NCS
During nerve conduction studies, the optimal lower limb temperature is which of the following?
A 33 degrees Celsius
B 32 degrees Celsius
C 31 degrees Celsius
D 30 degrees Celsius
D 30 degrees Celsius
The rhomboids are innervated by which trunk of the brachial plexus?
A Lower
B Middle
C Upper
D None of the above
D None of the above!!!
Explanation: The rhomboids are innervated by C4, C5 dorsal scapular nerve. This occurs just proximal to the brachial plexus - thus, no trunks or cords are involved in their innervation.
When a muscle recruits its first motor unit during initial contraction, it tends to fire at _____ Hz, and subsequent motor units are recruited for every ______ Hz increase in firing rate.
A 10, 15
B 5, 5
C 10, 10
D 5, 10
B 5, 5
Explanation: The initial motor unit is recruited at 5 Hz; every 5 Hz an additional motor unit is added to the recruitment pattern. Thus, your EMG screen should at some point show you 4 motor units firing at once with gradually increased muscle contraction by the patient: these units will be firing at 20, 15, 10, 5 Hz respectively.
A 27-year-old male suffers a gunshot wound to the right arm and presents for EMG/NCS after persistent weakness and sensory loss. Nerve conduction studies show reduced ulnar CMAP amplitude, reduced ulnar SNAP amplitude, and absent medial antebrachial cutaneous nerve sensory responses. Electromyography shows abnormal spontaneous activity and reduced recruitment in the first dorsal interosseous, flexor digitorum profundus to digit 3, and abductor pollicis brevis. There is no abnormal activity in the deltoid, biceps brachii, triceps brachii, or cervical paraspinals. What is the most likely diagnosis?
A C8 radiculopathy
B Posterior cord plexopathy
C Ulnar neuropathy
D Medial cord plexopathy
D Medial cord plexopathy
Explanation: The medial antebrachial cutaneous nerve is a direct branch of the medial cord. Each of the muscles and nerve distributions affected arise from the medial cord (either median or ulnar distributions). None of the other answer choices account for all of the abnormalities within this vignette.
Years following a neurotmesis injury in the forearm, what is the most reasonable expectation on EMG/NCS?
A Normal CMAP
B “Repaired” CMAP
C Absent CMAP
D Equally any of the above
C Absent CMAP
Explanation: Neurotmesis is the complete transection of a nerve. Thus, axons have no connective tissue nerve path through which they may find their former muscle fibers, and the CMAP most likely will not be normal or partially repaired.
Once a patient has reached age 50, the conduction velocity of their nerves will decrease by approximately how many meters per second for each subsequent decade?
A. 15
B. 8
C. 4
D. 2
D. 2
Explanation: After age 50, conduction velocity normally decreases by about 2 m/s per decade.
Which of the following muscles is NOT innervated by the posterior interosseous nerve?
A Extensor digitorum
B Extensor carpi radialis longus
C Extensor indicis proprius
D Extensor carpi ulnaris
B Extensor carpi radialis longus
Explanation: The ECRL is innervated by the radial nerve, not the posterior interosseous nerve (which arises from the radial nerve).
Complex repetitive discharges (CRD) occur via which of the following mechanisms?
A Myopathy
B Potassium depletion
C Ephaptic transmission
D Active denervation
C. Ephaptic transmission
Explanation:A CRD is an involuntary discharge that is very wide and serrated (complex in appearance, and repetitive in firing) and occurs due to a motor unit being denervated and then reinnervated by another motor neuron, which itself then becomes denervated. Ephaptic transmission is the process by which these muscle fibers all fire regularly together. CRDs are seen in chronic radiculopathy, anterior horn cell disease, and some normal patients.
You are reviewing EMG/NCS findings. You note abnormal superficial fibular and sural SNAPs, normal medial and lateral plantar nerve SNAPs, and abnormal CMAP to the Extensor digitorum brevis. The needle EMG results demonstrate decreased recruitment in tibialis anterior (TA), EDB, and fibularis longus, with normal activity in abductor hallucis, gastrocnemius, semimembranosus, biceps femoris, rectus femoris, tensor fascia lata, and lumbar paraspinals. Which of the following is the most likely diagnosis?
A Sciatic neuropathy
B Superficial fibular neuropathy
C Common fibular neuropathy
D Deep fibular neuropathy
C Common fibular neuropathy
Explanation: Common fibular neuropathy is the best answer. Deep fibular neuropathy would spare the fibularis longus (abnormal in this case). Superficial fibular neuropathy would spare the EDB and TA (abnormal in this case). Sciatic neuropathy would show abnormalities in gastrocnemius, abductor hallucis, and plantar SNAPs potentially, as well as potentially hamstring muscles.
A patient presents with numbness in digits 4 and 5 and hand weakness. Nerve conductions of the ulnar nerve recording from the ADM reveal low amplitudes with normal latencies and velocities. Which of the following conductions would be most helpful to more precisely localize the lesion?
A. Lateral antebrachial cutaneous
B. Radial sensory study
C. Median sensory study
D. Dorsal ulnar cutaneous
D. Dorsal ulnar cutaneous
Explanation: The dorsal ulnar cutaneous would be abnormal in ulnar neuropathy at the elbow but normal in ulnar neuropathy at the wrist so would be the best choice to help localize the lesion.
During a nerve conduction study you realize that the patient’s limb being studied is cold. Which of the following effects will this have on the waveform?
A. Increased amplitude, prolonged latency, increased duration
B Increased amplitude, prolonged latency, decreased duration
C Decreased amplitude, shortened latency, decreased duration
D Decreased amplitude, prolonged latency, increased duration
A. Increased amplitude, prolonged latency, increased duration
Explanation: A cold limb will result in increased amplitude, prolonged latency (slow conduction velocity), and increased duration. When the limb is cold, channels stay open longer, causing a larger, heftier amplitude, and longer time of depolarizing.
Which of the following muscles receives its innervation via the following neural pathway?
C7, C8, T1 → middle and lower trunk → medial and lateral cord → median nerve → anterior interosseous nerve.
A Lumbricals 1 and 2
B Flexor carpi radialis
C Flexor digitorum superficialis
D Pronator quadratus
D Pronator quadratus
EXPLANATION: Pronator Quadratus is innervated by the above pathway. FDS, FCR, and lumbricals 1 and 2 are all directly innervated by the median nerve, not the anterior interosseous nerve.
Which of the following EMG/NCS findings is associated with post-polio syndrome?
A Large amplitude CMAPs
B Giant MUAPs
C Absent SNAPs
D Increased recruitment MUAPs
B Giant MUAPs
Explanation: Post-polio syndrome (“burning out” and death of anterior horn cells in a patient with a history of polio who recovered decades ago) is associated with giant MUAPs on EMG. This is essentially because a few anterior horn cells are doing all the work of the contracting the muscles, and so there are very few MUAPs to study, as all the muscle fibers belong to just a few anterior horn cells. SNAPs are unaffected in anterior horn cell disease. CMAP amplitudes are low in post-polio syndrome. Increased recruitment in MUAPs can be seen in myopathys
A 72 year-old male is currently being treated in the ICU for urinary tract infection (UTI) leading to sepsis. Having survived the infection, the patient is ordered physical therapy (PT). However, the patient is unable to participate in PT, citing profound weakness and numbness/tingling. He denies a history of this problem. You are called to perform an EMG/NCS to discover the cause of the patient’s symptoms. You decide to study the right hemibody. NCS reveals normal latencies, but decreased amplitude of median, ulnar, radial, tibial, and fibular SNAPs and CMAPs (sensory nerve and compound motor action potentials). EMG reveals long-duration, large amplitude motor units with decreased recruitment and 2+ fibrillations extensor digitorum brevis, abductor hallucis, tibialis anterior, and first dorsal interosseous. Which of the following is the most likely diagnosis?
A Critical illness neuropathy
B Diabetic polyneuropathy
C Critical illness myopathy
D Chronic inflammatory demyelinating polyradiculopathy (CIDP)
A Critical illness neuropathy
Explanation: This patient presents with critical illness neuropathy (CIN). It usually involves a history of a critical illness, typically a sepsis/SIRS scenario, and as the patient recovers from the medical aspects of this, they begin to complain of new-onset profound weakness with numbness/tingling. CIN is an axonal sensorimotor polyneuropathy; thus, it will decrease the amplitudes of both SNAPs and CMAPs while leaving the latency/conduction velocity largely intact. EMG shows a neuropathic recruitment pattern (long-duration, large amplitude units with decreased recruitment; the axonal loss causes the active denervation reflected as fibrillations and positive sharp waves). Critical illness myopathy presents classically as proximal>distal weakness in a critical illness setting with normal sensation and normal SNAPs, and short-duration, small amplitude, early recruitment motor units on EMG. Diabetic PN manifests chronically in patients with diabetes, not acutely in the ICU setting. CIDP presents much more gradually as well.
During a blink reflex study, you recall that the Vm nucleus is located within which of the following structures?
A Neocortex
B Medulla
C Pons
D Midbrain
C Pons
Explanation: The blink reflex study tests CN V and CN VII in the brainstem and peripherally. CN V has two nuclei being tested (Vm and Vs). The Vm nucleus lies within the pons. It accepts CN V input from the face and conducts it to the ipsilateral CN VII nucleus, causing an ipsilateral blink (R1 response). It also simultaneously conducts the impulse to the medulla where the Vs nucleus lies. Vs accepts the impulse from Vm and sends the impulse to the bilateral CN VII nuclei, which then causes a bilateral blink via the orbicularis oculi muscles (R2 response).
Raising the low frequency filter during nerve conduction studies will cause which of the following?
A No effect
B Increased amplitude
C Prolonged peak latency
D Shortened peak latency
D Shortened peak latency
Explanation: Raising the low frequency filter will shorten the peak latency and decrease the amplitude.
While examining a patient you notice that their right scapula sits more medially than the left scapula. Needle EMG would most likely reveal abnormalities of musculature belonging to which nerve?
A. Thoracodorsal nerve
B. Spinal accessory nerve
C. Dorsal scapular nerve
D. Long thoracic nerve
D. Long thoracic nerve
Explanation: The two nerves associated with winged scapula (this patient has a medially winged scapula) are the long thoracic nerve, which innervates the serratus anterior and whose injury can cause a medially winged scapula, and the spinal accessory nerve, which innervates the trapezius and whose injury can cause a laterally winged scapula. The dorsal scapular nerve innervates the rhomboids, and the thoracodorsal nerve innervates the latissimus dorsi.
During an EMG, with the muscle at rest, you decide to advance your needle until you hear a “seashell” sound. What does this sound represent?
A Ephaptic transmission
B Miniature endplate potentials
C Active denervation
D Fasciculations
B Miniature endplate potentials
Explanation: The “seashell” sound is the sound of miniature endplate potentials (MEPPs), which are heard when the needle is very close to endplates, which is a painful needle location for the patient. You should try to relocate quickly.
A patient’s right upper limb EMG/NCS results are as follows. Routine median, ulnar, and radial sensory nerve conduction studies are normal except for reduced amplitude of the median and radial SNAPs to the thumb. EMG reveals decreased recruitment and 1+ positive sharp waves in supraspinatus, deltoid, biceps, triceps, brachioradialis, and flexor carpi radialis, but normal activity in rhomboids, extensor indicis proprius, flexor digitorum superficialis, abductor pollicis brevis, and first dorsal interosseous. Which of the following is the most likely diagnosis?
A Lateral cord brachial plexopathy
B Upper trunk brachial plexopathy
C C6 radiculopathy
D C5 radiculopathy
B. Upper trunk brachial plexopathy
Explanation: All upper trunk muscles and sensory nerves noted are affected (median SNAP to the thumb is from C6/upper trunk fibers!). Radiculopathy will show normal sensory nerve action potentials (SNAPs). Lateral cord plexopathy would affect biceps, brachialis, coracobrachialis, pronator teres, flexor carpi radialis, flexor digitorum superficialis, pronator quadratus, flexor pollicis longus, pectoralis major, and lateral antebrachial cutaneous nerve. Upper trunk plexopathy is the best answer. Please refer to the innervation chart at the beginning of the Upper and Lower Extremity Peripheral Nervous System Diseases chapters for a detailed innervation guide.
A 72 year old patient is seen in clinic three months after a multilevel anterior cervical discectomy and fusion (ACDF) surgery. On examination, the patient has mild lower extremity weakness with bilateral foot drop, decreased sensation in the hands and feet, and mild hyperreflexia. What would you expect EMG findings to look like in the lower extremities?
A. Reduced amplitude in motor and sensory evoked potentials
B. Myopathic motor unit action potentials
C. Prolonged F waves and prolonged sensory evoked potentials
D. Normal motor unit action potentials with reduced activation
A. Reduced amplitude in motor and sensory evoked potentials
Explanation: The patient most likely has a diagnosis of cervical myelopathy given the physical examination, which includes upper motor neuron findings (e.g. hyperreflexia) as well as his history of ACDF. Electrodiagnostic studies should be essentially normal, including sensory evoked potentials, motor evoked potentials, and motor unit action potentials. As this is a central process, there may be reduced activation.
You are performing an NCS on a patient with hand numbness. NCS reveals no response on SNAP to digit 2, and normal SNAP to digit 5. The CMAP to the abductor pollicis brevis (APB) demonstrates decreased amplitude and prolonged latency. Patient declines an EMG. Which of the following is the best step?
A Ibuprofen, repeat EMG in 6 months to monitor progression
B Hand surgeon referral
C Corticosteroid injection
D Wrist bracing
B Hand surgeon referral
Explanation: This patient with absent median nerve SNAP, reduced median CMAP amplitude, prolonged motor latency, and active denervation to the APB (median nerve-innervated muscle, active denervation as evidenced by fibrillations and positive sharp waves on EMG) should be referred to a hand surgeon for carpal tunnel release. This case represents severe CTS due to active denervation and decreased CMAP amplitude, together representing ongoing axon loss. (moderate to severe cases are usually most appropriate for surgeon referral). Wrist bracing and ibuprofen with EMG monitoring is appropriate for mild cases (prolonged SNAP latency only). Injection is appropriate for mild-moderate cases that fail to improve with conservative treatment (moderate CTS: prolonged SNAP and CMAP latency but normal CMAP amplitude).
Which of the following correctly states the innervation of the flexor digitorum superficialis of the forearm?
A. C6, C7, upper and middle trunk, medial and lateral cord, median nerve
B. C7, C8, middle and lower trunk, medial and lateral cord, median nerve, AIN (ant. interosseous nerve)
C. C7, C8, T1, middle trunk, medial cord, median nerve
D. C7, C8, middle and lower trunk, medial and lateral cord, median nerve
D. C7, C8, middle and lower trunk, medial and lateral cord, median nerve
its just the way it goes
When completing an EMG, you appreciate spontanous discharges of rhythmic motor unit action potentials separated by short periods of silence, creating a "marching" sound on electromyography. These are commonly seen in which of the following?
A. Lumbar radiculopathy
B. Limb-Girdle Muscular Dystrophy
C. ALS
D. Radiation Plexopathy
D. Radiation Plexopathy
Explaination: Myokymic discharges are rhythmic, involuntary bursts of motor unit action potentials (5–60 Hz) separated by short periods of silence, creating a "marching" sound on electromyography (EMG).
Commonly seen in Radiation plexopathy (most common), compression neuropathy, rattle snake venom
You are performing an EMG on a patient with hand numbness. NCS reveals reduced amplitude of the SNAP (sensory nerve action potential) to digit 2 and the CMAP to the abductor pollicis brevis (APB). The SNAP to digit 5 and the CMAP to the abductor digiti minimi are normal. Needle EMG reveals 1+ fibrillations in the APB and flexor carpi radialis (FCR). The remaining muscles, including deltoid, brachioradialis, flexor carpi ulnaris, first dorsal interosseus, and cervical paraspinals, are normal. What is the most likely etiology of this condition?
A. Compression between the two heads of the flexor carpi ulnaris (FCU)
B. Compression underneath the flexor retinaculum
C. Compression underneath the ligament of Struthers
D. Compression within the Arcade of Struthers
C. Compression underneath the ligament of Struthers
Explanation:This case represents findings of median neuropathy at the elbow (due to active denervation observed in the FCR, a median nerve muscle located at the elbow). This can be caused by compression of the median nerve by a tight pronator teres muscle, bicipital aponeurosis, or ligament of Struthers, all in the elbow. The Arcade of Struthers and the two heads of the FCU can compress the ulnar nerve and cause ulnar neuropathy at the elbow. The normal ulnar nerve studies in this case rule out these answer choices. Compression of the median nerve under the flexor retinaculum causes median neuropathy at the wrist, but the +1 fibs and sharps in the FCR rule out median neuropathy at the wrist as the most likely cause of this patient’s symptoms.
In a Klumpke palsy, which of the following electrodiagnostic findings would most likely be present? SNAP: sensory nerve action potential. PIP: proximal interphalangeal joint.
A Negative “OK” sign
B Sensory loss of lateral forearm
C Intact PIP flexion strength
D Normal median nerve SNAP to the thumb
D Normal median nerve SNAP to the thumb
Explanation: Klumpke palsy (lower trunk brachial plexopathy) is a C8-T1/lower trunk plexopathy. Thus, downstream nerves and muscles will be affected, including all intrinsic hand muscles, flexor digitorum superficialis and profundus, flexor carpi ulnaris. Sensory loss occurs in the medial arm, medial forearm (MAC territory), and hand (excluding the median nerve’s C6 fiber innervation of the thumb territory - hence the normal median SNAP to the thumb). Flexor digitorum superficialis is affected, thus causing weak PIP flexion strength. The lateral antebrachial cutaneous nerve (LAC) is innervated by C5-C6 upper trunk fibers; thus, lateral forearm sensation would be normal in this case. The “OK” sign is an anterior interosseous nerve test, whose muscles involve C8 and T1 innervation; thus, the “OK” sign would be expected to be positive in this case.