EMG Questions from 2019, 2015, 2013, 2012, and 2011 SAE

In patients with steroid myopathy, the needle electromyographic study usually reveals

• Small motor unit potentials with early recruitment

• Small motor unit potentials with reduced recruitment

• Positive waves and fibrillation potentials in proximal muscles

• Normal motor unit potentials and normal recruitment

Answer explanation

Needle exam in patients with steroid myopathy usually reveals normal insertional and no abnormal spontaneous activity. Usually no motor unit abnormalities. This is because steroid myopathy preferentially affects type 2 muscle fibers which are recruited after type 1 fibers which typically mask observation of the type 2 fibers.

Of the muscles listed, the most distal muscle receiving innervation from the C5 root is

• Brachioradialis

• Pronator quadratus

• Brachialis

• Biceps brachii

A 33-year-old pregnant woman complains of pain in the groin radiating along the medial thigh. Needle exam shows denervation in gracilis and adductor muscles. She most likely has a lesion in the

• Femoral nerve

• Obturator nerve

• Sciatic nerve

• Genitofemoral nerve

Answer explanation

Gracilis, adductor brevis, and adductor longus are supplied by obturator (adductor magnus has some innervation by a branch off of the sciatic nerve).

Which electrodiagnostic finding is most consistent with neurogenic thoracic outlet syndrome?

• Small median motor response from the thenar muscles

• Abnormal response of the lateral antebrachial cutaneous nerve

• Abnormal median sensory responses

• Abnormal spontaneous activity in the pronator teres muscle

Answer explanation

Neurogenic TOS involves the lower trunk of the brachial plexus; hence, sensory and motor loss develops C8-T1. Thumb abduction is often affected. Sensory changes are usually in ulnar and medial antebrachial cutaneous nerves.

Which of the following sensory nerves has fibers that come from the tibial nerve

• Calcaneal

• Lateral sural cutaneous

• Superficial peroneal (fibular)

• Saphenous

Answer explanation

The calcaneal nerve has medial and lateral branches. Medial comes off tibial nerve and lateral off sural. Saphenous is branch off of femoral nerve. Lateral sural cutaneous and superficial fibular come off common fibular.

The best objective evidence of cubital tunnel syndrome is

• Reproduction of symptoms with percussion of the ulnar nerve at the elbow

• Drop of compound motor unit action potential amplitude of ulnar nerve stimulated above elbow

• Positive ulnar neural tension sign

• X-ray showing sclerosis at the medial epicondyle

Answer explanation

This is best objective finding, rest may indicate finding suggestive of cubital tunnel syndrome

When comparing CMAP latency with F-wave latency, which finding is most likely to occur as you move the stimulating electrode more proximal on the limb?

• Both CMAP and F-wave latency decrease

• Both CMAP and F-wave latency increase

• CMAP latency will increase and F-wave latency decrease

• CMAP latency decrease and and F-wave latency increase

Answer explanation

F response is a late response that becomes evident after CMAP response. F-wave is derived from antidromic simulation of a peripheral nerve that propagates an impulse proximally to the anterior horn cells of the spinal cord and returns an orthodomic response to the distal muscle. It is a pure motor response creating a small action potential representing 1-5% of the muscle fibers. Moving stimulator proximal shortens distance to spinal cord resulting in decreased latency of F-wave and increased latency of CMAP as the muscle/recording electrode are further from stimulation.

While performing electrodiagnostic study for carpal tunnel syndrome, you notice median, ulnar, and radial sensory nerve conduction studies demonstrate prolonged peak latencies with increased SNAP amplitudes and durations. What should be evaluated next?

• Additional NCS of lower extremity

• F responses on the median and ulnar nerves

• Limb temperature

• Needle exam

Answer explanation

Prolonged distal latency with increased SNAP amplitudes may be indicative of cool limb temperatures. Effects include slowed nerve conduction velocity, prolonged distal and peak latency, increased SNAP/CMAP amplitude, and phases of MUAP. >32C upper limb and >30C lower limb.

Which electrodiagnostic finding would be most helpful in differentiating critical illness myopathy from critical illness polyneuropathy?

• Normal CMAP amplitudes

• Abnormal spontaneous activity

• Normal SNAP amplitudes

• Decreased CMAP amplitudes

Answer explanation

CIM has normal SAP amplitudes and CIP has axon loss pattern. Both have decreased CMAP and may have abnormal spontaneous activity on NEE.

Which phenomenon occurs when an axon is depolarized

• Intracellular region becomes more negatively charged

• Sodium channels open, allowing sodium to rush out of the axon

• Large anions and potassium permeate into the axon

• Sodium movement is driven by electrical and concentration gradients

Answer explanation

Resting potential is about -90mV. With depolarization an influx of sodium occurs, driven by electrical and concentration gradients. Axon becomes more positive internally. The axon membrane is impermeable to large negatively charged anions and relatively impermeable to sodium ions in resting state. Concentration of sodium is higher outside of the membrane and potassium concentration is higher inside.