Distal bimelic amyotrophy (DBMA): Phenotypically distinct but identical on cervical spine MR imaging with brachial monomelic amyotrophy/Hirayama disease

Our objective was to characterize the MR imaging features in a large and distinct series of distal bimelic amyotrophy (DBMA) from India. We utilized a retrospective and prospective study on 26 cases. Results demonstrated that upper limb distal muscle wasting and weakness was predominantly symmetrical in onset. Mean age at onset was 20.9 ± 7.0 years, mean duration 83.0 ± 102.6 months. MRI carried out in 22 patients with flexion studies showed forward displacement of posterior dura in 19 (86.4%). Crescent shaped epidural enhancement on contrast was seen in 20/24 cases (83.3%), and bilateral T2W hyperintensities of cord in17 (65.4%) - symmetrical in15 cases. Maximum hyperintensity was noted at C5-C6, C6-C7 levels. Cord atrophy was noted in 24 (92.3%) cases (most affected: C5-C6, C6-C7) - symmetrical atrophy in 21cases. Cervical spine straightening occurred in six (23.1%) cases and reversal of lordosis in 15 (57.7%). In conclusion, this study confirms that DBMA is phenotypically distinct but pathophysiologically the same as brachial monomelic amyotrophy (BMMA) on MR imaging. Typical MRI features were seen in all. It is important to differentiate this disorder from ALS, which could present at a younger age as often seen among Indians. The clinical and MR imaging features are highly suggestive that DBMA, as with BMMA/Hirayama disease, occurs due to dynamic alterations at the cervical spine level.