 Hirayama Disease

 Aka Juvenile Muscular Atrophy of the Distal Upper Extremity  Rare disease that affects predominantly males in their 2 nd or early 3 rd decade of life  Features include muscular weakness and atrophy in the hand and forearm  Brachioradialis muscle is spared, therefore the pattern of forearm involvement is referred to as an oblique amyotrophy  Unilateral involvement vs asymmetric and symmetric bilateral involvement

Hirayama Disease  Insidious onset and clinical course is steadily progressive  After a period of initial deterioration, a stable stage is reached  Pathogenesis is constantly debated.  Dynamic compression of the spinal cord felt to be an important finding in the diagnosis

Hirayama Disease: Pathogenesis  Dynamic cord compression during neck flexion with forward displacement of the posterior dura is an unequivocal finding in the progressive stage  However, this finding is absent in older patients who have reached a stable stage  Some believe that a disproportional length between the vertebral column and the dural canal leads to a “tight” dural canal.

Hirayama Disease: Pathogenesis  The normal cervical dura is slack and consists of transverse folds during neck extension.  With flexion, the length of the cervical canal increases  In normal patients, the dural slack compensates for the increased length in flexion  Patients with Hirayama dz may have an imbalance in growth of vertebrae and dura, and the short dural canal cannot compensate; therefore, the dural canal becomes tight when the neck is flexed.  Results in an anterior shift of the posterior dural wall, causing spinal cord compression

Hirayama Disease: Pathogenesis  The pathogenesis of cervical myelopathy may be ischemic changes or chronic trauma with repeated neck flexion.  Compression may cause microcirculatory disturbances in anterior portion of the cord, leading to necrosis of anterior horns  Changes are often greatest at C6 vertebral level  Primarily affects the anterior horn cells, and in later stages of the disease, spinal cord atrophy ensues.  Some autopsy studies report ischemic changes in the anterior horn cells, with asymmetric cord thinning.

Hirayama Disease: MR findings  Flexion-extension images demonstrate forward migration of the posterior wall of dura  Posterior epidural space enlarges with flexion and is seen as a crescent of high signal on T1 and T2 images  Likely reflects congestion of posterior internal vertebral venous plexus  Uniform enhancement of epidural space with contrast  May have flow voids in epidural space  Compressive flattening of the spinal cord with forward shifting of the posterior dura.

Hirayama Disease: MR findings  Spinal cord flattening is asymmetric in majority of cases.  This is an important finding on routine nonflexion MR images and should raise the suspicion.  Spinal cord atrophy limited to the anterior horn cells found in later stages of the disease.  Morphologic changes on MR images correlate well with clinical and electromyographic data

Hirayama Disease Two flexion T1 post Gd images show increase thickening of posterior epidural space (arrows) which enhances and contains flow voids.

Hirayama Disease Axial T1 post Gd images in same patient show thick epidural space, particularly posteriorly, compressing the spinal cord.

Hirayama Disease: Treatment  Avoidance of neck flexion can stop the progression of the disease  Some advocate application of a cervical collar for 3 to 4 years since the progressive stage usually ceases in a few years.  Surgical intervention, including cervical decompression and/or fusion with or without duraplasty, may be an option.