Second Impact Syndrome

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Continuing Education Activity

Second impact syndrome (SIS), also known as repetitive head injury syndrome, describes a condition in which individual experiences a second head injury before complete recovery from an initial head injury. It is thought that athletes who sustain a concussion and return to their sport early are at particularly high risk. Though it is a relatively rare condition, clinicians should be aware of SIS and educate patients who have experienced or are at risk of experiencing a head injury, as the syndrome is often deadly. This activity describes the pathophysiology, evaluation, and management of second impact syndrome and highlights the role of the interprofessional team in the management of affected patients.


  • Describe the risks associated with second impact syndrome.
  • Identify the risk factors for second impact syndrome.
  • Review the controversy surrounding the proposed mechanism of injury in second impact syndrome.
  • Explain the importance of improving care coordination amongst interprofessional team members to improve outcomes for patients affected by second impact syndrome.


Second impact syndrome (SIS), or repetitive head injury syndrome, describes a condition in which individual experiences a second head injury before complete recovery from an initial head injury[1]. Recently SIS has gained increased attention as many cite athletes sustaining a concussion and returning to the sport early as being particularly at risk. Though it is a relatively rare condition, physicians should be aware of SIS, and educate patients who have experienced or are at risk of experiencing a head injury, as the syndrome is often deadly. The term second impact syndrome entered the medical lexicon in 1984 when Saunders and Harbaugh wrote an article describing a case report of a football player who died four days after suffering a head injury after he returned to play on the day of his death. He collapsed and died after a presumed second head injury.[2]


There have been very few confirmed cases of SIS to date; therefore, the exact incidence, risk, and pathophysiology of the condition are not well known. According to the Centers for Disease Control and Prevention, an estimated 1.6 to 3.8 million sports-related concussions occur in the United States each year.[2]While the etiology of a concussion is generally understood, the etiology of a second impact syndrome is not well understood.  The generally accepted cause relates to sustaining a second concussion before the brain has a chance to recover from the initial insult fully.  The athlete will rapidly develop altered mental status and a loss of consciousness within seconds to minutes of the second hit resulting in catastrophic neurological injury.[3] The catastrophic injury results from the dysfunctional cerebral blood flow autoregulation leading to an increase in intracranial pressure. The pressure rapidly develops and eventually results in brain herniation. The herniation may occur either medially across the falx cerebri or inferiorly through the foramen magnum, resulting in brain stem injury and rapid deterioration and leading to death within 2 to 5 minutes. A recent case report sheds some light on the injury pattern[4]. An athlete returned to hitting drills five days after his initial concussion. He had a normal CT scan on day four but had a persistent severe headache. After getting hit at practice one day later, he collapsed several plays later after complaining of headaches and not being able to feel his legs. A second CT done in a local emergency room demonstrated bilateral but thin subdural hematomas. However, a subsequent MRI demonstrated caudal displacement of midline structures with injury to both thalami resulting in transtentorial herniation.[4]


A review article in 2016 completed a PubMed search and found only been 36 cases reported in 15 publications with 17 cases meeting inclusion criteria.[3] The characteristics of an athlete susceptible to a second impact injury were male gender, ages 13-24, contact sports like American football, boxing, and hockey. Only seven of the 17 receiving a direct blow to the head and the others were thought to have received a blow to the body and the forces transmitted to the head. CT imaging demonstrated diffuse cerebral edema with midline shift leading to herniation in 4 of the cases. Additionally, all of the cases had either thin or moderate subdural hematomas, and 2 of the cases had a subarachnoid hemorrhage, and three suffered an ischemic stroke.


Patients who have experienced a concussion show a poorly understood but complex array of neuronal, metabolic, and ionic changes. The mechanism of injury seems to be associated with axonal shearing. This causes rapid depolarization, neurotransmitter release, and ionic shifts where potassium ions leak extracellularly, and they are replaced with sodium and calcium intracellularly. This results in dysregulation of the cerebral blood flow leading to edema. Thus increased glucose utilization combined with the injury-related decrease in resting cerebral blood flow creates an energy mismatch. [5] All these changes require time and energy for a return to normal neurotransmitter physiology. Generally, this is thought to take about 7-10 days but may be longer in younger athletes. Metabolic abnormalities after an initial concussion may leave the brain more susceptible to further injury. [6]

History and Physical

Any athlete who has returned to play after a concussion should be watched carefully, even if they have had an appropriate recovery period and completed a return to play protocol. A concussed patient will report any combination of symptoms including a headache, nausea, memory loss, dizziness, blurry vision, confusion, fatigue, photo- or phonophobia, motor or sensory loss, poor hand-eye coordination, or emotional irritability/labiality. On physical examination, the patient may have an altered level of consciousness, retrograde, or post-traumatic amnesia, but generally, difficulty concentrating and balance seems to be consistently impaired.  Additionally, there may be sensory or motor abnormalities, visual abnormalities. Unless the player has been knocked out, if they remain on the field of play may show signs of ataxia, running in the wrong direction, or slowed reaction time.

As concussions have been more widely recognized and the complications related to insufficient recovery have become better recognized, sideline evaluations have developed. Sideline evaluation of cognitive function is an essential component in the assessment of this injury. Brief neuropsychological (NP) test batteries that assess attention and memory function have been shown to be practical and effective. Such tests include the SCAT5, which incorporates the Maddocks' questions and the Standardised Assessment of Concussion (SAC).[7]

For an athlete who has collapsed with a suspected SIS, a Glasgow coma scale score, pupillary reactivity, deep tendon reflexes, upper/lower extremity clonus, and the presence or absence of a Babinski reflex can be done prior to transport or upon arrival at the emergency room.  

Due to the challenges of what constitutes a second impact syndrome, the paucity of research, and the rapidly progressing nature of the condition, it is difficult to provide good guidance on an appropriate history and physical exam. Perhaps clinicians should change their mindset and think every concussion they see, especially in male athletes ages 13-24, could be a second impact syndrome and observe the injured athlete closely for progressing signs or symptoms.  


Obtaining a thorough history is critical in evaluating a patient with a suspected brain injury. Though the patient will unlikely be able to provide a good history, one should gather as much information from those who may have witnessed the incident. It is especially important to determine how the injury occurred, if there is any seizure or concussion history, if alcohol or illicit drug use was involved, whether or not there was any loss of consciousness, any resultant weakness or paresthesias, difficulty walking, or incontinence of the bladder or bowel.

Patients who have experienced significant injury or loss of consciousness, continued symptoms, deterioration in neurologic function, or neurologic deficits should be evaluated with imaging studies.

Screening for alcohol or illicit drugs would be recommended. Computed tomography (CT) scanning is the preferred imaging modality for acute head trauma. It is a more sensitive imaging modality for detecting acute hemorrhage (e.g., non-enhanced CT). It provides better delineation of bone (e.g., for detecting any skull fractures) and more sensitive in detecting acute hemorrhage and identifying any surgically reversible injuries. 

Current guidelines recommend a CT for a suspected skull fracture, intracranial bleeding, or other intracranial disorders based on physical exam findings.[8] Since second impact syndrome injuries generally cause loss of consciousness, it would seem prudent to start with a CT scan of the head. 

Treatment / Management

The management of second impact syndrome is limited due to the limited understanding of the condition, paucity of research, and the controversial nature of the condition. The management of the condition starts with prompt recognition of a sports-related concussion and protecting the athlete until they recover from the initial injury. The current treatment guidelines consist of relative rest, both cognitive and physical, and then a graded return to play. This should be supervised ideally by the team physician (with experience in managing concussions) and athletic training staff. The athlete should not be cleared to return to full participation until they have completed a return to play protocol, and the medical staff gives clearance.[5]

Differential Diagnosis

  • Subarachnoid bleed
  • Ischemic stroke
  • Subdural hematoma
  • Basal skull fracture
  • Skull fracture

Deterrence and Patient Education

At this point, prevention seems to be the most logical answer for a second impact syndrome. Parents and athletes should be educated on the potential complications of a concussion, symptoms, and an expected time course for recovery. Included in this would be discussing the potential problems related to returning to play too soon, including prolonged recovery, persistent symptoms, and a second impact syndrome. Encouragement from parents and coaches will help the athlete to avoid minimizing symptoms and from returning to play too soon. Most return to play protocols will take at least seven days before the athlete is cleared to participate.

Enhancing Healthcare Team Outcomes

The diagnosis of concussion remains a clinic one as it is based on a constellation of symptoms. Because of this, it would be recommended the team physician have experience in evaluating and managing concussions. An interprofessional team with clear communication between the athlete, parents, coaches, athletic training staff (if available), and the physician or nurse practitioner would be essential. It appears at this time a second impact syndrome is a preventable injury. It is thought if an athlete is allowed to fully recover from an initial concussion and not be allowed to return too quickly, the second impact event will be unlikely to occur. School nurses should participate in the education of patients and their families.

If a player goes down on the field from a suspected head injury, the on-field medical staff, emergency personnel, and the emergency room should communicate clearly in order to optimize the care of the injured athlete. Utilizing a standard communication tool like SBAR will allow for efficient and clear communication amongst the health care team. [Level 5]

Article Details

Article Author

Todd May

Article Author

Lisa Foris

Article Editor:

Chester Donnally III


7/6/2021 9:02:53 PM

PubMed Link:

Second Impact Syndrome



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