Electrocardiogram and cardiac effects of neurological disorders


ECG, cardiomyopathy, subarachnoid hemorrhage, stroke, QT interval

Changes in electrocardiographic rhythm and morphology and cardiac structure may occur with acute or chronic central nervous system diseases. In subarachnoid hemorrhage (SAH), large upright or deeply inverted T waves and prolonged QT intervals are characteristic. These changes may be mediated by a sympathetic surge associated with hypothalamic involvement and can cause myocardial ischemia, stunning, and infarction (with creatine kinase elevations, troponin elevations, and regional wall motion abnormalities). Electrocardiogram changes can be seen in neuromuscular disorders and muscular dystrophies (e.g., Friedreich ataxia, myotonic dystrophy, mitochondrial disorders, Pompe disease), migraine, brain tumor, head injury, and stroke (both as cause and effect). Many other conditions affect autonomic function (postural orthostatic tachycardia syndrome, Lewy body disease), and the electrocardiogram may be affected by medications that have cholinergic effects or prolong the QT interval. In epileptic patients, cortical stimulation of the left insula leads to bradycardia and depressor effects, but the opposite effect can be seen with right insular stimulation.

Abnormal cardiac rhythms, most commonly atrial fibrillation, are associated with embolic strokes along with increased stroke risk with patent foramen ovale, and cardiomyopathies. The prevalence of atrial fibrillation below age 55 years in the United States is less than 0.5%, rising to 6% for individuals older than 65 years.



Electroencephalography, Alpha rhythm, Beta activity, Mu rhythm, Lambda waves, PLEDs, FIRDA, Sleep spindles

The electroencephalogram (EEG) is the difference in voltage between two different recording locations plotted over time. An EEG signal consists of inhibitory and excitatory postsynaptic potentials of pyramidal cells generated in the brain cortex. This activity reflects the major influence of subcortical structures, especially the brainstem reticular formation and intralaminar and reticular nuclei of the thalamus, generating the three normal states of consciousness: waking, non-rapid eye movement sleep, and rapid eye movement (REM) sleep. EEG is clinically useful in large part because it provides real-time information regarding brain physiology, rather than structure.

  1. I. The normal, adult, waking EEG may contain the following:

    1. A. Alpha rhythm (8 to 13 Hz): Present occipitally in nearly all adults, it appears during relaxed wakefulness with eyes closed and attenuates with eye opening or mental effort. EEG attenuation and reactivity is a sign of better prognosis when evaluating encephalopathy. Posterior rhythm below 8 Hz is considered abnormal after age 8, abiding by the “eight by eight” rule.
    2. B. Beta activity (> 13 Hz): This is a normal finding unless its amplitude consistently exceeds 25 mV, which may suggest the presence of benzodiazepines, barbiturates, or chloral hydrate. Beta is enhanced over skull defects (breach rhythm, sometimes appearing quite “spiky”) and depressed in areas of focal brain injury and over subdural, epidural, or subgaleal fluid collections.
    3. C. Slow wave activity: Theta and delta activity. The presence of theta activity (4 to 7 Hz) waves in an awake adult’s EEG recording is generally considered abnormal—concerning for mild encephalopathy. Their appearance is one of the hallmarks of the onset of drowsiness. Normal elderly individuals may have a limited amount of intermittent temporal theta. Activity slower than 4 Hz (delta) should not be present in the waking adult record.
    4. D. Mu rhythm: A rhythm of alpha frequency that is located centrally. It attenuates with contralateral extremity movement. It originates from the sensorimotor cortex.
    5. E. Lambda waves: Low voltage, occipital sharps that appears only with eyes open, associated with searching eye movements.
    6. F. Features that prompt considerations of normal variants: high frequency spiking (6 Hz and above), monomorphic rhythms (repetitive waves of similar shape and wavelength), and their disappearance during deeper sleep. Other benign patterns appearing during waking or drowsiness include posterior occipital sharp transients of sleep, small sharp spikes/benign epileptiform transients of sleep, positive occipital sharp transients of sleep, rhythmic temporal theta bursts of drowsiness (psychomotor variant), posterior slow waves of youth, subclinical rhythmic electrographic discharge of adults, 14- and 6-Hz positive bursts, 6-Hz phantom spike and wave, and wicket spikes.
    7. G. Hyperventilation (HV) response: A physiologic increase in generalized slowing occurs with prolonged HV, particularly in children, and is accentuated with hypoglycemia. Abnormalities during HV include focal slowing or epileptiform discharges.
    8. H. Photic stimulation response: Normal photomyoclonic responses consist of muscular contractions, typically orbicularis oculi, elicited by each flash. Abnormal photoparoxysmal responses, bursts of generalized epileptiform discharges that may outlast the flash stimuli, are indicative of generalized epilepsy or an inherited EEG trait.

  2. II. The normal, adult, sleep EEG may contain the following:

    1. A. Wakefulness EEG: Features include (1) posterior alpha rhythm is present when eyes are closed, disappears with eye opening; (2) anterioposterior gradient of voltage and frequency. Anteriorly, waves are of lower voltage and higher frequency. Posteriorly, waves are of higher voltage and lower frequency.
    2. B. Drowsiness: Features include subtle slowing of the posterior rhythm and slow roving lateral eye movement of drowsiness.
    3. C. Stage I sleep: Defined by midline sharp waves called vertex waves.
    4. D. Stage II sleep: Marked by the appearance of sleep spindles, rhythmic 12- to 15-Hz waves with a waxing and waning morphology, and K complexes, large biphasic sharp transients maximal over the vertex, often precipitated by external stimuli.
    5. E. Stages III and IV sleep: Defined by the presence of delta waves 2 Hz or slower, greater than 75 μV, occurring between 20% and 50% of a 30-s epoch (stage III) or over 50% (stage IV).
    6. F. REM sleep: Defined by relatively low-voltage desynchronized EEG, muscular atonia, and bursts of REMs.

  3. III. EEG artifacts:

    1. A. Eyeblink artifact: Sharp, downward waveform in the frontal leads (Fp1-F3 and Fp2-F4) with rapid decrease in amplitude in the subsequent electrodes, F3-C3 and F4-C4 respectively.
    2. B. Muscle artifact: Fast wave that “turns the channel black” and does not have an electric field, i.e., not present in adjacent electrodes.
    3. C. Other EEG artifacts include a lateral eye movement artifact, nystagmus artifact, electrode pop, sweat artifact, pulse artifact, 60 Hz artifact, EKG artifact and special movement artifact (hiccup, chewing, and glossokinetic).

  4. IV. EEG abnormalities (Fig. 24i–v)
    Figure 24 Examples of electroencephalogram abnormalities.
    (i) Typical 3 seconds spike and wave seen in absence seizure. (ii) Left mesial temporal epilepsy with interictal focal discharges. (iii) Triphasic waves. (iv) Periodic lateralized epileptiform discharge. (v) Alpha coma. (vi) Burst suppression.

    1. A. Epilepsy: Interictal epileptiform activity, consisting of spikes, sharp waves, or spike-wave complexes, is strongly but not absolutely correlated with epilepsy. Thus, the presence of such activity does not unequivocally indicate a diagnosis of epilepsy, nor does its absence exclude it. Nevertheless, their presence, in combination with clinical information, frequently allows one to make a diagnosis in terms of recognized electroclinical syndromes (see Epilepsy). Ictal discharges, or electrographic seizures, provide irrefutable evidence of an epileptic seizure disorder. In generalized epilepsies, electrographic seizures may consist of a prolonged run of otherwise typical interictal discharges, but this is rarely the case in partial epilepsies, and the ictal patterns have their own morphology that is usually characterized by evolution in frequency and amplitude. The absence of an ictal EEG pattern during a typical, generalized convulsion provides strong evidence of a nonepileptic event (see Epilepsy), but this is less true for auras, focal motor, or sensory seizures, and complex partial seizures.
    2. B. Focal brain lesions: The presence of continuous, focal, polymorphic delta activity, especially in combination with depression of ipsilateral background rhythms, strongly suggests a focal lesion. However, an area of focal dysfunction, as may be seen following a complicated migraine or focal seizure, should also be considered. Periodic lateralized epileptiform discharges (PLEDs) are frequently associated with irritative lesions, such as acute cerebral infarcts or encephalitis.
    3. C. Diffuse encephalopathies: The EEG has a high sensitivity for detecting global cerebral dysfunction but is nonspecific as to etiology. Exceptions include Alzheimer disease and human immunodeficiency virus encephalopathy, in which the EEG may remain normal until late in the course of the disease. Early changes include slowing of the alpha rhythm and the appearance of generalized theta activity; more severe cases show generalized polymorphic delta, frontal intermittent rhythmic delta activity (FIRDA), and a lack of normal reactivity. Triphasic waves are seen in hepatic or other metabolic encephalopathies and may be periodic. Other conditions associated with periodic discharges include Creutzfeldt-Jakob disease (most patients have periodic sharp discharges, occurring with a period of about 1 second, within 12 weeks of diagnosis) and subacute sclerosing panencephalitis (periodic, generalized, slow waves, or sharp and slow complexes, with a period of about 5 to 10 seconds).
    4. D. Coma: Findings include lack of normal background, reactivity, or state changes, in combination with continuous generalized polymorphic delta activity, FIRDA, low-voltage patterns, and periodic discharges. These may include PLEDs or triphasic waves or a burst-suppression pattern (bursts of electrical activity separated by periods of diffuse voltage suppression, indicative of severe diffuse cerebral dysfunction). In patients with a coma following seizures, electrographic status epilepticus should be ruled out. Alpha coma, with generalized, invariant, unreactive alpha frequency activity, is associated with toxic or metabolic insults and cerebral anoxia and must be distinguished from normal alpha rhythms present in those with a “locked-in state.”
    5. E. Brain death: Confirmatory evidence includes the demonstration of electrocerebral silence (ECS), under proper technical conditions, in the appropriate clinical context. Conditions associated with reversible ECS include overdose of central nervous system depressants, hypothermia, cardiovascular shock, metabolic and endocrine disorders, and very young age.


André-Obadia N., et al. Continuous EEG monitoring in adults in the intensive care unit (ICU). Neurophysiol Clin. 2015;45(1):39–46.

Marcuse L.V., et al. Rowan’s primer of EEG. ed 2 London: Elsevier; 2016.

Electrolyte disorders


Sodium, potassium, calcium, magnesium, phosphate. acid base balance, dehydration, serum osmolality

Symptoms are usually more severe with acute changes than chronic alterations in electrolyte levels. Occasionally, chronic disturbances may produce signs and symptoms opposite from the acute state. In general, central nervous system (CNS) dysfunction occurs with abnormalities of sodium, peripheral nervous system dysfunction with abnormal potassium levels, and combinations of both with abnormalities of calcium, magnesium, and phosphate. Management is directed at treatment of the primary disorder and correction of the electrolyte abnormality. Neurologic findings usually disappear with appropriate therapy (Table 51 lists signs and symptoms).

Table 51

Comparison of electrolyte disorders
Electrolyte Disorder Causes Symptoms
Hypernatremia Water loss
Sodium retention
Impaired thirst or access to water
Poor release of ADH
Poor arousability
Seizures (rare)
Hyponatremia Fluid loss (sweat, polyuria, diarrhea, third space)
Fluid overload (CHF, renal failure)
Medications (diuretics)
Iatrogenic (excess intravenous fluids)
Headache, nausea
Muscle cramps
Reduced consciousness
Hyperkalemia Poor elimination (renal failure, mineralocorticoid insufficiency)
Excess potassium release from cells (trauma, burns, status epilepticus)
Rare symptoms
Nonspecific weakness
Hypokalemia Excess gastrointestinal or urinary excretion
Distribution away from extracellular space (insulin, β-agonists)
Hypercalcemia Hyperparathyroidism (primary, secondary, or tertiary)
Psychiatric symptoms
Hypocalcemia Renal failure
Parathyroid deficiency
Acute pancreatitis
Tumor lysis syndrome
Vitamin D deficiency
Critically ill patients
Chvostek sign
Trousseau sign
Altered mental status
Hyperphosphatemia Renal failure
Tumor lysis syndrome
Similar to effect of hypocalcemia (high serum concentration of phosphorous lowers the calcium level)
Hypophosphatemia Impaired absorption
Refeeding syndrome
Chronic alcoholism
Medications (catecholamines, thiazide diuretic, antacid)
Perioral paresthesias
Inability to wean from ventilator
Hypermagnesemia Excess magnesium intake (patients with eclampsia or renal failure) Nausea, vomiting
Dry mouth
Generalized weakness, hyporeflexia (severe)
Hypomagnesemia Critically ill patients
Low magnesium intake
Poor absorption
Excess excretion by kidneys (e.g., diuretics)
Tachycardia, sweating, dilated pupils
Seizures (severe)

ADH, Antidiuretic hormone; CHF, congestive heart failure; SIADH, syndrome of inappropriate secretion of antidiuretic hormone.

Adapted from Hocker, S. E. (2015). Electrolyte disturbance and acid-base imbalance. In K. D. Flemming & L. K. Jones (Eds.), (2015). Mayo Clinic Neurology Board review: clinical neurology for initial certification and MOC (pp. 741–745). Oxford University Press.


Sodium is the main determinant of serum osmolality (Osm) and extracellular fluid volume. Neurologic symptoms are dependent on the time lag necessary for the brain to compensate for rapid changes in serum Na+ concentration, and Osm.

  1. I. Hyponatremia: Acute decreases of Na+ levels to 130 mEq/L may produce symptoms, whereas chronic changes to 115 mEq/L may be asymptomatic. Acute hyponatremia (< 115 mEq/L) with seizures carries a high mortality rate and necessitates rapid (over a 6-hour period) correction to 120 to 125 mEq/L (with hypertonic saline or normal saline and furosemide). Rapid correction to levels greater than 120 to 125 mEq/L may result in central pontine myelinolysis, a disorder described in alcoholics but also occurring in children and adults with liver disease, severe electrolyte imbalances, malnutrition, anorexia, burns, cancer, Addison disease, and sepsis.
    There is symmetrical focal myelin destruction predominantly involving the basal central pons. Asymptomatic chronic hyponatremia usually requires no immediate intervention and is managed by correction of the underlying condition.
  2. II. Hypernatremia: Neurologic symptoms develop when the serum Na+ level rises above 160 mEq/L or serum Osm is greater than 350 mOsm/kg. Level of consciousness correlates well with the degree of hyperosmolality. Sudden increases in serum Osm may produce decreased brain cell volume, with mechanical traction on cerebral vessels causing subcortical, subdural, or subarachnoid hemorrhage. Cerebrospinal fluid protein levels may be high without pleocytosis, and the electroencephalogram (EEG) is normal or mildly slowed. Hypernatremia resulting from diabetes insipidus may occur with tumors involving the hypothalamus or pineal region, as well as with basilar meningitis, encephalitis, ruptured aneurysms, sarcoidosis, trauma, or surgery. Treatment for hypernatremia includes isotonic solutions to reduce the serum Na+ level by no more than 1 mEq/L every 2 hours during the first 2 days of treatment. Rapid infusion of hypotonic solutions may cause cerebral edema and seizures.


Almost 60% of total body K+ is located within muscle; therefore, predominantly muscular symptoms occur with altered K+ levels.

  1. I. Hypokalemia most commonly occurs with diuretic use but also occurs with gastrointestinal losses, mineralocorticoid excess, and rarely, thyrotoxicosis. Muscle weakness usually develops with serum levels of 2.5 to 3.0 mEq/L, with structural muscle damage occurring at levels below 2.0 mEq/L. Hypokalemia and hypocalcemia frequently coexist, with cancellation of neuromuscular manifestations. Treatment of one condition in isolation may produce symptoms of the other. Electrocardiogram (ECG) and cardiac abnormalities are common and may require intensive care unit (ICU) monitoring and treatment. Treatment includes increasing dietary K+, supplements of potassium chloride (KCl), and the use of K+-sparing diuretics.
  2. II. Hyperkalemia is relatively uncommon but may occur in familial hyperkalemic periodic paralysis (see Periodic Paralysis). Quadriparesis may develop with levels greater than 6.8 mEq/L, and levels greater than 7.0 mEq/L are life-threatening due to cardiac toxicity. Potassium levels greater than 6.0 mEq/L require ICU monitoring and immediate therapy with administration of glucose and insulin, cation exchange resins, or calcium gluconate.


Plasma Ca2+ is a stabilizer of excitable membranes in the central and peripheral nervous systems and in muscle. Ca2+ concentrations are closely controlled through the combined effects of parathyroid hormone, calciferol, and calcitonin on intestine, kidney, and bone.

  1. I. Hypocalcemia occurs in neonates, patients with renal failure, and after thyroid or parathyroid surgery. The “tetany syndrome” originates in the peripheral nerve axon and initially becomes evident with distal and perioral tingling. Distal tonic spasms (carpopedal spasms) may progress to laryngeal stridor and opisthotonus if severe. The EEG is diffusely slow with an exaggerated response to photic stimulation. ECG abnormalities are also common. Treatment consists of oral calcium supplements. Acute treatment of hypocalcemia with tetany or seizures may require 10% IV solutions of calcium gluconate or CaCl. Underlying disorders should be corrected, if possible. Hypocalcemia often coexists with hypomagnesemia. In such cases, total serum calcium levels may be normal, but ionized calcium levels may be low.
  2. II. Hypercalcemia: Malignant neoplasms are the most common cause of increased serum Ca2+ levels. Mental status alterations occur with total serum levels greater than 14 mg/dL. Serum calcium levels need to be adjusted for serum albumin levels which are often low in chronically ill patients. Myopathy or carpal tunnel syndrome may occur in association with hyperparathyroidism.

Treatment: Saline hydration and furosemide are recommended. Occasionally, mithramycin (suppresses bone resorption) or calcitonin (suppresses bone resorption and increases urinary Ca2 + excretion) are required.


Ninety-eight percent of Mg2 + is intracellular. Magnesium is necessary for the activation of various enzymes. Extracellular Mg2+ affects central and peripheral synaptic transmission. Acute changes in serum levels may not reflect total body stores.

  1. I. Hypomagnesemia occurs most commonly as a result of excess renal loss (chronic alcoholism, diuretics), but may also be the result of decreased intake or absorption. Neurologic symptoms usually develop at levels below 0.8 mEq/L. The presence of seizures requires treatment with parenteral MgSO4. Oral Mg2 + supplements may suffice in less severe cases. Calcium gluconate should be available when giving IV MgSO4, as transient hypermagnesemia may cause respiratory muscle paralysis (see also hypocalcemia, above).
  2. II. Hypermagnesemia, an uncommon disorder, usually occurs with increased intake and renal failure. Deep tendon reflexes may be lost at levels of 5 to 6 mEq/L, and CNS depression occurs at levels above 8 to 10 mEq/L. Muscular paralysis is due to neuromuscular blockade.

Paralysis treatment may be accomplished by small amounts of parenteral calcium gluconate and hydration. Otherwise, discontinuation of Mg+-containing preparations is indicated. If renal function is severely impaired, dialysis may be necessary. Magnesium infusions are often given as treatment for seizures associated with eclampsia. Serum magnesium levels need to be closely monitored in this situation.


Hypophosphatemia is often complicated by multiple abnormalities of electrolytes, nutrition, and acid-base balance. The syndrome commonly occurs in malnutrition and chronic alcoholism, especially after the infusion of glucose or hyperalimentation solutions.

Acute hypophosphatemia may not reflect decreased total body stores and may produce neurologic symptoms if severe (< 1.5 mEq/L). Chronic hypophosphatemia is usually moderate (1.5–2.5 mEq/L) and may not be symptomatic unless acute stresses (alcohol withdrawal, burns, binding of phosphate in the gut) cause sudden decreases below the moderate level.

Electromyography and nerve conduction studies


Electromyography (EMG), nerve conduction studies (NCS)

Electrodiagnostic testing of nerve and muscle with nerve conduction studies (NCS) and electromyography (EMG) is used to localize lesions in the peripheral nervous system, to differentiate primary nerve and muscle disorders, to provide insight for underlying pathophysiology of peripheral nervous system disorders, and to assess its severity and temporal course.

Nerve conduction studies

NCS are performed by recording action potentials with a surface electrode over the skin (Table 52). Both motor and sensory components of nerves can be studied.

Motor NCS involve stimulating a peripheral nerve and recording the action potential from a muscle innervated by that nerve—because this induced action potential conducts in the same direction as physiologic motor nerve signals, it is referred to as “orthodromic.” Compound muscle action potential (CMAP) is signal recorded as results from depolarization of all muscle fibers innervated, hence it is termed compound.

Sensory NCS are usually done antidromically, by stimulating a peripheral nerve proximally and recording from a distal site innervated by that nerve; for some sensory nerves, orthodromic recording is also possible. The recorded response is called a sensory nerve action potential (SNAP).

The amplitude, duration, shape, and latency of CMAPs or SNAPs are all noted for comparison to expected normalized values or morphology. Conduction velocities are calculated for SNAPs by dividing the distance between stimulating and recording electrodes by the time required for action potential conduction. For CMAPs, because the latency recorded includes not only nerve transmission velocity but also neuromuscular junction transmission time, conduction velocity is calculated by dividing the distance between two stimulation sites (proximal and distal) by the difference in conduction time between the distal stimulus site and the recording site. Normal values of NCS vary with different physiologic factors, most importantly with temperature and age. Normal nerve conduction velocities are approximately 50 m/second in the upper limbs and 40 m/second in the lower.

Repetitive nerve stimulation is another form of electrodiagnostic testing that is helpful in the diagnosis of neuromuscular junction disorders. Repetitive stimulation focuses on the change in the CMAP amplitude, if any, that results from stimulating the motor nerve multiple times per second. With slow (3 Hz) repetitive stimulation, more than a 10% drop in CMAP amplitude is often seen in both myasthenia gravis and Lambert-Eaton myasthenic syndrome (LEMS). With fast (50 Hz) stimulation, an increase in CMAP amplitude of over 50% would be diagnostic of a presynaptic neuromuscular junction disorder such as LEMS or botulism. This increment is not seen in myasthenia gravis. The order of testing in suspected neuromuscular junction disease is therefore typically first slow then fast repetitive stimulation. These studies are challenging to perform: acetylcholinesterase medications must be withheld, tissue temperature carefully attended to, and for anatomic and technical reasons the study is usually done on facial nerve and musculature or the spinal accessory nerve and trapezius. Since 50 Hz stimulation is quite painful, a more commonly used test is to evaluate the increase in CMAP amplitude after 10 seconds of maximal muscle contraction. Single-fiber EMG is the most sensitive test of NMJ transmission (Table 53). Jitter is the variability in interpotential difference between two muscle fiber action potentials during consecutive discharges of the same motor unit. Increased jitter is present in patients with NMJ abnormalities, but cannot differentiate between MG and LEMS.

F waves are low-amplitude late responses due to antidromic activation of motor neurons (anterior horn cells) following peripheral nerve stimulation, which then cause orthodromic impulses to pass back along the involved motor axons, also called backfiring of axons. It is called the F wave because it was first noted in intrinsic foot muscles. The latency of the F wave is usually 25 to 32 milliseconds in the upper limbs and 45 to 56 milliseconds in the lower. F waves are useful for evaluating peripheral neuropathies with predominantly proximal involvement, such as the acute and chronic inflammatory demyelinating polyneuropathies, in which distal conduction velocities may be normal early in the disease. The H reflex is the other clinically recordable late response. It is the electrical homologue of checking ankle-jerk reflex on physical exam. It is the result of a monosynaptic reflex arc with an afferent component mediated by large, fast-conducting group 1a fibers, and an efferent component mediated by alpha motor neurons. For practical and anatomic reasons, the only routinely tested H reflex is in the S1 segment, recorded after tibial nerve stimulation in the popliteal fossa. Typical latency is 30 milliseconds. The loss of the H reflex is nonspecific—it can be simply a function of age (it is normally absent over age 60), or it can result from a very large number of disorders.


EMG records electrical activity in individual and collective muscle fibers, yielding more information on the localization and pathophysiology of peripheral nervous system disorders. NCS on the other hand record electrical activity from entire nerves at once and cannot study individual neurons. An EMG evaluation requires the examiner to carefully select the appropriate muscles to test on the basis of a thorough history and physical examination, and results of NCS.

For each of the muscles being studied, the first part of the examination is to assess insertional and spontaneous activity at rest. Once the insertional and spontaneous activity has been assessed, the examiner will ask the patient to slowly contract the muscle, and the motor unit action potentials (MUAPs) are evaluated. MUAPs are assessed for duration, amplitude, and numbers of phases. Then, the number of MUAPs and their relationship to the firing frequency (recruitment and activation pattern) are evaluated.

Insertional activity

Insertional activity occurs when a needle is quickly moved through the muscle and creates depolarization of muscle fibers which is visualized on the monitor as high-frequency, positive and negative spikes (with an associated “crisp” noise). Normal insertional activity typically lasts only a few hundred milliseconds. Decreased insertional activity is seen in muscle atrophy because fewer muscle fibers are available to respond to needle insertion. Any electrical activity lasting longer than 300 milliseconds is considered increased insertional activity which may be seen in neuropathic disorders that result in denervation and several myopathic conditions that result in necrosis of the muscle fibers, such as inflammatory myopathies.

Spontaneous activity

Recognition of abnormal spontaneous activity can provide helpful information for the diagnosis:

  1. 1. The distribution of abnormal spontaneous activity may suggest the neuroanatomic localization of the lesion (e.g., mononeuropathy, radiculopathy).
  2. 2. Certain types of spontaneous activity are associated with specific disorders, for example, myotonic discharges in myotonic dystrophy and hyperkalemic periodic paralysis (see below).
  3. 3. The amount of spontaneous activity or the presence of spontaneous activity may provide information regarding the time course and severity of the lesion (e.g., presence of fibrillation potentials begins 2 to 3 weeks after the acute nerve injury).

Spontaneous activity may originate from individual muscle fibers, or from entire motor units.

Fibrillation potentials and positive sharp waves are brief, rhythmic discharges from individual denervated muscle fibers, and indicate acute or ongoing impaired innervation. They are not seen on EMG testing until 2 weeks or more after denervation occurs. They are usually graded on a scale from zero to four, where zero means that no such potentials are present and 4 + means that these spontaneous potentials fill the entire screen. They can be seen in neurogenic disorders (neuropathies, radiculopathies, motor neuron disease, etc.), myopathic disorders (especially in inflammatory myopathy and muscular dystrophies), and in severe disorders of the neuromuscular junction (such as botulism or therapeutic chemodenervation with botulinum toxin).

Complex repetitive discharges (high-frequency, regular-firing, multiserrated repetitive discharges with abrupt onset and termination, creating a characteristic “machine-like” sound) result from the depolarization of a single muscle fiber followed by ephaptic spread to adjacent denervated fibers. This occurs in a wide variety of chronic neurogenic disorders (poliomyelitis, motor neuron disease, radiculopathies, and neuropathies) and myopathic disorders (Duchenne and limb-girdle dystrophy, polymyositis, and hypothyroidism). These potentials are usually not seen on EMG testing until 6 months or more after an injury.

Myotonic discharges are characterized by waveforms with waxing and waning amplitude and frequency, creating a “dive bomber” sound on the recording. They arise from single muscle fibers, but many fibers may fire myotonic discharges simultaneously. They are typically seen in myotonic dystrophy, myotonia congenita, paramyotonia congenita, hyperkalemic periodic paralysis, acid maltase deficiency, diazocholesterol toxicity, clofibrate toxicity, and rarely in polymyositis and colchicine toxicity.

Fasciculations are random single spontaneous discharges from a whole motor unit. On EMG, fasciculations have the morphology of single MUAP. They can appear as normal MUAPs, or they can be complex and large if they represent a pathologic motor unit. Fasciculations are nonspecific, and can be seen in radiculopathies, entrapment neuropathies, motor neuron disease such as amyotrophic lateral sclerosis (ALS), metabolic disorders such as thyrotoxicosis, and anticholinesterase overdoses. Although fasciculations involving superficial muscles can be visible on clinical examination, EMG helps record fasciculations from deeper muscles that are not clinically visible.

Myokymic discharges are rhythmic, grouped, spontaneous repetitive discharges of the same motor unit. These repeating bursts of MUAPs have a characteristic sound like that of soldiers marching. They may be recorded in facial muscles (facial myokymia) associated with Bell palsy or brainstem lesions resulting from multiple sclerosis, brainstem glioma, or vascular disease. Appendicular myokymia is associated with radiation plexopathy. Rarely, it may be seen in Guillain-Barré syndrome, radiculopathy, chronic entrapment neuropathy, and gold toxicity.

Neuromyotonic discharges are high-frequency (150–250 Hz) decrementing, repetitive discharges of a single motor unit that create a characteristic “pinging” sound on EMG recording. These are rare and are seen only with chronic neuropathic diseases (e.g., poliomyelitis and adult-onset spinal muscular atrophy) and syndromes of continuous motor unit activity, such as in Isaac syndrome.

Cramps are sustained involuntary muscle contractions caused by the activation of multiple motor units that occur in normal subjects (especially in distal lower extremity muscles) and in many neurogenic and metabolic disorders, including ALS, electrolyte imbalances, hypothyroidism, pregnancy, and uremia (see Cramps). Electrically, cramps are high-frequency discharges of motor units.

Voluntary motor unit potentials

Once the muscle has been assessed for insertional and spontaneous activity, the motor units are analyzed by asking the patient to slowly contract the muscle. The pattern of MUAP abnormalities will allow determination of whether the disorder is a neuropathic or a myopathic process and often helps to ascertain the time course and severity of the lesion. Assessment of MUAPs involves evaluation of the morphology or shape of individual units, and the timing of unit firing—that is, MUAP recruitment and activation.

The morphology of MUAPs provides much information into the health of the muscle being studied. Although the normal appearance of MUAPs will vary slightly from muscle to muscle, a typical MUAP is about 5 to 15 milliseconds in duration, between 0.1 and 2 mV in amplitude, and has 2 to 4 phases.

Short-duration, small-amplitude, polyphasic MUAPs occur in disorders with atrophy or loss of muscle fibers in the motor unit. Thus, they are present in myopathic disorders and in severe cases of neuromuscular transmission disorders (e.g., botulism). In early reinnervation, after severe denervation in which the newly sprouting axons only begin to reinnervate a few muscle fibers, the MUAP will also be small, short duration, and polyphasic but with reduced recruitment (“nascent” MUAP).

Long-duration, large-amplitude, polyphasic MUAPs occur with increased number or density of muscle fibers, or a loss of synchrony of fiber firing within a motor unit such as in chronic neuropathic processes (e.g., motor neuron disease, chronic radiculopathies, chronic axonal neuropathies, and chronic entrapment neuropathy).

MUAPs are considered polyphasic if they have 5 or more phases. Polyphasia is a measure of synchrony of the firing of muscle fibers within the same motor unit. This is a nonspecific measure and may be abnormal in both myopathic and neuropathic disorders. In normal muscles, up to 5% to 10% of MUAPs may be polyphasic (up to 25% in the deltoid).

Unstable MUAPs are MUAPs that change in morphology from one instance to the next. This may occur due to blocking of individual muscle fiber action potentials within the motor unit. This may be seen in disorders of neuromuscular transmission, myositis, muscle trauma, reinnervation, and rapidly progressive neurogenic atrophy.

The temporal characteristics of MUAP recruitment and activation are also important in the analysis of an EMG. Recruitment and activation are two different processes that the nervous system uses to increase the force of a contraction.

In activation, individual motor units are driven to fire at a faster rate. Poor activation of MUAPs is recognized by motor units firing slowly. As this process is centrally mediated, reduced activation is attributable to upper motor neuron lesions or lack of effort.

Recruitment refers to the orderly addition of motor units as activation increases. Decreased recruitment presents as a small number of units firing with a high frequency. Decreased recruitment occurs when there is a decreased number of available motor units; the remaining motor units will fire at a faster frequency to increase the muscle force. This occurs in any peripheral neuropathic process, including neuropathies, radiculopathies, motor neuron disease, and trauma. The term “early recruitment” is used to describe the recruitment pattern seen in myopathies; when the force generated by each individual motor unit is decreased, more motor units must be recruited to generate the same amount of force.

The normal firing rate of most motor units, before additional units are recruited, is 10 Hz. Recruitment ratio is another term used to describe the firing rate of a motor unit. This ratio is the rate of firing of the most rapidly firing motor unit (in Hz) divided by the number of units firing. A recruitment ratio of over 8 is considered abnormal and suggests a neurogenic process.

Embryology of the congenital malformations of the brain and spine


AVM embryology, Embryology, Congenital defects, encephalocele, anencephaly, meningocele

  1. I. Dorsal induction

    1. A. Primary neurulation: 3 to 4 weeks’ age of gestation (AOG); notochord and chordal mesoderm induce neural plate, neural plate closes forming neural tube, and tube closes beginning at medulla and proceeds rostrally and caudally. Defects at this stage may cause craniorachischisis; myeloschisis (failure of closure of neural tube or vertebral arch); anencephaly (absent calvaria and brain, brainstem and cerebellum present); encephalocele (meninges and brain parenchyma protrude through skull defect); myelomeningocele (meninges and spinal cord protrude through defect in vertebral arch); Chiari malformation; and hydromyelia (focal dilation of central spinal cord canal).
    2. B. Secondary neurulation: 4 to 5 weeks’ AOG; notochord and mesodermal interactions form dura, pia, vertebrae, and skull. Defects at this stage may cause myelocystocele; diastomyelia (splitting of spinal cord by mesodermal band); meningocele/lipomeningocele; lipoma; dermal sinus with or without cyst; tethered cord/tight filum terminale; anterior dysraphic lesions (neurenteric cyst); and caudal regression syndrome.

  2. II. Ventral induction: 5 to 10 weeks’ AOG; prechordal mesoderm induces face and forebrain; cleavage of prosencephalon; formation of optic vesicles and olfactory bulbs/tracts; telencephalon gives rise to cerebral hemispheres, ventricles, caudate, and putamen; diencephalon gives rise to thalami, hypothalamus, and globus pallidus; rhombencephalon gives rise to cerebellar hemispheres and vermis; myelencephalon gives rise to medulla and pons. Defects at this stage may cause holoprosencephaly (failure of cleavage of embryonic forebrain into paired cerebral hemisphere with absence of the interhemispheric fissure); septo-optic dysplasia (rudimentary septum pellucidum, hypoplasia of optic nerve and chiasm); arhinencephaly; olfactory bulb and tract aplasia; facial anomalies; cerebellar hypoplasias/dysplasias (Joubert syndrome, rhombencephalosynapsis, tectocerebellar dysplasia); and Dandy-Walker malformation (enlarged posterior fossa, hyogenesis or agenesis of the cerebellar vermis, and cystic dilatation of the fourth ventricle).
  3. III. Neuronal proliferation, differentiation, and histogenesis: 2 to 4 months’ AOG; germinal matrix forms at 7 weeks; cellular proliferation forms neuroblasts, fibroblasts, astrocytes, and endothelial cells; choroid plexus is formed; cerebrospinal fluid (CSF) production begins. Defects at this stage may cause microcephaly; megalencephaly; aqueductal stenosis; arachnoid cysts; and congenital vascular malformations.
  4. IV. Cellular migration: 2 to 5 months’ AOG; neuroblasts migrate from germinal matrix along radial glial fibers; cortical layers form from deep to superficial; gyri and sulci form; commissural plates form corpus callosum and hippocampal commissure. Defects at this stage may cause schizencephaly (lateral clefts through cerebral hemispheres extending from cortex to ventricles); lissencephaly (absence of gyri); pachygyria (abnormally wide and thick gyri); micro/polymicrogyria (small gyri with increased number and abnormal lamination); heterotropias (ectopic collections of gray matter); Lhermitte-Duclos syndrome (diffuse enlargement of the cerebellar cortex); and agenesis of the corpus callosum.
  5. V. Neuronal organization: 6 months postnatal; neuronal alignment, orientation, and layering; dendrites proliferate; synapses form.
  6. VI. Normal myelination begins during the fifth fetal month. It proceeds in a highly predictable and orderly manner: caudal to cephalad, dorsal to ventral, and central to peripheral. Sensory tracts myelinate first.

    1. A. Birth (full term): medulla, dorsal midbrain, inferior and superior cerebellar peduncles, posterior limb of internal capsule, and ventrolateral thalamus.
    2. B. One month: deep cerebellar white matter, corticospinal tracts, pre/postcentral gyrus, optic nerves, and tracts.
    3. C. Three months: brachium pontis, cerebellar folia, ventral brainstem, optic radiations, anterior limb of internal capsule, occipital subcortical U fibers, and corpus callosum splenium.
    4. D. Six months: corpus callosum genu, paracentral subcortical U fibers, and centrum semiovale (partial).
    5. E. Eight months: centrum semiovale (complete except some frontotemporal areas) and subcortical U fibers (complete except for most rostral frontal areas).
    6. F. Eighteen months: essentially like adults.
    7. G. Twenty years: peritrigonal region. Defects at this stage may be due to metabolic, demyelinating, and dysmyelinating disorders.

  7. VII. Acquired degenerative, toxic, or inflammatory lesions may occur at any stage, causing injury to otherwise normally formed structures. These may result in defects such as hydranencephaly (remnant cerebral hemisphere is a paper-thin membrane sac composed of glial tissue filled with CSF covered with leptomeninges); hemiatrophy; multicystic encephalomalacia; or periventricular leukomalacia.
Aug 12, 2020 | Posted by in NEUROLOGY | Comments Off on E
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