EDX Board Review

Front

A 66 year-old male develops right leg pain and weakness following a hospital stay for acute coronary syndrome requiring cardiac stenting. On exam he demonstrates isolated knee extension weakness. On EMG/NCS he demonstrates abnormal CMAP to the rectus femoris. Needle EMG reveals decreased recruitment and 1+ fibrillations in the vastus medialis. In order to cinch the diagnosis, you decide to perform additional studies, and expect to find which of the following findings?

Back

Decreased amplitude of saphenous SNAP

Answer explanation

This patient presents with femoral neuropathy due to cardiac catheter trauma (inserting the catheter into the femoral artery - next to the femoral nerve). The femoral nerve terminates as the saphenous nerve, supplying skin innervation to the medial leg below the knee. Thus, the saphenous nerve SNAP being abnormal (decreased amplitude would be expected) would nicely complete the diagnosis of femoral neuropathy in this patient.

Front

Normal upper and lower extremity conduction velocities are at least which of the following, respectively, in meters per second? Options: 50, 50; 50, 40; 40, 50; 40, 40

Back

50, 40

Answer explanation

Normal upper and lower limb CV is at least 50 m/s and 40 m/s respectively.

Front

During a blink reflex study, you recall that the Vm nucleus is located within which of the following structures? Neocortex, Medulla, Pons, Midbrain

Back

Pons

Answer 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).

Front

During nerve conduction studies, the optimal upper limb temperature is which of the following? 33 degrees Celsius, 32 degrees Celsius, 31 degrees Celsius, 30 degrees Celsius

Back

32 degrees Celsius

Answer explanation

Temperature above 32C for the upper extremities and above 31C for the lower extremities, measured at the dorsum of the hands and feet.

Front

You are reviewing EMG/NCS results of a patient. The results demonstrate decreased proximal and distal amplitude with normal latency. EMG reveals decreased recruitment. What is the most likely etiology of these findings?

Back

Axonal loss

Answer explanation

Decreased amplitude throughout an entire nerve’s length suggests axonal loss. EMG will show decreased recruitment in these cases. Conduction block is seen with proximally reduced amplitude, but stimulating distally beyond the spot of conduction block can demonstrate a normal “repaired” amplitude.

Front

A patient’s left upper limb EMG/NCS results are as follows. Routine median, ulnar, and radial sensory nerve conduction studies are normal. EMG reveals decreased recruitment and 4+ fibrillations in triceps, extensor indicis proprius, abductor pollicis brevis, and first dorsal interosseous, but normal resting activity and recruitment in deltoid, biceps, brachioradialis, and pronator teres. Which of the following is the most likely diagnosis? C8 radiculopathy, C7 radiculopathy, C6 radiculopathy, C5 radiculopathy

Back

C8 radiculopathy

Answer explanation

triceps - C7/C8 radial

extensor indicis proprius - C7/C8 radial

abductor pollicis brevis - C8/T1 median

first dorsal interosseous - C8/T1 ulnar

Front

In Erb palsy, which EMG/nerve conduction study finding is most likely to be discovered? SNAP: sensory nerve action potential. Options: Abnormal medial antebrachial cutaneous nerve SNAP, Normal median nerve SNAP to the thumb, Abnormal lateral antebrachial cutaneous nerve SNAP, Decreased recruitment of the extensor indicis proprius

Back

Abnormal lateral antebrachial cutaneous nerve SNAP

Answer explanation

Erb palsy is a C5, C6/upper trunk brachial plexopathy commonly caused by trauma as an adult or obstetrical trauma as an infant due to traction forces on these roots. Because the C5 and C6 roots are injured, changes can be expected to be found in any downstream SNAPs, CMAPs, or EMG needling of C5, C6 muscles. The Waiter’s Tip position is classic for this: arm is adducted, internally rotated, pronated, wrist flexed, all due to C5/C6 muscle weakness. There is sensory loss over the lateral arm and dorsolateral forearm. The musculocutaneous nerve (C5, C6, upper trunk) terminates as the lateral antebrachial cutaneous nerve (LAC); thus, if the C5,C6/upper trunk is injured, an abnormal LAC SNAP may be detected. The median sensory fibers to the thumb involve C5/C6 fibers; thus, this median SNAP would be abnormal on NCS. The extensor indicis proprius (EIP) does not contain C5, C6, or upper trunk innervation, and thus would show normal recruitment in this case. The medial antebrachial cutaneous (MAC) SNAP would be normal as well, as it arises from the lower trunk of the brachial plexus, which itself arises from C8 and T1 nerve roots.

Front

During an EMG, a patient is found to have active denervation of the pronator teres and abductor pollicis brevis. Which of the following statements is most likely to be true? The patient will demonstrate thumb flexion weakness, The patient will demonstrate distal interphalangeal joint flexion weakness to digits 4 and 5, The patient will demonstrate wrist flexion with medial deviation weakness, The patient will demonstrate wrist extension weakness

Back

The patient will demonstrate thumb flexion weakness

Answer explanation

This patient has median neuropathy at the elbow. Thus, flexor pollicis longus (FPL) will be affected and potentially weak, making "thumb flexion weakness" the correct choice. Wrist flexion with medial deviation weakness, and DIP flexion weakness to digits 4 and 5 represent ulnar nerve muscle functions (flexor carpi ulnaris and flexor digitorum profundus to digits 4 and 5), and wrist extension weakness represents a radial nerve muscle function. Remember that medial deviation of the wrist = ulnar deviation with respect to conventional anatomic position.

Front

Which of the following muscles is NOT innervated by the sciatic nerve (tibial division)?
Options:
Short head of biceps femoris
Semitendinosus
Long head of biceps femoris
Semimembranosus

Back

Short head of biceps femoris

Answer explanation

Semimembranosus, semitendinosus, and the long head of biceps femoris are all innervated by the sciatic nerve (tibial division). The short head of the biceps femoris is innervated by the sciatic nerve (common fibular division).

Front

A Martin-Gruber anastomosis is an anatomic variant consisting of which of the following nerve findings? Ulnar to median anastomosis, Median to posterior interosseous anastomosis, Ulnar to anterior interosseous anastomosis, Median to ulnar anastomosis

Back

Median to ulnar anastomosis

Answer explanation

The Martin-Gruber anastomosis involves median nerve fibers in the forearm crossing over and joining the ulnar nerve in the forearm. This means essentially that the median nerve innervates the abductor digiti minimi (ADM) and first dorsal interosseous (FDI), which are ulnar nerve muscles, in addition to innervating its own classic median nerve muscles (abductor pollicis brevis - APB, etc.). The ulnar nerve CMAP (compound muscle action potential) when stimulating at the elbow will show a low amplitude when recording over a distal ulnar muscle such as FDI. The ulnar CMAP will be “repaired”/normal if you stimulate the ulnar nerve at the wrist. Thus, it appears that there is a conduction block somewhere in the forearm when there actually is not. When you stimulate the ulnar nerve at the wrist, at this point the median nerve fibers have finally joined the ulnar nerve, so you are finally stimulating all the motor axons that are supplying the ulnar nerve hand muscles. When you stimulate the ulnar nerve at the elbow, you are not stimulating the median nerve fibers that have yet to contribute to the ulnar innervations, thus you only generate part of the full amplitude, which is what makes it look like there is a conduction block in the ulnar nerve, when in reality there is not. When suspecting a MGA, the physician should perform further testing by recording over the ADM or FDI and stimulating the median nerve at the elbow to see if a sizeable CMAP can be generated (this will not be the full “normal” CMAP, just a small “bump” that represents the small portion of median nerve axons that are destined to cross over and supply the ADM, whereas the lion’s share of median nerve fibers will innervate the classic median nerve muscles and thus will not produce any signal here, as we are recording over the FDI, an ulnar nerve muscle). If MGA is present, the small CMAP amplitude you generate with this will “add up” with the proximal ulnar CMAP to create a full normal-amplitude CMAP, just like the normal CMAP when stimulating the ulnar nerve at the wrist.