vasculitic neuropathies can be treated with immunosuppressive therapy. Painful neuropathies can initially be treated with symptomatic therapies.
