What are the symptoms of motor neurone atrophy?

There are many nerves in the human body, the most critical of which are the motor nerves. When the motor neurons are healthy, the human body can move normally. Motor neuron atrophy can lead to growth disorders and cause many hazards. Motor neuron atrophy can cause a variety of lesions, which are divided into many types, usually lower motor neuron type, upper motor neuron type, motor neuron mixed type, and other lesions.

Clinical manifestations

According to the location of the lesion and clinical symptoms, it can be divided into three types: lower motor neuron type (including progressive spinal muscular atrophy and progressive bulbar palsy), upper motor neuron type (primary lateral sclerosis) and mixed type (amyotrophic lateral sclerosis).

The relationship between them is not yet fully understood. For some patients, they are manifestations of this unit disease at different stages of development. For example, in the early stage, only muscular atrophy appears, and then pyramidal tract symptoms appear, presenting typical amyotrophic lateral sclerosis. However, some patients only have muscular atrophy during the course of the disease. In a very small number of patients, only slowly progressive pyramidal tract damage symptoms appear during the course of the disease.

Lower motor neuron type

Most cases occur around the age of 30. The disease usually begins with weakness and gradual atrophy of the small muscles in the hands, which may affect one or both sides, or start on one side and then spread to the other side. The palm becomes flat due to atrophy of the thenar and hypothenar muscles, and the hand becomes claw-shaped due to atrophy of the interosseous muscles. Muscle atrophy spreads upward, gradually affecting the forearm, upper arm and shoulder girdle. Muscle strength is weakened, muscle tone is decreased, and tendon reflexes are weakened or disappeared. Fasciculations are common and may be confined to certain muscle groups or widespread and are easily induced by tapping with the hands. In some cases, muscle atrophy starts from the tibialis anterior and peroneus muscles of the lower limbs or from the extensor muscles of the neck. In some cases, it may also start from the proximal muscles of the upper and lower limbs.

The tongue muscles are often the first to be affected by cranial nerve damage, causing atrophy of the tongue muscles accompanied by tremors. Later, the palate, pharynx, laryngeal muscles, and chewing muscles also gradually atrophy and become weak, resulting in slurred articulation, difficulty swallowing, and chewing weakness. Bulbous palsy may be the first symptom or may follow limb atrophy.

In the late stage, muscles throughout the body may atrophy, causing the patient to be bedridden, and respiratory insufficiency may occur due to paralysis of the respiratory muscles.

If the lesion mainly affects the anterior horn of the spinal cord, it is called progressive spondylolisthesis. Because it begins in adulthood, it is also called adult-onset spinal muscular atrophy, to distinguish it from infantile and juvenile spinal muscular atrophy that occur in infancy or adolescence. The latter two often have family genetic factors, and the clinical manifestations and course of the disease are also different, so they are not described in detail. If the lesion mainly affects the bulbar muscles, it is called progressive bulbar palsy or progressive bulbar palsy.

Upper motor neuron type

It manifests as limb weakness, tightness, and clumsy movements. Because the lesions often first invade the corticospinal tracts of the lower thoracic cord, the symptoms begin in the lower limbs and later spread to the upper limbs, with the lower limbs being the most severe. The limbs are weak, the muscle tone is increased, the walking is difficult, the gait is spastic, the tendon reflexes are hyperactive, and the pathological reflexes are positive. If the lesion involves the bilateral cortical brainstem, symptoms of pseudobulbar palsy will appear, manifested as clear pronunciation, swallowing difficulties, and hyperreflexia of the jaw. This disease is called primary lateral sclerosis. It is rare in clinical practice. It usually starts in adulthood and generally progresses very slowly.

Mixed upper and lower motor neuron type

The first symptoms are usually weakness and atrophy of the hand muscles, which usually start on one side and then spread to the other side. As the disease progresses, mixed damage to the upper and lower motor neurons appears, which is called amyotrophic lateral sclerosis. Generally, the lower motor neuron damage in the upper limbs is more serious, but the muscle tone may increase, the tendon reflexes may be active, and there are pathological reflexes. When the lower motor neurons are severely damaged, the symptoms of upper motor neuron damage in the upper limbs may be masked. In the lower limbs, the symptoms of upper motor neuron damage are prominent. In bulbar palsy, the tongue muscles atrophy and tremor are obvious, while the mandibular reflex is hyperactive and the sucking reflex is positive, indicating combined damage to the upper and lower motor neurons. In the late stage of the disease, muscles throughout the body become emaciated and atrophied, resulting in inability to lift the head, difficulty breathing, and being bedridden. The disease usually occurs between the ages of 40 and 60, and about 5% to 10% have a family history of the disease. The course of the disease progresses at varying speeds.

Motor neuron disease is a progressive disease of unknown cause that selectively damages the anterior horn of the spinal cord and the motor nerve nuclei of the brainstem. With the degeneration and loss of motor neurons, all muscles of the patient except the eyes and the muscles that control urination and defecation are affected.