The Importance of Flexion MRI in Hirayama Disease with Special Reference to Laminodural Space Measurements.

BACKGROUND AND PURPOSE: Hirayama disease is a benign focal amyotrophy of the distal upper limbs involving C7, C8, and T1 segmental myotomes with sparing of the brachioradialis and proximal muscles of the upper limb innervated by C5-6 myotomes. The objective of the present study was to study the utility of MR imaging in young patients presenting with weakness and wasting of the distal upper extremity and to evaluate the importance of the laminodural space during flexion cervical MR imaging.

MATERIALS AND METHODS: This was a prospective cross-sectional study conducted from January 2014 to July 2017 in a tertiary care center from Northeast India. Forty-five patients with clinically definite Hirayama disease underwent electrophysiologic evaluation followed by MR imaging of the cervical spine.

RESULTS: The mean age at recruitment was 22.8 ± 5.5 years. Forty patients (88.9%) had unilateral and 5 (11.1%) had bilateral upper extremity involvement. Cervical cord T2-weighted hyperintensities were demonstrated in 16 patients (35.6%), of which 15 (33.3%) had anterior horn cell hyperintensities. Flexion MR imaging showed loss of the posterior dural attachment, forward shifting of the posterior dural sac with postcontrast enhancement, and prominent posterior epidural venous plexus in all patients. The laminodural space at maximum forward shifting of the posterior dural sac ranged from 3 to 9.8 mm, with a mean distance of 5.99 mm (95% confidence interval, 5.42-6.57 mm).

CONCLUSIONS: Flexion cervical MR imaging is a very useful investigation in diagnosing Hirayama disease. The increase in the laminodural space and the presence of cervical cord flattening during flexion are essential for diagnosis.