neurologic disorders
Overview of Peripheral Nervous System Disorders
Evaluation
Deficits defined by history and examination Attention to clinical clues to peripheral nervous system disorders Usually nerve conduction studies and electromyography Sometimes nerve or skin punch biopsy Genetic testing (for hereditary neuropathies) History should focus on type of symptom, onset, progression, and location, as well as information about potential causes (eg, family history, toxic exposures, past medical disorders). Physical and Introduction to the Neurologic Examination should further define the type of deficit (eg, motor deficit, type of sensory deficit, combination). The following are evaluated: How to Assess Sensation (using pinprick and temperature for small fibers; using vibration and proprioception tests for large fibers) How to Assess Muscle Strength (noting whether motor weakness is proportional to the degree of atrophy) How to Assess Reflexes (noting type and distribution of reflex abnormalities) How to Assess the Cranial Nerves None Central and peripheral nerve function Overview of the Autonomic Nervous System None Physicians should suspect a peripheral nervous system disorder based on the pattern and type of neurologic deficits, especially if deficits are localized to particular nerve roots, spinal nerves, plexuses, specific peripheral nerves, or a combination. These disorders are also suspected in patients with mixed sensory and motor deficits, with multiple foci, or with a focus that is incompatible with a single anatomic site in the CNS. Physicians should also suspect peripheral nervous system disorders in patients with generalized or diffuse weakness but no sensory deficits; in these cases, peripheral nervous system disorders may be overlooked because they are not the most likely cause of such symptoms. Clues that a peripheral nervous system disorder may be the cause of generalized weakness include the following: Patterns of generalized weakness that suggest a specific cause (eg, predominant ptosis and diplopia, which suggest early Myasthenia Gravis ) Symptoms and signs other than weakness that suggest a specific disorder or group of disorders (eg, cholinergic effects, which suggest Organophosphate Poisoning and Carbamate Poisoning ) Deficits in a stocking-glove distribution, which suggest diffuse axonal disorders or Polyneuropathy None Fasciculations Hypotonia Muscle wasting without hyperreflexia Weakness that is progressive, chronic, and unexplained Clues that the cause may not be a peripheral nervous system disorder include Hyperreflexia Hypertonia These deficits suggest an Symptoms and Signs as the cause of weakness. Hyporeflexia is consistent with peripheral nervous system deficits but is nonspecific. For example, acute cervical transverse myelitis can mimic Although many exceptions are possible, certain clinical clues may also suggest possible causes of peripheral nervous system deficits (see table Clinical Clues to Causes of Peripheral Nervous System* Disorders ). Clinical assessment narrows diagnostic possibilities and guides further testing. Usually, Electromyography (EMG) and Nerve Conduction Studies (collectively called electrodiagnostic testing) are done. These tests help do the following: Differentiate peripheral nervous system disorders from neuromuscular junction and muscle disorders Localize site of peripheral nervous system dysfunction (eg, root, plexus, peripheral nerve) Distinguish demyelinating disorders (very slow conduction) from axonal disorders Patients with weakness but no sensory deficits can be evaluated with electrodiagnostic testing. Other testing, such as imaging, depends on whether a CNS lesion must be ruled out (eg, MRI if all limbs are affected, to rule out cervical spinal cord compression). Nerve biopsy is occasionally done to help differentiate demyelinating from vasculitic large-fiber neuropathies. If vasculitis is a consideration, the biopsy specimen should include skin and muscle to increase the likelihood of a definitive diagnosis. If a small-fiber neuropathy is suspected, skin punch biopsy can be done; loss of nerve endings supports that diagnosis. If clinical findings and electrodiagnostic test results are inconclusive, do a biopsy (nerve biopsy for suspected large-fiber neuropathy or skin punch biopsy for suspected small-fiber neuropathy). If all limbs are affected, consider MRI to rule out cervical spinal cord compression. Genetic testing is indicated if a hereditary neuropathy is suspected. 