Slow onset of asymmetric atrophy and weakness in mostly C7, C8, T1 myotomes, usually unilaterally but may occur bilaterally. It is typically a monophasic condition and disproportionately affects young males (15-25 years of life) of South Asian, Middle Eastern, or Asian descent. Patients develop progressive, painless weakness of one limb, typically the hand and forearm, secondary to focal anterior horn cell loss. Sensory function is spared and upper motor neuron signs are absent. In the majority of cases, weakness progresses in the affected limb in less than 5 years and then stabilizes. There is sparing of brachioradialis muscle . Patients may report worsening of symptoms in cold. Polyminimyoclonus and fasciculations may be seen in hands in addition to hand atrophy and weakness. Progression to the contralateral limb is less common.
Hypothesis: Patient may have disproportionate growth of vertebrae relative to dura, thus leading to forward displacement of the dura, especially in neck flexion. Neutral position of cervical MRI shows abnormal cord curvature, asymmetric cord atrophy, and increased intramedullary signal intensity. The cervical cord can be anteriorly displaced when an MRI is obtained in the flexed position. In the neutral position, loss of attachment between the posterior dural sac and adjacent lamina is appreciated. There is focal intermittent compression of the spinal cord during late adolescent growth. There is cord ischemia with the anterior horn cells or motor nerve roots.
EDX:
NCS and EMG show LMN pattern in the affected upper limbs only.
Other limbs should be normal for this diagnosis.
MRI (flexion/extension) may show atrophy in cervical cord region with anterior cord displacement, thus enlarging the epidural space. This is attributed to loss of attachment between the posterior dural sac and subjacent lamina
DDx: The presence of conduction block should initiate an evaluation of MMNCB, rather than monomelic amyotrophy.