Thread by @neurogenicpoet

Distraught parents bring their son to you. They tell you that he has been having weakness of his left hand since 1 year. With wasting. No sensory symptoms. They have been told, Motor Neurone Disease . You look at his hand and smile reassuringly. Why?

First things first, the demographics don't fit ALS. He's a young boy. Secondly, The disease seems quite restricted. Third, no signs of upper motor neuron disease. Finally, this OBLIQUE AMYOTRPOHY is classical and clinches a diagnosis of HIRAYAMA DISEASE

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Hirayama Disease typically affects young Asian males in their twenties. I have seen dozens of patients with Hirayama, never a girl (although case reports exist)
These boys tend to be lean and have long necks!
It is much more benign compared to ALS!

The disease usually presents as an insidious weakness, generally asymmetrical affecting C7, C8, T1 Myotomes
Generally progresses over a few years, more rapid at inset and then becomes static.
Sensory symptoms are ABSENT, Reflexes spared

Classically C7, C8, T1 myotomes are involved with sparing of C5, C6. Hence classically the BRACHIORADIALIS is spared. See the image again. Wasting of the ulnar side of the forearm and palm with sparing of Brachioradialis.
This is OBLIQUE AMYOTROPHY

Also note a REVERSE SPLIT HAND
Hypothenar muscles are more involved than Thenar Muscles (Reverse of ALS). One may also note fasiculatory tremors

HIRAYAMA is a "DUROPATHY"
It is hypothesised to occur due to tightening of dura due to long neck, disproportionate growth of cervical spine, leading to "dynamic compression" of cord during flexion. Anterior Horn Cells, being particularly susceptible to ischaemia, bare the brunt

Investigations:

Clinical Exam (including neck length!)

In #NeuroRadiology Contrast MRI Cervical Spine with neck flexion (35 Deg). should be sought. Look for anterior shifting of dura, leading to cord compression against vertebral body

NCS/EMG: Disproportionate reduced CMAP's of Ulnar compared to Median Nerve (opposite of ALS, electrophysiological reverse split hand, basically!)
All SNAPs normal with chronic dennervation in clinically affected muscles

Besides ALS, differentials include Syringomyelia, Brachial Plexitis, High Cervical Compression.

With history of Trauma keep in mind an entity called VLISFC (won't get into details here)

Treatment:
Cervical Collar
Surgery in select cases
Rehabilitation (Physiotherapy and Occupational Therapy)
Acetazolamide??

Patient's that have been following up have been doing well with Physiotherapy and Occupational Therapy. The disease usually gets static over time, and patients slowly adapt. In severe cases, there can be profound weakness and wasting, causing significant morbidity.

Over time, Hirayama becomes a spot diagnosis. A relatively benign disorder who's differentials are sinister!
Look for Oblique Amyotrophy and Reverse Split Hand

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