Hirayama Disease

Hirayama Disease

Aka Juvenile Muscular Atrophy of the Distal Upper Extremity

Rare disease that affects predominantly males in their 2nd or early 3rd 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.