EMG EMG Clinical Applications & Pathology 1 — Questions and Answers
Question 1: Which EMG finding is most characteristic of amyotrophic lateral sclerosis (ALS)?
- Widespread active denervation with fasciculation potentials in multiple limbs and the tongue (Correct answer)
- Purely sensory nerve conduction slowing with normal needle EMG
- Isolated proximal muscle fibrillations limited to one limb
- Normal EMG with prolonged F-wave latencies only
Correct answer: Widespread active denervation with fasciculation potentials in multiple limbs and the tongue
ALS produces widespread active denervation (fibrillations, fasciculations) across multiple body regions including bulbar muscles, reflecting both upper and lower motor neuron involvement.
Question 2: In carpal tunnel syndrome, which nerve conduction finding is typically the earliest and most sensitive indicator?
- Prolonged median sensory peak latency across the wrist (Correct answer)
- Absent median motor CMAP
- Prolonged ulnar distal motor latency
- Reduced sural sensory amplitude
Correct answer: Prolonged median sensory peak latency across the wrist
Prolonged median sensory peak latency across the wrist segment is the earliest and most sensitive electrodiagnostic finding in carpal tunnel syndrome.
Question 3: What pattern of abnormality on nerve conduction studies is most consistent with Guillain-Barré syndrome (AIDP subtype)?
- Prolonged distal latencies, slowed conduction velocities, conduction block, and temporal dispersion (Correct answer)
- Reduced CMAP amplitudes with normal conduction velocities
- Absent sensory responses with preserved motor conduction
- Normal motor studies with reduced sensory amplitudes only
Correct answer: Prolonged distal latencies, slowed conduction velocities, conduction block, and temporal dispersion
AIDP (the most common GBS subtype in the US) is a demyelinating neuropathy characterized by prolonged distal latencies, slow conduction velocities, conduction block, and temporal dispersion.
Question 4: Which myopathy typically shows short-duration, low-amplitude, polyphasic motor unit potentials (MUPs) with early recruitment on needle EMG?
- Inflammatory myopathy such as polymyositis (Correct answer)
- Chronic neurogenic disorder such as spinal muscular atrophy
- Acute denervation injury
- Neuromuscular junction disorder such as myasthenia gravis
Correct answer: Inflammatory myopathy such as polymyositis
Inflammatory myopathies such as polymyositis produce myopathic MUPs (short duration, low amplitude, polyphasic) with early recruitment due to loss of individual muscle fibers.
Question 5: A patient with bilateral foot drop, absent ankle reflexes, and stocking-glove sensory loss most likely has which electrodiagnostic pattern?
- Length-dependent polyneuropathy with distal sensorimotor involvement (Correct answer)
- Bilateral L5 radiculopathy
- Bilateral common peroneal mononeuropathy
- Lumbosacral plexopathy
Correct answer: Length-dependent polyneuropathy with distal sensorimotor involvement
Stocking-glove distribution with absent ankle reflexes is the hallmark of length-dependent polyneuropathy, where the longest nerve fibers are affected first.
Question 6: Which EMG finding best differentiates a C7 radiculopathy from a median nerve lesion at the elbow?
- Abnormal spontaneous activity in the cervical paraspinal muscles (Correct answer)
- Prolonged median motor distal latency
- Reduced median sensory nerve action potential amplitude
- Absent H-reflex
Correct answer: Abnormal spontaneous activity in the cervical paraspinal muscles
Abnormal spontaneous activity in cervical paraspinal muscles (which are innervated proximal to the plexus) localizes the lesion to the root level, distinguishing radiculopathy from a peripheral nerve lesion.
EMG EMG Clinical Applications & Pathology 1
Which EMG finding is most characteristic of amyotrophic lateral sclerosis (ALS)?