Hirayama disease - MRI and EMG synchronisation

Clinical History

The patient presented with gradually progressive right upper limb weakness for the past 18 months, with no sensory loss/movement restriction at the shoulder. It progressed from inability to lift heavy objects to difficulty in unbuttoning his shirt within twelve months. No h/o neck pain, trauma, preceding fever, bladder/bowel complains, hypo-aesthetic patches were noted.

Imaging Findings

MRI revealed: Unilateral cord atrophy at the right anterior aspect with flattening of the anterior surface of the cord at C6-C7 level (Fig. 1). Non-flexion T2-W images: Cord atrophy at C6-C7 vertebral level with the posterior wall of the dural canal in close proximity with the spinal canal (Fig. 2). Flexion T2-W sagittal image: Anterior shifting of the posterior wall of the dural canal below C5 level with marked flattening of the lower cervical cord (Fig. 3) with enhancing posterior epidural mass posterior to the anteriorly shifted dura mater with small flow voids within in the lower cervical region (C5-T1 level). (Fig. 4). Vertebral bodies and intervertebral discs were normal in signal intensities and alignment. Paravertebral soft tissues appeared normal. The above findings are consistent with Hirayama disease.

Discussion

Hirayama disease, initially recognized in Japan (1959) is also called mono-melic amyotrophy. The disorder has a distinct male predominance between 15-25 years of age, asymmetric involvement of upper extremity and a self-limiting course. [1] The atrophy predominantly involves the intrinsic hand muscles (interossei, thenar and hypothenar muscle groups) and ulnar side of the forearm. There is sparing of the brachioradialis muscle giving the impression of an oblique atrophy. The primary mechanism is forward displacement of the posterior wall of the lower cervical dural canal during flexion. Normally, the difference in the length of the dura (during flexion-extension) is 1.5 cm at the anterior wall and 5 cm at the posterior wall. The normal slack of the dura can compensate for the increased length in flexion in adults. In HD, the dural canal is no longer slack due to imbalance in growth of the vertebra and the dura mater. Therefore, a tight dural canal cannot compensate for the increased length of the posterior wall during flexion. This causes an anterior shifting of the posterior dural wall with consequent compression of the cord. This may cause microcirculatory disturbances in the territory of the anterior spinal artery or in the anterior portion of the spinal cord. [2]There is unilateral involvement in the majority of cases, but asymmetric and symmetric bilateral involvement is also observed. There is, however, no relationship between the patients’ handedness and side of greater muscular atrophy. Associated conditions like exacerbation of weakness in as cold environment and tremors may be present (minipolymyoclonia). [3] An EMG-NCV study of both upper limbs will reveal normal sensory conduction velocities of ulnar, median and other nerves (Motor and F wave: Very low amplitude signals in median, ulnar nerves of the affected side). However, nerve conduction study is normal on the unaffected side. The findings of the EMG-NCV study showed evidence of motor axon degeneration affecting T1>C8 fibres of the right>left upper limb suggestive of active + chronic motor axon degeneration. MR imaging reveals enhancing posterior epidural component noted on flexion in the lower cervical region (C5-T1 level). Atrophy of the lower cervical cord, more on the right with anterior shifting of the posterior dural sac on flexion. Vertebra and intervertebral discs are normal in signal intensities and alignment. Paravertebral soft tissues appear normal.
Avoiding neck flexion stops the disease progress. Some advocate application of a cervical collar for 3-5 years since the progressive stage usually ceases. Surgical intervention - cervical decompression or fusion with/without duraplasty are considerable.

Differential Diagnosis List

Final Diagnosis

Hirayama disease

URL:	https://www.eurorad.org/case/12184
DOI:	10.1594/EURORAD/CASE.12184
ISSN:	1563-4086