Age / Sex : /

Age / Sex : 17/M
C.C.: Weakness, Rt upper arm (onset: 1YA)
P.I.: progressive right arm weakness since 1 year
N/E: Motor power
Shoulder Abductor 5- Index: finger drop (+/)  thumb EPL: 4- IndexEDL: 0 Pronator:4 Sensory : intact

Hirayama disease = Nonprogressive juvenile spinal muscular atrophy of distal upper arm

Discussion

Findings:
• MR – forward displacement of the dural sac and flattening of the lower cervical cord in a fully flexed position of the neck. – Epidural enhancement & T2-hyperintensity behind the displaced lower cervical cord suggests circulatory changes in the spinal canal during neck flexion.

Diagnosis: Hirayama disease = Nonprogressive juvenile spinal muscular atrophy of the distal upper limbs = Benign juvenile muscular atrophy of upper extremities

Discussion: 
 - A focal amyotrophy with unilateral or asymmetric bilateral weakness and wasting of C7, C8, and T1 innervated muscles 
 - Insidious onset, chronic, often self-limiting course 
 - Male preponderance between the ages of 15 and 25 years 
 - Usually sporadic 
 - Mainly in Japan and other Asian countries 
 - The pathogenesis of the disorder is unknown - An assumption of imbalanced growth between the patient’s vertebral column and spinal canal contents has been suggested. This imbalanced growth will cause disproportional length between the patient’s vertebral column and the spinal canal contents, which will cause a "tight dural sac" or "overstretch of the cord" in the neutral position and an anteriorly displaced posterior dural wall when the neck is flexed. 
 - MRI of spinal cord - Atrophy of the lower cervical cord in a neutral position - Forward displacement and flattening of the lower cervical cord - Secondary congested posterior epidural venous plexus in a flexed position : Passive dilatation of the epidural venous plexus due to forward displacement of the dural sac 
 - Neutral-position MR - Lower cervical cord atrophy - Asymmetric cord flattening - Noncompressed intramedullary high signal intensity on T2WI - Abnormal cervical curvature (straight or kyphotic) - Loss of attachment (LOA) between the posterior dural sac and subjacent lamina: the most valuable finding

References:
1. Hirayama K. Non-progressive juvenile spinal muscular atrophy of the distal upper limb (Hirayama’s disease). In: De Jong JMBV, ed. Handbook of Clinical Neurology. Amsterdam, the Netherlands: Elsevier; 1991;15:107–120
2. Hirayama K, Toyokura Y, Tsubaki T. Juvenile muscular atrophy of unilateral upper extremity: a new clinical entity. Psychiatr Neurol Jpn 1959;61:2190–2197
3. Chen CJ, Hsu HL, Tseng YC, et al. Hirayama flexion myelopathy: neutral-position MR imaging findings--importance of loss of attachment. Radiology. 2004;231(1):39-44
4. Biondi A, Dormont D, Weitzner I, Jr, Bouche P, Chaine P, Bories J. MR imaging of the cervical cord in juvenile amyotrophy of distal upper extremity. AJNR Am J Neuroradiol 1989; 10:263-268