How is spinal muscular weakness diagnosed?

Spinal muscular weakness is a relatively common disease that may cause motor nerve disorders in patients and may even cause some complications. When doing this test to confirm the diagnosis, it is necessary to use genetic and serum diagnosis, including muscle scanning, to determine the specific problems of the disease and make a systematic diagnosis.

1. Serum CPK: The serum CPK of SMA-Ⅰ type is normal. Type II is occasionally elevated, and its CPK isoenzyme MB is often elevated. Type III CPK levels are often elevated, sometimes reaching more than 10 times the normal value, and isoenzyme changes are mainly MM; generally, CPK levels often increase with the development of muscle damage, and only begin to decline in the late stage when muscles are severely atrophied.

2. Genetic diagnosis For childhood SMA, the diagnosis can generally be made by amplifying exons 7 and 8 of the SMNt gene through the PCR method and combining it with single-stranded conformation polymorphism analysis (SSCP) or using DraI and DdeI to perform enzyme digestion map analysis of exons 7 and 8 of the SMNt gene.

Other auxiliary examinations:

1. CT muscle scan: This helps to differentiate SMA from other types of muscular dystrophy. SMA presents diffuse low-density changes with incomplete contours and loss of muscle tissue reflexes, while muscular dystrophy presents with a large number of low-density lesions, affecting all muscles. Pseudohypertrophy is generally rare in SMA patients.

2. Electrophysiological examination EMG can reflect the severity and progression of the four main types of SMA. However, the abnormal changes are similar, including increased amplitude and duration of fibrillation potentials and compound motor unit action potentials (MUAPs) and reduced interference phases. In cases of SMA-III and SMA-IV, neurogenic and myogenic potentials can sometimes be seen mixed in the same muscle. Myogenic MUAPs may be more obvious in patients with elevated CPK levels. In some SMA-III cases, muscle biopsy showed neurogenic damage, while EMG showed myogenic damage, suggesting that EMG and clinical features may be inconsistent. Fibrillation potentials and positive sharp waves were found in all types of SMA, but they were more obvious in SMA-Ⅰ, seen in all patients, while they were only seen in 60% of SMA-Ⅲ patients. Fasciculation potentials are positive in about 20% of SMA type I and 50% of type III. A unique manifestation of SMA-Ⅰ is that when the limbs are relaxed, spontaneous emission of MUAPs of 5 to 15 Hz can be seen. During voluntary movements, all types of SMA show a reduction in the interference phase, especially in SMA-Ⅰ type, which only presents a simple phase, which is evidence of motor unit loss. In more advanced cases of SMA-Ⅲ and Ⅳ, low-amplitude multiphasic potentials similar to myogenic damage can be seen, which is consistent with the secondary myogenic changes suggested by muscle biopsy.

3. Pathological examination Muscle biopsy is important for the diagnosis of SMA. Its pathological manifestations are characterized by denervation and reinnervation.