Second Impact Syndrome: A Medical Catastrophe that need not occur

In our most recent Newsletter, “Second Impact Syndrome,” is discussed at length. You can also sign up here to receive our Forensic Science Newsletter by email.

Definition of Second Impact Syndrome (SIS)

Occurs when an athlete who has sustained a mild traumatic brain injury, usually a concussion, with or without loss of consciousness, suffers a second traumatic brain injury before the symptoms associated with the first have cleared. Such patients are at risk for developing intense cerebral vascular congestion followed by diffuse brain swelling, which typically occurs immediately after the second impact to the head or to the trunk, the latter imparting acceleration to the brain.

Concussion is defined as a trauma-induced alteration in mental status, usually characterized by confusion and amnesia that may or may not involve loss of consciousness, link “Mild Traumatic Brain Injury (Concussion) in Infants, Toddlers, Children and Adolescents.”

General Information

Typically, the majority of athletes who experience second impact syndrome are under the age of 18, although it can also be seen in college athletes and rarely,  professional athletes.

The term “second impact syndrome of catastrophic head injury,” was coined in 1984 by R. L. Saunders and Harbaugh.

In July 2007, eMedicine summarized an article in the American Journal of Sports Medicine in which they identified 84 catastrophic head injuries following a study of American high school and college football players. Seventy-one percent of these high school players had suffered a previous concussion in the same season, with 39%
playing with residual symptoms.

A not uncommon associated injury in second impact syndrome is an acute subdural hemorrhage.

Clinical Presentation

Before the symptoms clear from the athletes first concussion, they resume a contact sport and sustain a second blow to the head, often very mild, with no loss of consciousness. The athlete may appear stunned, remaining on their feet for a short time, a few seconds to a minute or so, after which they suddenly collapse, lose consciousness, rapidly develop dilating pupils with no eye movement, which rapidly progresses to respiratory failure and death within 2 to 5 minutes.

An immediate CT scan shows vascular engorged hemisphere, which is followed by rapidly developing cerebral edema with flattening of the gyri and narrowing of the sulci, partial to complete collapse of the ventricles, midline shift, collapse of the basal cisterns,
distortion of the brainstem due to uncal and diencephalic herniation, and hemispheric asymmetry.

Mortality rate is approximately 47-50%. Those who survive have a virtual 100% morbidity (neurologic disability), which is primarily due to multifocal post-traumatic ischemic infarctions.


The athlete must be transported to the closes hospital and while in route be intubated, hperventilated and receive intravenous (IV) mannitol or hypertonic saline, preferably the latter, to address the rapidly evolving increase in intracranial pressure due to catastrophic vascular congestion and developing cerebral edema.


Research has suggested the symptoms of concussion, such as loss of consciousness after mild traumatic brain injury, the subsequent development of secondary brain damage, and the enhanced vulnerability of the brain after the initial concussion is largely due to ionic fluxes, acute metabolic changes, and cerebral blood flow alterations that occur immediately after the concussion.

The pathophysiology of the second impact syndrome is generally believed to be caused by a loss of autoregulation of the cerebral vasculature.

It has been shown following the primary direct mechanical injury to the brain, important injuries develop over hours to days from multiple mechanisms including hypoxemia, ischemia, and immunoexcitotoxicity. An in depth discussion of immunoexcitoxicity is given in “Postconcussive-Syndrome-Adults,” pages 4-11.


The overall incidence of secondary impact syndrome is unknown. Most of the information comes from case reports and series, which is given in the article, Second Impact Syndrome, pages 8-9.


The best prevention for second impact syndrome is to prevent the first concussion. Experts advise wearing protective head gear in high-impact contact sports.


Should an athlete sustain a concussion, with or without loss of consciousness, under no circumstances are they to return to play until all concussive symptoms have cleared.

For a more in depth understanding of second impact syndrome please review our latest Forensic Science Newsletter, “Second Impact Syndrome.” You can also sign up to automatically receive our Forensic Science Newsletter.

The website,, also has several books for your review under the heading of Neuropathology and Forensic Pathology.

Second-Impact-Syndrome Newsletter

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