Vasculogenic Myoclonus of Peripheral Origin after Whiplash Injury: A Case Report

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J Electrodiagn Neuromuscul Dis. 2023;25(1):42-45

Publication date (electronic) : 2023 April 26

doi : https://doi.org/10.18214/jend.2022.00192

Woo In Choi, Hyoung Seop Kim

Department of Physical Medicine and Rehabilitation, National Health Insurance Service Ilsan Hospital, Goyang, Korea

Corresponding author: Hyoung Seop Kim Department of Physical Medicine and Rehabilitation, National Health Insurance Service Ilsan Hospital, 100 Ilsan-ro, Ilsandong-gu, Goyang 10444, Korea Tel: +82-31-900-0137 Fax: +82-31-900-0343 E-mail: rekhs@nhimc.or.kr

Received 2022 September 19; Revised 2022 December 9; Accepted 2022 December 30.

Abstract

Introduction

Myoclonus is sudden, brief, involuntary muscle twitching that induces a simple jerky movement of a body part. Based on the neuroanatomical origin of the electrical discharge, myoclonus could be classified in terms of its etiology as cortical, subcortical, spinal, or peripheral [1]. Myoclonus can be easily diagnosed by clinical observation, but some cases are difficult to distinguish from other involuntary movements such as tremor, dystonia, and chorea. In those circumstances, electromyography (EMG) can be helpful, as one of the most specific characteristics of myoclonus is an abrupt and brief muscle contraction [2].

Information about the synchronization of motor unit discharges can be obtained from EMG [3]. Obtaining EMG recordings simultaneously from multiple muscles is useful, both to demonstrate the distribution of myoclonic jerks and to identify the best muscle for subsequent analysis.

Here, we report a rare case of peripheral origin myoclonus associated with vascular pulsation immediately after a whiplash injury.

Case Report

A 29-year-old female patient experienced continuous involuntary rhythmic twitching movement of the right elbow for 6 months after a whiplash injury due to a rear-end car accident. After the car accident, she was immediately transferred to the emergency department of a tertiary training hospital. There were no symptoms other than posterior neck pain and musculoskeletal pain in the right upper arm, and no other specific injuries or bone fractures were observed at that time. However, she soon noticed a continuous painless involuntary movement of the right elbow occurring at rest; this involved a rhythmic twitching movement of the muscles around the elbow, causing rhythmic elbow flexion. The symptom worsened 3 days after the accident.

Magnetic resonance imaging of the cervical spinal cord and brain showed no abnormal findings. Standard electroencephalography also showed no epileptiform activity. As it was difficult to determine the exact etiology, the patient received psychiatric treatment to determine whether there was a psychogenic cause. She was also hospitalized at an oriental medicine hospital and received acupuncture and herbal therapy. Since her symptoms did not improve, she was referred to our outpatient Department of Physical Medicine and Rehabilitation.

A continuous rhythmic movement of the muscles of the arm and forearm was observed, which was considered to be myoclonus (Supplementary Video 1). The patient remained conscious and communicated appropriately. The patient could not suppress the movement intentionally and did not complain of pain or any other discomfort. No motor weakness or sensory changes were observed on the neurological examination. This rhythmic movement occurred only at rest, lasting even during sleep, and was not exacerbated by movement. However, this myoclonic jerk stopped during elbow extension.

A nerve conduction study in both the motor and sensory nerve fibers of the median, ulnar, and radial nerves showed acceptable amplitude and velocity. An F-wave study showed acceptable latency at the bilateral median and ulnar nerves. Concentric needle EMG recordings of the right biceps, brachioradialis, and triceps presented bursts of spontaneous rhythmic activity synchronous to the clinical myoclonus (Fig. 1). Each burst included the activity of several motor unit action potentials. No abnormal activity was noted in other muscles, including the paraspinalis, deltoid, flexor carpi radialis, extensor carpi radialis, abductor pollicis brevis, and abductor digiti minimi (Table 1).

Fig. 1.

Electromyography recording of the triceps presents bursts of spontaneous rhythmic activity.

Table 1.

Needle Electromyography in the Right Upper Extremities

Doppler ultrasound (Philips EPIQ7, Bothell, WA, USA) performed at the right upper arm revealed a rhythmic twitching movement of the biceps and triceps. The muscle contraction coincided with the pulsation of the brachial artery, suggesting that the myoclonus was caused by vascular pulsation (Supplementary Video 2). Unusually, it was also observed that the brachial vein in the right arm pulsated along with the biceps contraction, which was thought to be a secondary effect of muscle contraction (Supplementary Video 3).

An ultrasound-guided brachial plexus block via the interscalene approach was performed to relieve the myoclonic jerk. An injection of 1 mL of 2% lidocaine along the upper and middle trunks of the right brachial plexus was performed between the anterior and middle scalene muscles. However, this did not alleviate the myoclonus.

Informed consent was obtained for the written case report and video to be published.

Discussion

Myoclonus can be classified based on the neuroanatomical origin of the electrical discharge as cortical, subcortical, spinal, or peripheral myoclonus. Different myoclonic patterns may be expressed depending on the neuroanatomical origin of the abnormal discharge, which can help determine the cause of clinical myoclonus. Cortical myoclonus is usually stimulus-sensitive and is characterized by an extremely short duration of the EMG correlates, usually less than 50 ms [2]. In contrast, spinal myoclonus tends to be rhythmic and involves a group of muscles innervated by a specific spinal segment [2]. Peripheral myoclonus is typically not stimulus-sensitive and is hypothesized to be caused by lesions of the peripheral nerves that may alter the sensory input and induce central reorganization [4].

Our case displayed several unique clinical characteristics. There was no direct trauma to the arm, only a history of whiplash injury to the neck. The myoclonus was confined to a single arm without any radiating pain. The movements were more rhythmic and the EMG bursts were longer in duration than what has been described in classic cortical myoclonus. An EMG study clearly revealed brief muscle jerks (lasting about 800 ms) with bursts of spontaneous rhythmic activity, synchronous to the clinical movement. This myoclonus was strongly supported to be of peripheral origin, as our patient had no clinical, radiological, or electrophysiological evidence of central nervous system origin. Furthermore, Doppler ultrasound revealed that the biceps and triceps muscle contraction coincided with the pulsation of the brachial artery, suggesting that the vascular pulsation caused the muscle contraction. Primary hemifacial spasms have a similar pathophysiology, which is attributed to benign compression of the facial motor nerve by a vessel within or close to its root exit zone from the brainstem [5].

A peripheral generator of myoclonus has been hypothesized in the brachial plexus and, rarely, in peripheral nerve lesions [6]. Kang et al. [7] reported a case of myoclonus of the ipsilateral upper extremity following supraclavicular brachial plexus block, and Hudson et al. [8] reported myoclonus after a routine peripheral nerve block in a healthy patient. Similarly, in our case, the peripheral ectopic activity could have been generated by the vascular structures at the site of these lesions. Considering the pathophysiology of primary hemifacial spasms [5], the history of traumatic whiplash injury might have affected the vascular structures around the upper and middle trunk, inducing vascular pulsation that stimulated peripheral nerves, eventually resulting in vasculogenic myoclonus.

In the case reported here, peripheral nerve trauma or nerve entrapment may have also induced peripheral myoclonus. Assal et al. [4] reported involuntary rhythmic movement following an injury to the cutaneous branch of the deep peroneal nerve. As xylocaine injection to the deep peroneal nerve suppressed the abnormal movement in that previous study [4], we injected lidocaine along the upper and middle trunk levels of the right brachial plexus; however, it had no effect on the myoclonic movement.

There are a few limitations in this case report. First, only a routine nerve conduction study including the median, ulnar, and radial nerves was conducted. A special nerve conduction study including axillary, thoracodorsal, dorsal scapular, musculocutaneous, and suprascapular nerves was not performed, as the myoclonus was limited to the upper extremity, not the shoulder girdle muscle. For this reason, there was a limited ability to accurately assess the possibility of brachial plexus lesion. Furthermore, considering the treatment options for hemifacial spasms, treatments such as botulinum toxin injection and microvascular decompression may also be considered. Unfortunately, as the symptom did not cause functional problems, the patient refused additional treatment before trying those interventions.

This case provides an opportunity to explore the pathophysiology of post-traumatic myoclonus induced by vascular pulsation. Doppler ultrasound revealed that the muscle contraction coincided with the vascular pulsation, suggesting that myoclonus was caused by vascular stimulation. Furthermore, EMG can be successfully used to analyze involuntary muscle activity in myoclonus, as it helps to determine the classification and distribution of myoclonic jerks.

Supplementary materials

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