Ian C. MacLean

Ischemic myelopathy: A review of spinal vasculature and related clinical syndromes

This article provides a comprehensive review of spinal vascular anatomy (arterial and venous) and clinical syndromes that result from the disruption of blood flow to or from the spinal cord. Blood is supplied to the spinal cord through three longitudinal channels: one anterior and two posterior spinal arteries. These vessels, which originate as branches of the vertebral arteries and run caudally along the spinal cord, are augmented by a variable number of medullary arteries. A vascular ring, or vasa coronae, surrounds the cord and connects these longitudinal channels. Central arteries from the anterior spinal artery and penetrating vessels from the vasa coronae provide blood directly to the cord. Venous flow from the spinal cord is also accomplished through a system of anterior and posterior spinal vessels draining through a variable number of medullary veins and an extensive valveless vertebral venous plexus. The disruption of blood flow to or from the spinal cord can result in infarction, with permanent neurologic loss and physical impairment. The clinical presentation in cases of ischemic myelopathy can be variable. Discrete syndromes based on the occlusion of specific vessels are reported and include venous infarction, anterior and posterior spinal artery syndrome, and central infarction.

1. Needle electromyography

Abstract This self-directed learning module highlights advances in the needle electromyographic (EMG) examination. It is a section of the chapter on electrodiagnosis for the Self-Directed Medical Knowledge Program Study Guide for practitioners and trainees in physical medicine and rehabilitation. This section contains referenced information regarding instrumentation and safety, spontaneous muscle activity, motor unit assessment in normal and pathologic states, single fiber EMG, and macro EMG. The electrical character of motor units is related to strength, age, fatigue, the histochemical type of the motor unit, and the particular muscle. Abnormal motor unit potentials can usually but not always be interpreted as neuropathic or myopathic, according to their distinguishing features, including satellite potentials. Myotonic potentials are distinguished from complex repetitive discharges, which single fiber EMG studies demonstrate as ephaptic transmission between muscle fibers. Myokymia and fasciculations are phenomena that can be clincally observed. Their EMG characteristics and pathophysiology are reviewed. Single fiber EMG study of neuromuscular junction disease and reinnervation states has improved understanding of these problems and can be used as an objective test of therapeutic response. Review herein of the proper use of filters and electrodes for best signal measurement emphasizes potential sources of error.

4. Central evoked potentials

Abstract This self-directed learning module highlights advances in the use of evoked potentials of the central nervous system. It is a section of the chapter on electrodiagnosis for the Self-Directed Medical Knowledge Program Study Guide for practitioners and trainees in physical medicine and rehabilitation. This section contains information regarding responses evoked by visual, auditory, and somatosensory stimulation. Reviewed are electrode placements for recording; instrumentation requirements, including amplifiers, filters, averaging, and signal storage; and common technical errors. The evoked response often assists in the anatomic and physiologic localization of pathology. Precise neural generation of some signals remains under debate. The discussion includes the use of these responses in central nervous system disease and trauma, including coma and brain death. The presence of the eighth nerve response due to auditory stimulation with absent brainstem responses suggests a grave prognosis. Intraoperative monitoring using somatosensory evoked potentials and, recently, motor potentials during scoliosis surgery is becoming a routine procedure. Somatosensory evoked potential studies are also useful in the evaluation of sensory neuropathies and proximal peripheral nerve abnormalities. The use of mechanical stimuli in somatosensory studies allows evaluation of specific sensory receptors and conduction pathways. Recently, cortical potentials related to voluntary activity and cognitive function have been evaluated. This area offers much promise in the understanding of neuromuscular control and in neuropsychiatry.

2. Peripheral evoked potentials

Abstract This self-directed learning module highlights advances in the clinical evaluation of nerve conduction through peripheral evoked potentials. It is a section of the chapter on electrodiagnosis for the Self-Directed Medical Knowledge Program Study Guide for practitioners and trainees in physical medicine and rehabilitation. This section covers instrumentation, neurophysiology of nerve conduction and neuromuscular transmission, techniques of performing the studies, and clinical applications. Technologic improvements in amplifiers and storage systems have made detailed measurement of sensory and motor responses a routine clinical procedure. The interaction of membrane action potentials and volume conduction is explained, and the effect of abnormal neuromuscular transmission on evoked responses is analyzed. Skin surface techniques for stimulation and recording of potentials are compared with transcutaneous needle techniques. Clinical examples are presented to clarify the usefulness of these studies in evaluating suspected problems, including localized compression neuropathies, peripheral axonopathies, demyelinating neuropathies, and disorders of neuromuscular transmission.

3. Late responses

Abstract This self-directed learning module highlights advances in late responses in nerve conduction testing. It is a section of the chapter on electrodiagnosis for the Self-Directed Medical Knowledge Program Study Guide for practitioners and trainees in physical medicine and rehabilitation. This section contains information regarding the H reflex, F wave, axon wave, and blink reflex in normal and pathologic states. The H reflex and F wave can often be elicited from the same motor nerve but are distinguished by amplitude and variability of response as well as by stimulation parameters required to evoke them. Both responses allow study of the entire length of a nerve, especially the otherwise inaccessible proximal plexus and intraspinal nerve segments. They are essential in the evaluation of neuropathic diseases. The axon wave is of shorter latency than the F wave and results from proximal axonal sprouting, often in an area of chronic nerve injury due to compression or chronic polyneuropathy with reinnervation. The blink reflex permits study of the facial and supraorbital nerves throughout their length, including their complex brainstem synapses. Technical aspects of all tests are carefully reviewed, and the reader is advised to practice these difficult techniques before applying them in diagnostic situations.