A woman with forearm amyotrophy

Abstract

We present the case of a 33-year-old woman with benign sporadic monomelic amyotrophy of the distal part of the arm, called Hirayama disease. Clinical features included forearm amyotrophy sparing the brachioradialis muscle, cold paresis and causalgia. Neck magnetic resonance imaging was normal in neutral and flexion position. Electromyography showed denervated patterns in the extensor digitorum communis, and conduction studies ruled out multifocal motor neuropathy. Motor evoked potentials were normal. Serum IgG anti-GM1 antibodies were moderately raised but were negative 8 months later. Outcome was favourable within 15 months, with partial motor recovery. Pathogenesis remains controversial: neck flexion induced myelopathy via chronic anterior horn ischaemia due to forward displacement of the posterior wall of the dura mater, or benign variant of lower motor neuron disease? Whatever the pathomechanism is, the clinical features and outcome are the same.

Background

This report describe a case of benign sporadic monomelic amyotrophy of the distal part of the arm, sometimes called Hirayama disease. Although Hirayama disease is uncommon, it is not a rarity, and has been extensively described in neurological reviews since its first description by Hirayama et al 50 years ago.¹

We think this case is important for several reasons:

• There is a peculiar clinical feature, the ‘oblique amyotrophy’, which is a forearm amyotrophy sparing the brachioradialis muscle (fig 1). Recognition of this feature should prompt appropriate investigations.

• The pathogenesis of the condition is still debated: neck flexion induced myelopathy or distal benign lower motor neuron disease?

Figure 1.

‘Oblique amyotrophy’: monomelic amyotrophy of the forearm, sparing the brachioradialis muscle.

Case presentation

In September 2007, a 33-year-old right handed woman was referred to our hospital with a 6 month history of progressive weakness and sensory disturbance of the left hand and forearm. She first noticed weakness of the index finger together with hyperesthesia and burning pain of the ulnar side of her forearm. Three months later she experienced cramps in her left forearm. These symptoms were worsened by coldness. Her past medical history was significant for a history of mononucleosis and a neck trauma in a car accident 5 years ago. She reported no allergy and no family history of neuromuscular disease.

The general physical examination was unremarkable. The neurological examination revealed mild muscular wasting of the distal muscles of the left forearm, but the brachioradialis muscle was spared from atrophy (fig 1). Muscle strength was decreased from 0 to 4/5 for the following muscles: flexor digitorum profondus was 0/5 on the index finger, 3/5 on the middle finger and 5/5 on the fourth and fifth finger; extensor digitorum communis was 4/5, extensor indicis proprius was 3/5; flexor pollicis longus was 4/5. There was no weakness in the proximal muscles or muscles of the other limbs, and no fasciculations. Sensory examination disclosed forearm hyperesthesia and mild trophic skin changes suggestive of causalgia. All deep tendon reflexes were present and symmetric.

Investigations

Electromyography showed actively denervated patterns in the left extensor digitorum communis, but all other muscles had normal recordings. Motor and sensory nerve conduction study in the upper and lower limbs was normal for ulnar, median, peroneus and tibialis nerves, except in the left ulnar nerve, where the amplitude was moderately reduced.

Motor evoked potentials were normal and neck flexion or extension did not show any change. The cerebrospinal fluid was acellular with a protein concentration of 0.27 g/l. Normal laboratory studies included creatinine, glucose, liver function tests, antinuclear antibodies, and serologic tests for hepatitis B and C, Campylobacter jejuni, HIV, Coxiella burnetii, and Toxocara canis. Serum IgG anti GM1 antibodies were moderately raised but were negative 8 months later; IgM anti GM1 antibodies were negative. Magnetic resonance imaging (MRI) of the neck in neutral and flexion position was normal.

Differential diagnosis

Differential diagnoses include a limited form of multifocal motor neuropathy or motor neuron disease, a radiculopathy or an intra-axial process of the lower cervical cord.

Outcome and follow-up

Partial motor recovery and spontaneous relief of pain occurred 15 months after the onset of symptoms, and no relapse occurred after a 30 month follow-up.

Discussion

Monomelic amyotrophy is a rare sporadic benign lower motor neurone disease affecting mostly young adults, with insidious onset, slow progression for 2–4 years followed by a stationary course.² When monomelic amyotrophy is restricted to the distal aspect of the upper limb it is called Hirayama disease.³ Core clinical features are wasting and weakness restricted to one limb, without involvement of cranial nerves, sensory or cerebellar systems. There are no pyramidal signs. Surprisingly, the brachioradialis muscle is spared and atrophy of the forearm is peculiar, referred to as ‘oblique amyotrophy’ (fig 1). Less common features include coldness of hands, hyperhidrosis, and aggravation of motor symptoms on exposure to cold.^2,4 Electromyography shows neurogenic pattern without conduction block.² Anti-GM1 antibodies can possibly be raised.^5,6

The pathogenesis of monomelic amyotrophy is still debated. Hirayama was the first to describe this entity 50 years ago, and hypothesised later that it was a flexion induced cervical myelopathy on the basis of forward displacement of the posterior wall of the dura mater on flexion-extension MRI images.⁴ Then 60 cases were described, linking MRI abnormalities to clinical features. However, a similar clinical entity was described without MRI abnormalities³ and similar MRI abnormalities were described in normal subjects.⁷ Therefore some authors consider Hirayama disease to be a motor neuron disease rather than a neck flexion induced myelopathy,^5,8 with more than 40 cases published. MRI abnormalities or not, long term follow-up reveals no worsening of disability or new neurological deficit.^2,5

Learning points

• Hirayama disease is a rare benign monomelic amyotrophy of the distal part of the arm.

• “Oblique amyotrophy”, an amyotrophy of the forearm and the hand sparing the brachioradialis muscle, is a peculiar clinical feature suggestive of Hirayama disease.

• Unlike other motor neuron diseases, outcome is favourable, with stabilisation over 3–5 years, without relapse or worsening of the condition.