Clinical Electrophysiology

1. Understand how events induced by alteration of membrane potential, synaptic transmission and neurotransmitter release can provide clinically relevant information about the functioning of the central and peripheral nervous system.

Neurofilaments provide structural conformity of the axon. Neurotubules maintain bidirectional transport of vesicles containing neurotransmitters and membrane proteins. Injury to the cell body leads to pathological changes in the axon.

In the peripheral nervous system, Schwann cells envelope one or several Schwann cells surrounding the peripheral nerve. The ratio of Schwann cells to neurons in the PNS is much higher than oligodendrocytes to neurons in the CNS. This allows breakdown of peripheral myelin to regenerate completely in 4-6 weeks; in the CNS, myelin loss doesn’t regenerate completely, so deficits accumulate. Death of the axon results in breakdown of meyline but not of the Schwann cell. Loss of myelin doesn’t result in disruption of axon.

Peripheral Axonopathy
Metabolic abnormality, disease, or toxins can result in disruption of the axonal transport system and cause the distal portion of the axon to degenerate. This degeneration spreads back proximally and larger diameter fibers are affected first. The loss of the axon results in muscle atrophy. That is, the integrity of the muscle depends on the axon. Sensory fiber loss such as in diabetes results in loss of sensation; wounds incurred may not be noticed, leading in infection and even osteomyelitis.

Demyelinating Neuropathy
The speed of conduction in the peripheral nerve is dependent on myelin. Demyelinating neuropathy is characterized by the slowing of conduction velocity but with minimal neurogenic changes on electromyography. That is, no muscle atrophy occurs in demyelinating neuropathy because the axon remains intact. Muscle movement after demyelination is slow, week and uncoordinated. Changes in acquired neuropathies are patchy while those in inherited types are more uniform. Demyelinating neuropathy is much like if the insulation is removed from wires and the wires keep short circuiting.

Demyelinating neuropathies include AIDP or Guillian Barre syndrome, CIDP, paraprotinemias (myeloma, MGUS), HIV, lyme disease, diphtheria. Genetic demylelinating neuropathies include HMSN and Refsums disease.

Asymmetric Neuropathy/Entrapment Mononeuropathy
Entrapment of nerves is often asymmetric and often happens at the median nerve at the wrist, ulnar nerve at the elbow, radial nerve in the spiral groove, sciatic nerve in the sciatic notch, and the peroneal nerve at the fibular head.

Neuromuscular Junction Disease
Diseases such as myasthenia gravis are disease at the NMJ. In myasthenia gravis, ACh receptors are lost, resulting in muscle weakness. Many patients have involvement of small muscles first, such as the ocular and bulbar muscles. Symptoms include ptosis, diplopia, dysphagia, and dysarthria. Weakness is episodic, precipitated by exertion, and relieved by rest. As disease progresses, limb and respiratory muscles become involved. Sensations and deep tendon reflexes remain normal.

Muscle Diseases
Muscle disease involves larger muscles first, such as the proximal muscles of the arms and legs. Patients often had difficulty going up stairs, getting up from sitting, and combing hair. However, sensations remain normal while deep tendon reflexes remain normal until late stages. Muscle disease can be inherited or acquired.

Inherited muscle disease are long duration, have positive family history, and a typical distribution of weakness. Inherited muscle diseases or dystrophies include x-linked diseases such as Duchenne and Becker, autosomal dominant such as FSHD, autosomal recessive such as limb girdle dystrophy, metabolic myopathies, and congenital myopathies.

Acquired muscle diseases are shorter in duration and there is usually no family history. Acquired muscle diseases can be inflammatory, infectious such as trichinosis and cysticercosis, pharmacologic through steroids, hypolipidemic agents, and endocrine such as cushing’s disease, hyperthyroidism, and hypothyroidism.

Root Disease
Root diseases are related to spinal disks and are usually asymmetric involvement. Root pains are a characteristic feature and motor and sensory defects occur in the distributionof the affected root. Basically, the annulus fibrosis ruptures and the nucleus pulposus spills out and compresses the roots.

2. Became familiar with techniques used in clinical neurophysiology to study such activity in the central and peripheral nervous systems. These include electromyography (EMG), nerve conduction studies, and recording of CNS potentials evoked by sensory stimulation.

Electrodiagnosis is used to assess dynamic functioning or physiology. Imaging studies reflect static anatomy. Using these two complementary modalities, diagnoses can be improved.

Motor Nerve Conduction
Nerve is stimulated at two points while recording responses over the muscle. By measuring the latency of responses and distance between the stimulation points, one can calculate the conduction velocity. Amplitude, latency, and conduction velocity is compared with normal values. Motor nerves typically studied include the ulnar, medial, peroneal, and tibial nerves.

Sensory Nerve Conduction
Sensory nerve conduction studies use a similar procedure as motor nerve conduction studies, though technically more difficult because sensory neuron action potentials are much smaller than motor neuron action potentials. Measurements can be orthodromic (action potentials traveling down axon away from soma) or antidromic (action potentials traveling up axon toward soma). Sensory nerves routinely studied include the median, ulnar, and sural nerves. Normal conduction velocity is 40-60 m/s.

Late Reponses – H&F reflexes
Nerve conduction studies evaluate only the distal PNS and do not study proximal elements such as roots. Late responses study the proximal PNS but their utility is limited.

Repetitive Stimulation Studies
Repetitive stimulation studies involve repeated stimulation of the nerve. Normally, each stimulation evokes action potentials of the same amplitude. However, in pathology such as myasthenia gravis, each additional stimulation results in a decremental response, decreasing action potential amplitude and muscle fatigue.

Electromyography (EMG)
EMG detects changes in muscle induced by denervation or primary muscle disease. Pathological findings consist of abnormal activity at rest and changes in motor unit potential due to alterations in muscle membrane excitability or motor unit architecture. EMG studies insertional activity, spontaneous activity, and motor unit potential.

In insertional activity, electrical activity is mechanically provoked by insertion of needles. This activity is usually brief (less than 300 seconds). However, this activity maybe prolonged with denervation and reduced in muscle fibrosis.

After the insertional activity subsides, there should be no activity if the patient doesn’t move. However, pathology may cause positive sharp waves, fibrillations, complex repetitive discharges, and fasiculations:

Root disease frequently causes positive sharp waves and fibrillations. Also, denervated muscle will give off spontaneous discharge after a latent period of 10-14 days.

Fibrillations and positive sharp waves occur in denervated single fibers while fasiculations and complex repetitive discharges occur in denervated fiber bundles and more chronic lesions.

Fasiculations are muscle twitches that occur in amyotrophic lateral sclerosis or Lou Gehrig’s disease due to denervation; eventually muscles will atrophy.

Benign fasiculations can occur in normal people and are common in medical students. These fasiculations present with localized twitching and no muscle weakness/atrophy.

Motor Unit Analysis
Motor unit analysis studies amplitude, duration, polyphasia, and recruitment/firing rate. In muscle disease or necrosis, motor fibers in a motor unit can be destroyed, leading to small motor unit potentials reflecting a smaller motor unit. Conversely, denervation of motor fibers can be reinnervated by surviving motor units after injury, producing a giant motor unit and giant motor unit potentials.

3. Learn how to use of such methods aids the clinician in diagnosis, treatment, and prognostication of disease.

Root disease manifests abnormal F waves, H reflexes, neurogenic EMG, and normal nerve conduction. Nerve disease can have axonal or demyelinating patterns: axonal patterns present with muscle atrophy while demyelinating patterns present with decreased conduction velocity but no muscle atrophy. NMJ disease is characterized by decremental response on repeated stimulation. Muscle disease manifests with a myopathic pattern on EMG.

Electrodiagnosis is an extension of clinical exam but a good pertinent history and examination is very important to plan electrodiagnosis studies. Electrodiagnosis can confirm disease of distal motor neurons, establish site of lesion, classify disease process, evaluate treatment, detect subclinical disease, and prognosticate.

Axonal disease recovery is slow with regeneration rate at only 2-3 mm a day, taking months or years, and, sometimes, never completing. Demyelinating neuropathy recovery can be rapid and complete as Schwann cells bring about remyelination.