One of the symptoms of spinal muscular atrophy is weakness in the movement of the hands. It has an insidious onset and usually occurs in middle-aged men. It is characterized by weakness of the hands and atrophy of the intrinsic muscles of the hands, which may have a “claw-shaped hand” or “ape hand” deformity. Here is how to check the weakness of the hands. About 1/3 of SMA-I cases develop in utero, and the mother may notice weaker fetal movements. Half of the cases start within 1 month of birth, and almost all cases develop within 5 months. The incidence is about 1/10,000 births and is equal in both sexes. Most of them show hypotonia soon after birth, and muscle weakness is mainly in the proximal muscles of the limbs, but also in the trunk muscles. The infant has weak sucking and swallowing, low crying, shallow breathing, and may have abnormal thoracic movement. Difficulty in turning over and raising the head. Tendon reflexes are absent. Muscle atrophy of the extremities can be detected on palpation, but is often masked by subcutaneous fat. Eye movements are normal. Sphincter function is normal. Tongue muscle atrophy and fasciculations may be seen. 10% of cases may have joint deformities or contractures. The prognosis for this type is poor. Approximately 95% of cases die in the first 18 months of life. The onset of SMA-II is slightly later than that of SMA-Ⅰ, usually within 1 year of age, and rarely between 1 and 2 years of age. The incidence is similar to that of SMA-Ⅰ. Infants have normal early growth, but after 6 months of age, motor development is delayed, and although they can sit, they cannot stand alone or walk at a normal level. 1/3 of the children cannot walk, and 20% to 40% of the children still have the ability to walk before the age of 10. The majority of cases show severe proximal muscle weakness, with the lower extremities more than the upper extremities, while the respiratory and swallowing muscles are generally not involved. The facial muscles are involved in 1/3 of cases, and the tongue and other muscles are fibrillated in more than 50% of cases. Tendon reflexes are diminished or absent. This type has a relatively benign course, with most cases surviving into childhood and some into adulthood. SMA-III is also known as Kugelberg-Welander disease. The disease usually begins in early childhood to adolescence, with the majority starting before age 5. The onset is insidious and presents with progressive proximal limb muscle weakness and atrophy. Early on, muscle weakness in the thighs and hips is more pronounced, causing the child to walk with a duck stance and difficulty in climbing ladders, gradually involving the scapular girdle and upper extremity muscles. The muscles innervated by the brain are not usually involved, but weakness of the facial and soft palate muscles may occur. The extraocular muscles are normal. Pseudohypertrophy of the gastrocnemius muscle is present in about one quarter of cases, almost always in males. In half of the patients, muscle bundle tremor is seen early. Arching of the foot may also be seen. Tendon reflexes are diminished or absent. Sensation is normal. The prognosis for this type is good, especially in women. Survival is usually into adulthood, and many patients have a normal life expectancy. The more severe cases tend to be male patients. In addition to neurogenic changes, EMG can be confused with myogenic damage and should be differentiated from myotonic dystrophy. SMA-IV is collectively referred to as adult SMA, with the age of onset ranging from 15 to 60 years, mostly around 35 years. The onset and progression are relatively insidious, but cases of progressive exacerbation or relative quiescence have been reported. The prognosis for this type is relatively good, and the ability to walk is often maintained for life. The incidence is less than 0.5 per 100,000. About 1/3 of the cases are autosomal dominant and present with proximal muscle weakness that progresses slightly faster, with loss of running ability after about 5 years. There are also autosomal recessive types, which generally have a more benign course. The other type is X-linked recessive, also known as spinal brainstem SMA (Kennedy disease), which has a variable age of onset, but often develops before the age of 40. The early presentation of painful muscle spasms may precede the onset of muscle weakness by several years. Proximal muscle weakness often begins in the lower extremities and gradually spreads to the scapular girdle, facial muscles, and medullary innervated muscles. Fibrillation of the lower muscles and lingual muscles may be seen. Dysphagia and nagging may appear after several years. In about 50% of cases, some endocrine dysfunction is combined with gynecomastia and primary testicular lesions.