What is the split hand syndrome?
Many scientists subscribe to the “dying back” hypothesis, whereby degeneration begins at the neuromuscular junction when motor neurons retreat from the synapse. A few others, prefer the “dying forward” or upper motor neuron hypothesis. They believe ALS begins in the brain, before spreading to lower motor neurons. In medicine, the split hand syndrome is a neurological syndrome in which thumb hand muscles undergo mass loss, while the muscles on the side of the little finger are spared. This makes it difficult to grab small objects between thumb and forefinger. If there are no lesions affecting the branches of the ulnar nerve that are directed to the unused muscles, it is almost certain that the lesion is located in the anterior horn of the spinal cord at C8-T1.This area is often associated with ALS and is the place where higher motor neurons join lower motor neurons. This syndrome has been proposed as a relatively specific sign of amyotrophic lateral sclerosis, but it can also occur in other anterior horn disorders, such as spinal muscular atrophy, Charcot-Marie-Tooth disease, poliomyelitis and progressive muscle atrophy. The phenomenon is observed in more than half of ALS patients, and the underlying mechanism is not fully understood. To a certain extent, these characteristics can also be observed during normal aging. The term split-hand syndrome was coined for the first time in 1994 by a Cleveland Clinic researcher named Asa J. Wilbourn.
How does this syndrome relate to ALS?
Composite motor action potential (CMAP) is an electromyographic study (electrical study of muscle function). Several studies have shown a significant reduction in the amplitude of motor action potentials during low frequency repetitive nerve stimulation (RNS) of muscles involved in ALS.
The motor plate is a type of synapse that allows the transmission of a nerve message from a nerve fiber to a muscle fiber in the form of a chemical message by neurotransmitters that will bind to the specific receptors on the surface. muscle fibers. It is not known if the dysfunction of the motor plate is involved in the formation of the divided hand.
A study showing that neuromuscular junction degradation is linked to this syndrome
Dong Zhang, Yuying Zhao, Yan Chuanzhu, Lili Cao and Wei Li have studied the dysfunctions of the neuromuscular junction in different muscles of the hand in patients with ALS, to determine if these dysfunctions are related to the phenomenon of the divided hand. This clinical study at Shandong University's Qilu Hospital enrolled 51 ALS patients, 24 patients with myasthenia gravis had a decrease in RNS, and 20 patients with Lambert Eaton Myasthenia Gravis Syndrome (LEMS).
Who were the patients?
The mean age at onset of the 51 patients with ALS was 58 years old. This group included 23 women and 28 men. The evolution of their disease has varied from 5 to 24 months. Of these, 36 patients had upper limbs, 10 lower limbs and 5 patients had a bulbar form. Patients with myasthenia gravis included 9 men and 15 women, and the mean age was 44 years. The LEMS patients included 16 men and 4 women, and the average age was 59 years old.
What did this study find?
Among the fifty-one ALS patients, thirty-one patients had a split of the hand, 24 patients with the upper limb form and 6 patients with the lower limb form. There was no statistical difference in the frequency of hand splitting between the upper limb group and the lower limb group. There was no hand fracture in patients with bulbar-type ALS. This study showed that more than 60% of the hand muscles of ALS patients had a negative △ D similar to that of patients with myasthenia gravis, but significantly different from that of patients with LEMS, suggesting that Postsynaptic abnormalities could play a major role.
A dysfunction of neuromuscular transmission has been found in the hand muscles of patients with ALS, it is confirmed that the abductor pollicis brevis (short abductor muscle of the thumb) is involved in this syndrome. The dysfunction of the neuromuscular transmission of this muscle could be involved in the formation of the split hand phenomenon. While not a breakthrough, this study highlight a disease starting at the neuromuscular junction, not at the interface between upper neurons and lower neurons in the spine.