Introduction of Hirayama Disease
Hirayama Disease is also known as Benign Focal Amyotrophy
It is also known as:
- Benign focal amyotrophy
- Brachial monomelic amyotrophy
- Benign calf amyotrophy
- Hirayama disease
- Juvenile segmental muscular atrophy
This LMN disease is clinically restricted to one limb.
The etiology of Hirayam Disease is unknown.
On autopsy it is found that the spinal cord has been flattened at the affected region, the anterior horn is atrophied and there is a reduction in number of motor neurons.
Based on these studies Hirayama has proposed a mechanically induced limited form of ischemic cervical myelopathy, because of local compression of the dura and spinal cord against vertebrae during repeated neck flexion/extension, in turn due to disproportionate growth between the contents of the dural sac and the vertebral column.
However, surgical decompression has not altered the course of the Hirayama Disease, and this theory is no longer widely held.
Another school of thought is that this is a segmental, perhaps geneti- cally determined, SMA, but the actual cause is still unknown.
The disease usually begins in the late teens, but many cases can pres- ent in the fourth decade. More than 60% of patients are men.
Although originally described in Indian and Japanese patients, Hirayama Disease is now recognizable around the world.
The most common presentation is one of an idiopathic, slowly progressive, painless weakness and atro- phy in one hand or forearm.
The distribution of muscle weakness varies markedly from case to case, but a characteristic feature is that the condition remains limited to only a few myotomes in the affected limb.
The most common pattern of Hirayama Disease is unilateral atrophy of muscles supplied by C7–T1spinal segments, with sparing of the brachioradialis (the “oblique atrophy” pattern).
Muscle stretch reflexes are invariably hypoactive or absent in the muscles innervated by the involved cord segment but are normal elsewhere. UMN signs are not present, and if they are, one should con- sider the onset of ALS instead.
Approximately 20% have hyperesthesia to pinprick and touch, usually located on the dorsum of the hand. The cranial nerves, pyramidal tracts, and the autonomic nervous system are normal.
Weakness and atrophy may progress steadily for the initial 2–3 years, but most patients have stabilized within 5 years.
The arm is the affected limb in approximately 75% of the patients and the leg in the remaining 25% (benign calf amyotrophy).
Spread may occur to the contralateral limb in about 20% of cases (Gourie-Devi and Nalini, 2003), and rare patients later develop an ALS-like picture.
No pathognomonic laboratory or electrodiagnostic tests exist for this condition; their main purpose is to exclude alternative diagnoses.
Motor nerve conduction studies are either normal or reveal only reduction in the maximum CMAPs; a modest reduction in SNAPs occurs in up to one-third of patients.
The EMG examination may show some fibrillation and fasciculation potentials, and chronic neurogenic motor unit changes are prominent. The C5–T1 myotomes are most commonly involved when the arms are affected. Careful EMG exam- ination may reveal mild neurogenic changes on the asymptomatic con- tralateral side.
The serum creatine kinase (CK) concentration may be modestly elevated, but other routine laboratory test results are normal.
Cervical MRI may reveal segmental spinal cord atrophy or occasionally an area of increased signal on T2-weighted scans of the cervical spinal cord enlargement.
“Incidental” spondylosis and cervical spinal canal stenosis detected by MRI require careful evaluation before the diagno- sis of benign focal amyotrophy is established.
Differential Diagnosis of Hirayama Disease
Two diseases require distinction from benign focal amyotrophy: ALS, which is almost always a relentlessly progressive terminal disease, and MMNCB, which is a treatable peripheral motor neuropathy.
A small proportion of ALS presents as an LMN monomelic disease, albeit in an older patient population. It is only with follow-up examination that the more widespread anterior horn cell disorder becomes apparent and UMN signs appear.
Deep tendon reflexes are almost always hyperactive early in the evolution of ALS.
Furthermore, the electrodiagnostic finding of generalized widespread acute and chronic motor neuron loss distinguishes ALS from the segmental motor neuron involvement of benign focal amyotrophy.
The slowly progressive focal weakness that is distinctive of benign focal amyotrophy may also be the present- ing picture of MMNCB, but detailed motor nerve conduction studies and serum tests for elevated titers of anti-GM1 antibodies can differ- entiate these two conditions.
Cervical or lumbosacral radiculopathy may alsopresent in a similar manner. However, radicular pains and sensory impairment are suggestive of radiculopathy.
Neuralgic amyotrophy aka Parsonage-Turner syndrome- It typically begins with severe pain before the onset of weakness and wasting in the distribution of muscles supplied by the brachial plexus. It may also involve sensory nerves. Most cases are monophasic and do not progress over years.
Cervical syringomyelia or a benign tumor involving nerve roots or the spinal cord may also cause progressive weakness in a monomelic fashion. Careful EMG studies and neuroimaging should differentiate these diseases.
These above entities are the differential diagnosis of Hirayama Disease.
Treatment
Hirayama Disease is a benign condition. It is unlike malignant condition like that of ALS.
This condition is not life threatening, but it impairs motor function in the involved limb, however patients adapt to the diability very well.
Supportive care consists of physical and occupational therapy and effective use of assistive devices (splinting and braces).
Tendon transfers are a consideration in selected patients with focal weakness in a mus- cle group whose function is crucial for certain activities of daily living.
Images of Hirayama Disease:
The above image is suggestive of spinal cord compression at cervical vertebral level in the posterior aspect.
The compression is visible on MRI during the flexion of the neck. This is known as dynamic flexion- extension MRI.
This results in muscle wasting in the hands and forearm.