Prior muscle activation affects the compound muscle action potential

INTRODUCTION

This study was performed to evaluate the effect of prior voluntary activation of a muscle on the subsequently-recorded compound muscle action potential (CMAP).

METHODS

The CMAPs from the hypothenar, thenar, and extensor digitorum brevis muscles were recorded in 6 healthy volunteers at rest and for up to 30 min following 5 separate epochs of up to 20 s of voluntary muscle activation.

RESULTS

There was consistent, significant (P < 0.02) enhancement of the negative area, amplitude, and duration of the CMAP after activation. The enhancement was maximal, up to 144% of baseline, within about 1 min post-activation; thereafter, the CMAP gradually returned to baseline over about 15 min.

DISCUSSION

Activation of a muscle within several minutes prior to testing enhances the subsequently-recorded CMAP. This observation highlights prior muscle activation as a physiological variable that influences the size of the CMAP during motor nerve conduction studies. Muscle Nerve, 2019.

Electrodiagnostic features of true neurogenic thoracic outlet syndrome

INTRODUCTION

We report the electrodiagnostic (EDX) features of 32 patients with surgically verified true neurogenic thoracic outlet syndrome (TN-TOS).

METHODS

Retrospective record review.

RESULTS

We found uniform EDX evidence of a chronic axon loss process that affected the lower portion of the brachial plexus and disproportionately involved the T1 more than the C8 sensory and motor fibers. Because of this relationship, the medial antebrachial cutaneous sensory nerve (T1) and median motor (T1 > C8) study combination was abnormal in 89%, whereas response combinations that primarily assessed the C8 fibers were less frequently affected.

CONCLUSIONS

The characteristic EDX features of TN-TOS are T1 > C8 nerve fiber involvement. A comprehensive EDX examination of the lower plexus with contralateral comparison studies is imperative to diagnose this disorder accurately.

Phrenic neuropathy due to neuralgic amyotrophy

The authors reviewed the medical records of 33 patients diagnosed with idiopathic phrenic neuropathy and found that 17 patients had clinical features of neuralgic amyotrophy. They concluded that a careful clinical and electrodiagnostic evaluation may implicate neuralgic amyotrophy as a causative disease in patients with apparently isolated phrenic neuropathy.

Infraclavicular brachial plexus injury following axillary regional block

Infraclavicular brachial plexopathy is a potential complication of axillary regional block. We retrospectively reviewed 13 such injuries and found the median nerve most often affected, followed by combined median and ulnar neuropathies, and then by various combinations involving the median, ulnar, radial, and musculocutaneous nerves. All were axon-loss in type and most were severe in degree electrophysiologically. The clinical and electrodiagnostic features of these injuries are strikingly similar to those sustained after axillary arteriography, which has been associated with the medial brachial fascial compartment (MBFC) syndrome. This syndrome is characterized by the evolution of neurologic deficits and pain following hematoma formation within a compartment of the upper arm. Thus, we believe that this mechanism underlies most nerve injuries that result from axillary angiography or axillary regional block. This has important treatment implications, as timely surgical intervention may lead to improved outcome.

The medial brachial fascial compartment syndrome following axillary arteriography

OBJECTIVE

To review clinical and electrodiagnostic features of the medial brachial fascial compartment syndrome, a complication of percutaneous axillary vessel puncture.

METHODS

The authors reviewed electrodiagnostic examinations over a 20-year period.

RESULTS

This syndrome presents with weakness, pain, and numbness during or following the percutaneous procedure. Injury is characterized by axon loss and involves terminal nerves of the infraclavicular brachial plexus-most often the median nerve alone, followed by combinations of the median, ulnar, radial, and musculocutaneous nerves.

CONCLUSIONS

Early recognition of the medial brachial fascial compartment syndrome may lead to prompt surgical intervention, which, in turn, may prevent permanent nerve injury. Late diagnosis generally results in poor outcome and often results from delayed symptom onset and lack of overt compartment syndrome signs.

Comparison of surgical and electrodiagnostic findings in single root lumbosacral radiculopathies

To identify the segmental innervation of L-2-S-1 muscles, we compared the preoperative electrodiagnostic examinations of 45 patients with single-level lumbosacral radiculopathies confirmed radiologically and surgically. The electrodiagnostic findings were classified as abnormal only by the needle examination and only if muscles demonstrated active denervation or a marked neurogenic motor unit potential firing pattern. In comparison to other surgical, intraoperative root stimulation, and clinical studies, we found several differences. Overall, there was little overlap among L-2-4, L-5, and S-1 radiculopathies. The tibialis anterior was predominantly L-5 innervated, the gastrocnemius (medial and lateral head) predominantly S-1 innervated, and the biceps femoris (short and long head) exclusively S-1 innervated. The two heads of biceps femoris were not affected in any patients with L-5 radiculopathy in whom they were examined. These findings help guide both the clinician and surgeon in the diagnosis and treatment of lumbosacral radiculopathies.