Do longer necks predispose to Hirayama disease? A comparison with mimics and controls

Background: Dynamic changes in cervical spine during flexion is a proposed mechanism for Hirayama disease [HD], a localized form of anterior horn cell disorder. Apparent shortening of dura as compared to vertebral column leading to dural shift on flexion is considered to be the primary mechanism in this hypothesis. Whether this disproportion is a result of short dura or longer cervical segment is not known and neck length has not been studied in HD. Also, all patients with segmental motor weakness and wasting of upper limbs do not show dural changes; hence comparative evaluation of HD and its mimics is important.

Material and methods: Patients with segmental wasting and weakness limited to upper limbs were subjected to flexion MRI. A special pillow was designed to provide fixed flexion angle of 35°. Patients showing dural changes formed the HD group while rest formed the non-HD group [mimics]. Both groups were analyzed on clinical, electrophysiological and radiological parameters. Whole spine to neck ratio of patients in HD group was compared to the non-HD group and age matched controls.

Results and conclusions: Patients with HD had longer cervical segments as compared to the non-HD group and age matched controls [p=0.001]. The longer cervical segment, in combination with dural changes probably contributes to the pathophysiology of dynamic flexion hypothesis of HD and the onset around the growth spurt. Patients with HD had cold paresis and finger extensors were consistently weaker than flexors. Apart from longer necks, cold paresis and pattern of weakness may help to differentiate HD from its mimics.