Neuromuscular

Question 1

Brachial Plexus Diagram

Answer 1

Question 2

Trapezius

Nerve innovation

Action

Answer 2

CN XI, C3, C4.

Question 3

Giant Axonal Neuropathy

Inheritance, age, nerve fiber type, Signs of neuropathy, gait, phenotype, pathognomonic pathology, mutation, prognosis.

Answer 3

rare autosomal recessive

manifests in early childhood

Intermediate filaments of CNS and PNS

Sensorimotor neuropathy (CST and UMN signs), predominately axonal

Optic atrophy leading to vision loss

Gait - walking on the inner edges of the feet

Tightly curled hair

Pathognomonic - large focal axonal swelling that contain tightly packed disorganized neurofilaments.

GAN gene mutation (encodes gigaxonin, involved in cross linking of intermediate filaments.

Progressive and death typically occurs by adolescence.

Question 4

Refsum Disease

Inheritance, type of disorder, pathophysiology, general signs, Neurological signs, treatment

Answer 4

Autosomal dominant

Peroxisomal disorder

Enzyme defect -> involved in fatty acid metabolism - > accumulation of phytanic acid (an intermediate)

Retinitis pigmentosa (with night blindness and visual field constriction), Cardiomyopathy, Skin changes.

Neuropathy, hearing loss, anosmia, ataxia, and cerebellar signs.

Large-fiber sensorimotor neuropathy.

Overriding toes due to a shortened fourth metatarsal may aid in the diagnosis.

Treatment - Reduce dietary intake of phytanic acid.

Question 5

MNGIE

Name, features, pathophysiology

Answer 5

myoneurogastrointestinal encephalopathy (MNGIE)

Features - Intestinal pseudoobstruction, ophthalmoparesis, demyelinating neuropathy.

Pathophysiology - Thymidine phosphorylase gene mutation

Question 6

Abetalipoproteinemia

AKA, Inheritance, Pathophysiology, Dx (Labs), Clinical presentation

Answer 6

Bassen–Kornzweig syndrome

Autosomal recessive

Defective triglyceride transport -> abnormal very low-density lipoprotein secretion

Fat malabsorption -> vitamins A, E, D, and K deficiencies

Dx

Low levels of serum β-lipoprotein and vitamin E in the serum

Peripheral smear shows acanthocytes

Retinitis pigmentosa, neuropathy, and ataxia.

Question 7

Mitochondrial cytopathy syndrome

Clinical presentation, pathophysiology

Other mitchondrial cytopathies

Answer 7

Neurogenic muscle weakness, ataxia, and retinitis pigmentosa syndrome

Adenosine triphosphate 6 gene mutation

Predominantly sensory axonal neuropathy

Other mitchondrial cytopathies

Mitochondrial encephalopathy with lactic acidosis and stroke-like symptoms (MELAS)

Disorders due to polymerase γ mutations

Question 8

KSS

Pathophysiology, onset, clinical features

What is not a prominent feature?

Answer 8

Kearns–Sayre syndrome (KSS)

Mitochondrial DNA mutation

< 20 yo

Retinitis pigmentosa, progressive ophthalmoplegia, cardiac conduction defects, ataxia, myopathy, and hearing loss

Neuropathy is not a prominent feature of KSS.

Question 9

Significant axon loss lesions

Reduce/increase action potential amplitudes?

Increased/preserved/mildly reduced conduction velocities?

Answer 9

Reductions in action potential amplitudes and tend to have preserved or mildly reduced conduction velocities

Question 10

Define F-Wave on EMG

Where is it best obtained?

What is the F-wave always preceded by?

Answer 10

Long latency muscle action potential seen after supramaximal stimulation to a nerve.

Best obtained in the small foot and hand muscles.

F-wave is always preceded by a motor response and its amplitude is rather small, usually in the range of 0.2-0.5 mv.

The electrical impulse travels antidromically (conduction along the axon opposite to the normal direction of impulses) along the motor axons toward the motor neuron, backfiring and then traveling orthodromically (conduction along the motor axon in the normal direction) down the nerve to be recorded at the muscle.

Question 11

3 main measures from NCS

Answer 11

1. Sensory nerve action potential (SNAP) amplitude
2. Sensory latency (onset and peak)
3. Conduction velocity.

Question 12

Define SNAP

Answer 12

SNAP amplitude (uV) = number of axons conducting between the stimulation site and the recording site.

Question 13

Define Sensory Latency

Answer 13

Sensory latency (mS) = time it takes for the action potential to travel between the stimulation site and the recording site of the nerve.

Question 14

Define conduction velocity

Answer 14

Conduction velocity is measured in meters per second and is obtained dividing the distance between stimulation site and the recording site by the latency: Conduction velocity = Distance/Latency.

Question 15

How is a motor nerve conduction study obtained?

Define CMAP and what determines its response?

Answer 15

Stimulating a motor nerve and recording at the belly of a muscle innervated by that nerve.

Compound motor action potential

Depends on the motor axons transmitting the action potential, status of the neuromuscular junction, and muscle fibers.

Question 16

What is routinely assessed and analyzed in a motor NCS?

Conduction velocity calculation and reason?

Answer 16

The CMAP amplitudes, motor onset latencies, and conduction velocities are routinely assessed and analyzed.

The distance between two stimulation sites is divided by the difference in onset latencies of those two sites, providing the conduction velocity in the segment of nerve between the two stimulation sites.

This avoids being confounded by time spent traversing the neuromuscular junction and triggering a muscle action potential (since these are subtracted out).

Question 18

Correlations and associations for sensory and motor responses

Decrease in the amplitudes - >

Prolonged latencies

Slow conduction velocities

Answer 18

In general, for sensory and motor responses:

1. Decrease in the amplitudes correlates with axon loss lesions. 2. Prolonged latencies and slow conduction velocities correlate with demyelination
2. Low amplitudes can result from demyelinating conduction block when the nerve stimulation is proximal to the block.

Question 20

Define H reflex

Site of stimulationa and recording

Direction of impulse travel

Why is the H-reflex test helpful?

Answer 20

The H-reflex is the electrophysiologic equivalent of the ankle reflex (S1 reflex arc)

Stimulate the tibial nerve at the popliteal fossa while recording at the soleus.

The electrical impulse travels orthodromically through a sensory afferent, enters the spinal cord, and synapses with the anterior horn cell, traveling down the motor nerve to be recorded at the muscle.

The H-reflex is the electrophysiologic equivalent of the ankle reflex, which is an S1 reflex, and this test is helpful in the evaluation of S1 radiculopathies.

Question 21

When are large polyphasic motor unit potentials (MUPs) seen?

How is insertional activity recorded and when is it increased/decreased?

How is spontaneous activity assessed?

Answer 21

Chronic neuropathic lesions

Insertional activity is recorded as the needle is inserted into a relaxed muscle.

It is increased in denervated muscles and myotonic disorders, and is decreased when the muscle is replaced by fat or connective tissue and during episodes of periodic paralysis.

Spontaneous activity is assessed with the muscle at rest, and examples include fibrillation potentials, fasciculation potentials, and myokymia and myotonic potentials.

All spontaneous activity is abnormal.

Question 22

Define:

Myokymia potential

Myotonia potential

Answer 22

Myokymia is a type of abnormal spontaneous electrical activity characterized by grouped discharges of motor unit action potentials. Myokymia originates from areas of axonal demyelination. The audio signal of myokymia is ‘marching.

Electrical myotonia is the spontaneous discharge of muscle fibers that waxes and wanes in both amplitude and frequency on electromyography (EMG). Myotonia is thought to be due to increased excitability of muscle fibers, leading to discharge of repetitive action potentials in response to stimulation.

Question 23

How are MUPs obtained?

What is recruitment?

Describe what changes with MUPs in axon loss lesions or myopathic processes?

Answer 23

MUPs are obtained while the needle is inserted into the muscle during voluntary contraction.

Recruitment is a measure of the number of MUPs firing during increased force of voluntary muscle contraction.

In axon loss lesions, reduced recruitment is characterized by a less-than-expected number of MUPs firing more rapidly than expected.

Early or rapid recruitment occurs in myopathic processes with loss of muscle fibers, in which an excessive number of short-duration and small-amplitude MUPs fire during the muscle contraction.

Question 24

MUPs in relation to neuropathic and myopathic disorders.

Answer 24

With poor voluntary effort or with CNS disorders causing weakness, recruitment is reduced with normal MUPs firing at slow or moderate rates, sometimes in a variable fashion.

In neuropathic disorders with denervation and reinnervation, MUPs disclose increased duration and amplitude, and may be polyphasic.

In myopathic disorders, MUPs are of reduced duration and amplitude, and may also be polyphasic.

Question 25

Why are there normal SNAPs in a radiculopathy? Where is the dorsal root ganglion located? What kind of neuron is in the DRG? Define radiculopathy

Answer 25

SNAPs are recorded distal to the lesion, in the postganglionic projections from the dorsal root ganglion. Radiculopathies are usually diagnosed with the needle EMG component The dorsal root ganglion is located just outside the spinal canal within the intervertebral foramen. It has sensory unipolar neurons with preganglionic fibers that extend proximally and enter the spinal cord through the dorsal horns, projecting rostrally in the spinal cord. A radiculopathy occurs from an intraspinal canal lesion resulting in damage of the **preganglionic** fibers. The cell body in the dorsal root ganglia and the postganglionic fibers remain unaffected, and therefore, even though sensory symptoms are prominent, the SNAPs are normal.

Question 26

Axon loss radiculopathy - Define What can be seen on EMG at 3 weeks vs 3-6 months In what direction does reinnervation occur and what muscles are more likely to be successfully innervated?

Answer 26

An axon loss radiculopathy will also injure motor fibers in the intraspinal canal region affecting the respective myotome. 1. This leads to denervation, with fibrillation potentials seen 3 weeks after the onset of motor axon loss, decreased recruitment. 3 to 6 months later: - Large and polyphasic motor unit potentials (MUPs). The presence of these large and polyphasic MUPs is dependent on reinnervation and collateral innervation, typically occurring in a proximal to distal fashion, with proximal muscles more successfully reinnervated as compared to distal muscles.

Question 27

- Describe how to help make the diagnosis of myasthenia gravis with EDx - Are NCS abnormal or normal? - Pathophysiology of myasthenia gravis

Answer 27

Low frequency (2 to 3 Hz) repetitive nerve stimulation demonstrates more than 10% decremental response of compound muscle action potential (CMAP) - Sensory NCS are normal in MG, and motor NCS are usually normal as well - antibody-mediated destruction of postsynaptic nicotinic acetylcholine receptors - The decremental responses occur due to a normal reduction in the release of acetylcholine after subsequent stimulation and the reduced availability of receptors from the disease, leading to a loss of end-plate potentials and reduction of the motor action potentials.

Question 28

What characterstics of comound muscle action potential suggest presynaptic disorders? Provide examples of presynaptic disorders

Answer 28

- Whenever CMAPs are found to be low in amplitude, a presynaptic disorder such as Lambert–Eaton syndrome or botulism should be suspected. - An increment in the CMAP amplitudes after exercise or rapid repetitive stimulation is a feature of a presynaptic disorder, and not of MG.

Question 29

What kind of EDx is required to investigate presynaptic disorders and how does this work?

Answer 29

In presynaptic neuromuscular junction disorders, rapid stimulation with frequencies of 20 to 50 Hz produces an incremental response by overcoming the efflux of calcium (which occurs within 100 to 200 ms)

Question 30

How is Jitter analysis by SFEMG performed? What is a jitter and how does this relate to myasthenia gravis? How is neuromuscular blocking measured this way? Is SFEMG sensitive or specific for myasthenia gravis?

Answer 30

Jitter analysis by single-fiber EMG (SFEMG) is performed by recording with a single-fiber needle electrode positioned to detect potentials from two muscle fibers of the same motor unit. The variability of the interpotential interval between these two potentials is the jitter, and it is abnormal in MG due to delayed neuromuscular transmission. Neuromuscular blocking can also be detected, and is measured by the percentage of discharges in which one of the potentials is missing. SFEMG is highly sensitive but not specific for MG, being frequently abnormal in other neuromuscular junction disorders.

Question 31

Discuss MUPs in relation to myasthenia gravis?

Answer 31

Moment-to-moment variation of the motor unit potentials (MUPs) may be present in MG, meaning that the MUPs vary in amplitude and configuration with successive discharges, due to blocking at some of the neuromuscular junctions of the muscle fibers composing the MUP.

Question 32

LEMS Pathophysiology Associative underylying comorbity

Answer 32

Lambert–Eaton myasthenic syndrome (LEMS) is a disorder of neuromuscular transmission characterized by a reduced release of acetylcholine, and antibodies to presynaptic voltage-gated calcium channels. LEMS is frequently detected as a paraneoplastic syndrome associated with small cell lung carcinoma

Question 33

NCS in LEMS EDx of LEMS Is needle EMG and NCS abnormal or normal in LEMS?

Answer 33

Sensory NCS are normal, but CMAP amplitudes are usually low to borderline low at rest because many fibers fail to reach threshold after a stimulus, given inadequate release of acetylcholine vesicles. Incremental response with rapid repetitive stimulation, given that the calcium availability in the presynaptic terminal is enhanced with repetitive stimulation, resulting in a larger release of quanta of acetylcholine and larger end-plate potentials. - Rapid stimulation (20 to 50 Hz) because the frequency of stimulation is faster than the time it takes for calcium to leave the presynaptic terminal (100 to 200 ms), leading to higher levels of calcium influx and larger end-plate potentials. - The incremental response has to be more than 50% to be considered diagnostic. - Needle EMG is usually normal in LEMS, as are nerve conduction studies.

Question 34

Skeletal muscle types (I, IIa, IIb) - AKA, ATPase activity, glycolytic capacity, oxidative capacity, color and size.

Answer 34

Type 1 muscle fibers, also called slow-oxidative, have slow ATPase activity and large oxidative capacity, with large numbers of mitochondria. They are red in color and small in diameter. Type IIa fibers are also called fast oxidative glycolytic fibers, and have fast ATPase activity, with high glycolytic capacity and moderate oxidative capacity. These fibers are fast and resistant to fatigue. They are red in color and large in diameter. Type IIb fibers are also called fast-oxidative-glycolytic fibers, and have fast ATPase activity, with high glycolytic capacity but low oxidative capacity. These fibers are fast and fatigable. Their color is pale and diameter is large.

Question 35

# Define chronic inflammatory demyelinating polyneuropathy Onset, Time course fo Dx, CSF EDx findings - What if the above is not suggestive of CIDP?

Answer 35

- symmetric demyelinating polyneuropathy presenting with proximal and distal weakness with or without sensory loss and hypo- or areflexia. - 40 and 60 years of age, and it is progressive and/or relapsing, with a time course of at least 8 weeks necessary for the diagnosis to be made - CSF may demonstrate albuminocytologic dissociation - demyelinating polyneuropathy, including prolongation of distal motor latencies, reduction of motor conduction velocities, prolongation of F-wave latencies or absence of F-waves, partial motor conduction block, and abnormal temporal dispersion. - Sensory NCS do not play a major role in the diagnosis of CIDP. - Perform a sural nerve biopsy (evidence of demyelination and remyelination with onion-bulb formation, and sometimes evidence of inflammation.)

Question 36

Time course of GBS Provide time course of subacute inflammatory demyelinating polyradiculoneuropathy

Answer 36

- Symptoms do not progress after 4 weeks - When symptoms relapse after treatment and/or symptom progression extends beyond 4 weeks (but \<8 weeks)