Effect of neck flexion on somatosensory and motor evoked potentials in Hirayama disease

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Abstract

Hirayama disease (HD) is a rare motor disorder mainly affecting young men, characterized by atrophy and weakness of forearm and hand muscles corresponding to a C7–T1 myotome distribution. The weakness is usually unilateral or asymmetric and progression usually stops within several years. The etiology of HD is not well understood. One hypothesis, mainly based on MRI findings, is that the weakness is a consequence of cervical flexion myelopathy. The aim of this study was to explore the function of corticospinal and ascending somatosensory pathways during neck flexion using evoked responses.

Materials and methods

15 men with HD and 7 age-matched control male subjects underwent somatosensory evoked potentials (SSEP) and motor evoked potentials (MEP) studies with the neck in neutral position and fully flexed. SSEP studies included electrical stimulation of median and ulnar nerves at the wrist, and tibial nerve at the ankle with recording over the ipsilateral Erb's point, cervical spine, and contralateral sensory cortex. MEP recordings were obtained by magnetic stimulation of the motor cortex and the cervical lower spinal roots; the evoked responses were recorded from the contralateral thenar and abductor hallucis muscles.

Results

MEP recordings demonstrated significant lower amplitudes, and slightly prolonged latencies in HD patients on cervical stimulation, compared to control subjects. During neck flexion, MEP studies also demonstrated a statistically significant drop in mean upper limb amplitude on cervical stimulation in HD patients, as well as in control subjects, although to a lesser degree. In contrast, no significant differences were found in SSEP studies in HD patients compared to control subjects, or between neutral and flexed position in these groups.

Conclusion

The study shows a negative effect of cervical flexion on MEP amplitudes in HD patients as well as in control subjects, requiring more studies to investigate its significance. Neck flexion did not have an influence on any SSEP parameters in patients or controls.

Introduction

Hirayama disease (HD) is a localized disorder of the lower motor neurons occurring predominantly in young men. Weakness and wasting mainly affect the musculature of the hand and forearm within C7–T1 myotomes, thus typically sparing the brachioradialis muscle. The amyotrophy is unilateral in most patients, asymmetrically bilateral in some, and rarely symmetric [1]. Weakness develops in both the extensor and flexor muscles; the finger extensors and wrist flexors are usually predominantly involved. There is no cranial nerve, sensory, or sphincter dysfunction and pyramidal signs are absent, although hyperreflexia of the lower limbs may be present. The initial slowly progressive course is followed by a spontaneous arrest within 5 years after onset in more than 70% of patients [2].

The etiology of HD is not well understood, but various possibilities have been suggested, such as ischemia of the spinal cord as a result of neck flexion [3], [4], [5], [6], autoimmune, atopic [7], and genetic factors [8], [9]. Genetic factors are suggested by the young age of the patients, the remarkable male predominance and the increased frequency of the disorder in the Asian as compared to the Caucasian population [3], but no clear genetic cause has been established until recently [10], [11], [12], [13], when exome sequencing identified two possible susceptibility genes for HD [9].

The hypothesis of flexion injury of the spinal cord is supported by the frequent finding of forward displacement of the dural sac and compressive flattening of the lower cervical cord on MRI [3], [14], [15]. Although mild reduction in the anteroposterior diameter was also noted in normal controls [16], forward shifting of the posterior cervical dural sac is probably not suggestive of HD, unless there is also an associated cord compression [15]. Under normal circumstances, the anteroposterior diameter of the cervical spinal canal decreases during neck extension, and cervical myelopathy associated with compression during neck flexion is uncommon, suggesting HD is a category of cervical flexion myelopathy.

A further characteristic finding on MRI is a high signal in the posterior epidural space on T1 and T2 weighted images, suggesting circulatory changes in the spinal canal during neck flexion. These changes were attributed to several possible mechanisms, including passive dilatation of the posterior internal vertebral venous plexus due to forward displacement of the cervical dural sac, congestion of the venous plexus, abnormal drainage in the vertebral venous plexus, or epidural vascular malformation [2].

Neurophysiological and pathological studies in HD have produced conflicting results. Amplitude decrease of the N13 cervical response of somatosensory evoked potentials (SEP) was described in HD patients with the neck in fully flexed as opposed to standard position [6], [17], [18]. In contrast, other researchers did not find evidence of N13 abnormalities during neck flexion [8], [19]. In addition, normal and pathological motor evoked potential (MEP) findings (decreased or increased MEP amplitude, longer central motor conduction time), were reported with the neck in standard and flexed neck positions in few single case studies and small series [4], [18], [20], [21].

In this paper, in order to examine whether cervical flexion leads to compromise of motor and/or sensory pathways, electrophysiological (SEP and MEP) studies were carried out both in neutral position, and with the neck flexed, in a cohort of 15 HD patients and 7 control subjects. A significant change in these parameters on neck flexion, could yield important physiological information, and therefore was the main goal of this study.

Section snippets

Patients and controls

SEP and MEP studies in neutral position and with the neck fully flexed were performed in 15 male patients, suffering from HD for more than two years. Most of them already reached the stable phase of the disease, with typical clinical features of juvenile muscular atrophy of the distal upper extremity, without sensory deficit or sphincter involvement. All patients underwent a comprehensive neurological examination to rule out atypical features. None of our patients had lower limb symptoms or

Results

Mean age of the patients at the time of examination was 22 ± 3.8 years (range 17–31). The mean age at onset of symptoms was 18 ± 1.9 years (range 16–21). Weakness was unilateral in 10 patients, asymmetrically bilateral in 4, and bilateral in one. The controls had a mean age of 22 ± 1 years (range 22–24).

Discussion

The etiology of HD is not known. One hypothesis, mainly based on MRI findings, is that the disorder is due to cord compression during neck flexion [3], [4], [5], [6]; on this basis some experts have recommended cervical collar application [2], [21], or even surgical decompressive treatment [5]. According to the “flexion myelopathy” hypothesis, the microcirculatory disturbances in the anterior spinal artery lead to segmental anterior horn neuronal loss [14]; the susceptibility may stem from

Conflict of interest

On behalf of all authors, the corresponding author states that there is no conflict of interest.

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