Persistent Ultrasonographic Nerve Constriction in Clinically Improving Neuralgic Amyotrophy

Neuralgic amyotrophy (NA) is an uncommon neurological disorder characterized by acute pain followed by muscle weakness and atrophy. Advanced imaging techniques, such as ultrasound and MRI, can reveal structural nerve abnormalities including fascicular enlargement [1, 2]. Nerve ultrasound findings of fascicular entwinement (the abnormal rotation of nerve fascicles), or fascicular constriction, are quite specific to NA and may have implications for diagnosis and management [2]. We present a case of NA following coronary artery bypass grafting (CABG), with persistent nerve constriction noted on ultrasound despite clinical improvement. A 68-year-old male with a history of diabetes and idiopathic thrombocytopenic purpura developed severe left arm and forearm pain lasting for a week, beginning four days after undergoing coronary artery bypass grafting (CABG), followed by progressive weakness in left wrist and finger extension and hand numbness. Electrodiagnostic testing conducted 2 months post symptoms onset revealed a non-localizing, subacute, proximal left radial nerve injury. Brachial plexus MRI was unrevealing. Upon referral to a neuromuscular specialist four months post-surgery, he exhibited persistent weakness in left finger extension, grade 3/5 on the Medical Research Council (MRC) Scale for Muscle Strength, along with atrophy involving forearm and intrinsic hand muscles. Although the radial nerve was most involved, there were milder, widespread motor deficits in the left arm, in keeping with NA. A repeat electrodiagnostic study five months post-symptom onset demonstrated a low radial compound muscle action potential (CMAP), which had improved compared to the prior study, and preserved left radial sensory response. Clinically, he improved in left finger extension (MRC grade 4/5). An ultrasound of the left radial nerve, using a 12–22 MHz linear transducer, revealed two focal regions of nerve constriction between the spiral groove and antecubital fossa. Figure 1 demonstrates one of these regions, with an incomplete fascicular constriction and an hourglass deformity. Surrounding the constriction, the fascicle demonstrated increased cross-sectional area and reduced echogenicity (Figure 2). Increased Doppler signal within or surrounding a nerve, representative of abnormal vascularity, is often seen in inflammatory nerve conditions. In this case, the absence of a Doppler signal along the course of the left radial nerve may indicate chronicity [2]. In NA, nerve enlargements are the most common ultrasound finding, followed by constriction, and fascicular entwinement is the least common [3]. There was no fascicular entwinement noted through the extent of the left radial nerve. The superficial radial and posterior interosseous nerves appeared normal distally. This case highlights the utility of ultrasound in identifying sonographic features of NA. The presence of both nerve constriction and enlargement in our patient raises the question of whether these findings reflect differential involvement of individual fascicles versus the entire nerve or represent stages in a stepwise pathological progression. The nerve lesions were distal to the brachial plexus, and out of field for the brachial plexus MRI. Surgical treatment for nerve constrictions may be considered, but this patient had clinical improvement despite persistent nerve constrictions. Understanding the imaging characteristics of NA can aid in diagnosis and may inform treatment decisions. Further research is needed to clarify the prognostic implications of nerve constriction in NA. The authors declare no conflicts of interest.