Second-impact syndrome
(重定向自Second Impact Syndrome)
![Types of brain herniation[25] 1) Uncal 2) Central - The brainstem herniates caudally. 3) Cingulate herniation - The brain squeezes under the falx cerebri. 4) Transcalvarial herniation - through a skull fracture 5) Upward herniation of the cerebellum 6) Tonsillar herniation - the cerebellar tonsils herniate through the foramen magnum.](/uploads/202502/09/Brain_herniation_types-2.svg5554.png)
Second-impact syndrome (SIS) occurs when the brain swells rapidly, and catastrophically, after a person suffers a second concussion before symptoms from an earlier one have subsided. This second blow may occur minutes, days or weeks after an initial concussion, and even the mildest grade of concussion can lead to SIS. The condition is often fatal, and almost everyone who is not killed is severely disabled. The cause of SIS is uncertain, but it is thought that the brain's arterioles lose their ability to regulate their diameter, and therefore lose control over cerebral blood flow, causing massive cerebral edema.