Nosology of juvenile muscular atrophy of distal upper extremity: from monomelic amyotrophy to Hirayama disease--Indian perspective

Abstract

Since its original description by Keizo Hirayama in 1959, "juvenile muscular atrophy of the unilateral upper extremity" has been described under many nomenclatures from the east. Hirayama disease (HD), also interchangeably referred to as monomelic amyotrophy, has been more frequently recognised in the west only in the last two decades. HD presents in adolescence and young adulthood with insidious onset unilateral or bilateral asymmetric atrophy of hand and forearm with sparing of brachioradialis giving the characteristic appearance of oblique amyotrophy. Symmetrically bilateral disease has also been recognized. Believed to be a cervical flexion myelopathy, HD differs from motor neuron diseases because of its nonprogressive course and pathologic findings of chronic microcirculatory changes in the lower cervical cord. Electromyography shows features of acute and/or chronic denervation in C7, C8, and T1 myotomes in clinically affected limb and sometimes also in clinically unaffected contralateral limb. Dynamic forward displacement of dura in flexion causes asymmetric flattening of lower cervical cord. While dynamic contrast magnetic resonance imaging is diagnostic, routine study has high predictive value. There is a need to lump all the nomenclatures under the rubric of HD as prognosis in this condition is benign and prompt diagnosis is important to institute early collar therapy.

Figure 1

Wasting of hands and forearms with oblique amyotrophy (black arrows) (a); electromyography showing normal motor unit action potential in right deltoid (b); neurogenic motor unit action potential in right abductor pollicis brevis (c); incomplete interference pattern on submaximal volition in right abductor pollicis brevis (d).

Figure 2

Localised cord atrophy (white arrow) on neutral sagittal T2WI (a), pear shaped asymmetric cord flattening on neutral transverse T2WI (green arrow) (b), asymmetric cord flattening with loss of attachment of dura from subjacent lamina (blue arrow) on neutral position axial T2WI MRI (c), and anterior displacement of posterior dura from C3–D8 on postcontrast imaging showing intense enhancement of posterior epidural space from C6–D3 (red arrow) (d).

Figure 3

A 23-year-male with weakness and wasting of left hand and forearm with anteroposterior pear-shaped cord flattening with loss of attachment of dura from subjacent lamina (red arrow) on T2WI axial MRI (a), midsagittal image showing loss of cervical lordosis (white arrow) but not showing cervical cord atrophy on neutral position sagittal T2WI MRI (b), dorsal epidural voids (green arrows) on flexion sagittal T1WI and T2WI MRI ((c), (d)), and posterior epidural crescentic enhancing mass (blue arrow) on flexion contrast sagittal T1WI MRI (e).

Figure 4

Two young males (a) and (b) with proximal left upper limb weakness and wasting without the typical findings described in Figures 1 to 3: marked wasting proximal left upper limbs (black arrow) (A) and mild/no wasting distal left upper limbs in same patients (B), preserved or mild loss of cervical lordosis and absence of localized cord atrophy at corresponding C5, 6 segments (C), normal cervical cord outline without loss of attachment of dura from subjacent lamina at all levels on neutral position axial T2WI MRI (D), and no posterior epidural crescentic enhancing mass on flexion contrast sagittal T1WI MRI (E).