What to do next?

An EMG was done. This showed normal sensory studies and normal nerve conduction velocities with low-amplitude ulnar responses. There was denervation in the lower myotomes with chronic changes and this was bilateral.

What should be done?

An MRI of the spine showed flattening of the cervical spinal cord with mild spondylosis and “snake eye” finding ( Fig. 33B-2 ). No flexion contrast MRI was done. This could show dilatation of the cervical epidural space ( Fig. 33B-3 ).

T2-weighted MRI image of the cervical spine showing “snake eyes “lesions (arrow)

Dilatation of the cervical epidural space (arrow).

Reproduced with permission by Khadilkar SV, Yadav RS, Patel BA. Neuromuscular Disorders: A Comprehensive Review With Illustrative Cases . Springer; 2018.

Differential Diagnosis

This patient could have cervical spine disorder such as cervical spondylosis affecting multiple roots, but the lack of pain and sensory deficits is somewhat against this. Other than tumor, other possibilities would be intermedullary tumor or syringomyelia. This produces wasting in the hands, but the patients usually have a cape anesthesia, which is similar to intermedullary tumors.

There are cases of multifocal motor neuropathy that are predominantly axonal. Although they do not have a noticeable conduction block, this condition should be taken into consideration when making a differential diagnosis because it is not present in the patient’s case of Hirayama disease.

Spinal muscular atrophy of distal loss has been reported, and this is unusual; however, Welander muscular dystrophy affects mainly the upper extremities and should be considered in the differential diagnosis. Usually these patients have an elevated creatine kinase. Ulnar decompression was most likely not necessary in this patient who also had involvement of other muscles innervated by the ulnar nerve. If he had nerve conduction slowing at that time, HNPP should also be a consideration, although again there was no other nerve involvement.

Summary

This patient had bilateral Hirayama disease based on the clinical and EMG findings and had “snake eye” findings on the MRI. He was treated conservatively.

Discussion for Cases 33A and 33B

Since the initial description by Hirayama in 1959 of a group of patients with juvenile muscular atrophy of a unilateral upper extremity, there have been several reports of similar cases given various descriptive terms, including benign focal amyotrophy (BFA) and monomelic amyotrophy. Although the total number of reported cases is small, one study concluded this to be a benign focal atrophy which represented 11% of motor neuron diseases. The condition appears to be a more benign form of motor neuron disease with a fairly distinct course. The differential diagnoses include juvenile spinal muscular atrophy, ALS, late motor neuron degeneration after poliomyelitis, syringomyelia, radiculopathy or brachial plexopathy, multifocal motor neuropathy, and multifocal acquired motor axonopathy. Juvenile spinal muscular atrophy is unlikely as this characteristically is bilateral and symmetrical involving arms and legs with atrophy. Weakness seen as a late sequela of poliomyelitis typically occurs several years after a polio attack and is progressive. Syringomyelia usually includes sensory abnormalities, as well as characteristic CT and myelographic findings. Brachial neuritis often has severe pain, selective sensory involvement, and no fasciculations and does not progress. Multifocal motor neuropathy affects individual nerves, showing evidence of conduction block in nerve conduction tests. Multifocal axonal neuropathy also affects individual nerves, but no conduction block is demonstrated. Structural lesions, such as arteriovenous malformation, tumor, or a central herniated disk, can give rise to a variety of findings referable to the upper extremities and have characteristic radiologic findings.

Due to the selective involvement of predominantly one limb, most patients undergo imaging studies to rule out the presence of such a structural lesion. Previous case reports of BFA have shown normal radiologic studies, although Mukai et al. recently reported in the Japanese literature that their patients with distal and segmental muscular atrophy showed focal cord atrophy, as in Case 33A. The pathologic findings of typical motor neuron diseases include degeneration of large anterior horn cells in the spinal cord with resulting atrophy of the anterior roots, degeneration of the pyramidal tracts, and diffuse involvement of the anterolateral columns. Due to the benign nature of BFA, pathological findings are scarce, but this shows shrinkage and necrosis of the anterior horns. MRI imaging, which was not available at the time that patient 33A was studied, provides less traumatic and more objective means to demonstrate cord atrophy and other features which could include the presence of “snake eyes” as well as shrinkage of the cord and dilatation of the epidural space and veins.

We concluded that these patients had a syndrome consistent with BFA, although in this condition distal arm involvement is more characteristic. The limited monomelic involvement in Case 33A, age of onset, electrophysiologic findings, and clinical course are quite consistent with descriptions of the disorder.

Increased reflexes in atrophic denervated limbs have been reported and occur in up to 15% of patients. The hyperreflexia could suggest involvement of the upper motor neurons, but the lack of upper motor neuron signs is against that possibility. A bilateral presentation as in Case 33B is being reported.

The radiologic finding on CT myelography showing a defect in the anterolateral cervical cord in Case 33A is consistent with cord atrophy. This defect could represent localized degeneration of anterior horn cells thought to be involved in the pathogenesis of the syndrome. Spinal cord atrophy has been documented by CT in cases of cervical spine trauma, spondylosis, and syringomyelia.

MR imaging, which was not available at the time that patient 33A was studied, provides a less traumatic and more objective means of demonstrating localized spinal cord atrophy and rule out other focal diseases.

The etiology of BFA is unclear but has been postulated to be caused by immobilization of the neck after trauma. This is supported by the findings that neck flexion flattens the cord and stretches its vessels, which could produce ischemia. Alternatively, the dura and cord may be compressed against the body of the vertebrae during flexion. Neck flexion has been reported to cause electrophysiologic dysfunction. A recent study documented an anterior displacement of the dural sac with flattening of the cord during flexion, associated with engorgement of the epidural venous plexus by venography. Rare familial cases have been reported, one was associated with a mitochondrial DNA deletion. There has also been an association with airway allergy, suggesting an autoimmune-mediated process in some patients. A benign monomelic amyotrophy of the lower extremity has been reported.

There is no specific therapy for this disease, although cervical collars may slow the progression in early cases.

Important Points

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  Focal atrophy of muscles in a proximal upper extremity could be caused by a C5–C6 radiculopathy, upper trunk plexopathy, and/or a motor neuron disorder such as ALS, or by multifocal motor neuropathy and BFA or monomelic amyotrophy or Hirayama disease.

  
  
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  Distal upper extremity weakness, atrophy and fasciculations, and the lack of sensory findings with a very slow progression are characteristic of BFA. This condition demonstrates that not all acquired motor neuron disorders have a grim prognosis.

  
  
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  The EMG in patients with this presentation should include several myotomal levels, at least one upper and one lower extremity, and the opposite limb if a motor neuron disorder is suspected. Paraspinal muscles should also be studied. Needle testing should include muscles innervated by motor cranial nerves such as those of the tongue and, if necessary, thoracic paraspinals to rule out ALS.

  
  
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  BFA is a nonprogressive, or slowly progressive motor neuron disorder that affects mainly young individuals with involvement of distal upper extremities but may manifest in the proximal arm and in the lower extremities and can be bilateral.

  
  
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  Post-myelography CT scan or MRI may show segmental cord atrophy and dilatation of veins in the epidural space.

  
  
  
  
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    Hirayama disease is thought to be caused by epidural venous distension of the cervical spine which can be diagnosed with flexion MRI with contrast. MRI findings in Hirayama disease can also show “snake eye” findings.

    
    
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    The treatment of Hirayama disease is conservative and braces can help.