What is Hirayama Disease?
Hirayama disease, also known as monomelic amyotrophy, Sobue disease or Juvenile non-progressive amyotrophy is a rare and incurable disease that was first described in 1959 by Dr. Keizo Hirayama, a neurologist, attached to the Chiba University School of Medicine in Japan. It mainly affects young males aged between 15-25 years, generally after they experience their adolescent growth spurt. Technically, it is termed as a lower motor neuron disorder.
Classical findings include muscle wasting (atrophy) and weakness of the forearms and hands, either on one or both sides. There is, however, no change in sensation and the patient doesn’t experience any pain. There may be involuntary muscle twitching (fasciculations), but these are rare. The disease usually progresses for 2-5 years before stabilizing or plateauing. After plateauing, improvement in the symptoms does not occur. However, it does not worsen either.
Even though Hirayama disease is regarded as a self-limiting and non-progressive disease, it can be severely debilitating is certain individuals. For this reason, early intervention is vital for halting disease-progression and reducing the severity of the disability.
Salient Features of Hirayama Disease
- Muscle weakness in the hand and forearm
- Muscle wasting in the hands
- Usually affects the hands on one side (unilateral)
- Legs are not affected
- Occurs between the ages of 15-25 years
- Occurs suddenly, progresses gradually for several years, then stabilizes
- Pain or sensory changes do not occur
Epidemiology of Hirayama Disease
Hirayama disease predominantly occurs in Asian countries, especially in Japan, where it was first reported. Other Asian countries where the disease has been reported, include China, Malaysia, Taiwan, Sri Lanka, and India. However, very few cases have been reported from Europe and USA. In the case of India, most patients are from eastern and southern India, and to a lesser extent from northern India. The major affected states include West Bengal, Andhra Pradesh, and Telangana.
Clinical Presentation of Hirayama Disease
Hirayama disease generally starts at puberty and progresses to mid-teens and early adulthood, then reaches a plateau and does not progress any further. But by that time, irreversible damage has already been done to the hands, which become totally useless. Therefore, early diagnosis and medical intervention are vital.
The disease affects males more than females (male to female ratio – 3:1). In most patients, the right hand is predominantly affected at a ratio of 2.8:1, irrespective of hand dominance. However, in severe cases, both hands can be affected. This is especially evident in case of a rare condition called O’Sullivan-McLeod syndrome.
Hirayama disease progresses very slowly, usually up to 5 years, but sometimes, may be as long as 10 years. After this period, the symptoms stabilize, but in rare instances, they can persist till the age of 40 years. Other rare clinical features include worsening of symptoms upon exposure to cold (cold paresis), cold hands, muscle cramps, and occasionally tremors and twitching. However, there is no difficulty in passing urine.
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