Concussion and Traumatic Brain Injury

Primary Injury Mechanisms

• Mechanical disruption and ionic shifts: Traumatic axonal injury (TAI) results from shearing forces between gray and white matter at different densities. The initial biomechanical deformation of neuronal membranes triggers uncontrolled opening of voltage-gated ion channels, leading to influx of calcium (Ca²⁺) and sodium (Na⁺), with efflux of potassium (K⁺). This ionic dysregulation is fundamental to post-concussive dysfunction and may persist for hours to days.
• Excitotoxicity and neuroinflammation: Excessive glutamate release from damaged neurons overstimulates NMDA and AMPA receptors, perpetuating Ca²⁺ influx and activating intracellular proteases, phosphatases, and endonucleases. Microglial activation occurs within minutes, producing pro-inflammatory cytokines (IL-1β, TNF-α, IL-6) that amplify neuronal dysfunction and contribute to blood-brain barrier (BBB) disruption. This neuroinflammatory cascade peaks at 24-72 hours post-injury.
• Metabolic crisis and axonal dysfunction: The ion pump ATPase becomes hyperactive in an attempt to restore electrochemical gradients, dramatically increasing cellular metabolic demand while cerebral blood flow (CBF) is paradoxically reduced (termed hypoperfusion) in the acute phase. This creates an energy mismatch with increased glucose utilization, lactate accumulation, and mitochondrial dysfunction. Coupled with transient alterations in cerebral autoregulation, this metabolic vulnerability explains the heightened susceptibility to secondary injury.

Secondary Injury Processes

• Axonal degeneration and tau pathology: Initial TAI triggers active axonal degeneration that progresses over weeks, characterized by accumulation of phosphorylated tau (p-tau), amyloid-beta (Aβ), and ubiquitinated proteins. Repeated concussions may promote abnormal tau aggregation and seeding, mechanistically linking recurrent TBI to CTE, particularly in contact sports athletes.
• Cerebrovascular dysfunction: Beyond acute hypoperfusion, TBI induces endothelial dysfunction with impaired vasodilatory responses to metabolic demand, increased vascular permeability, and BBB disruption. Magnetic resonance imaging (MRI) studies demonstrate persistent microstructural white matter abnormalities on diffusion tensor imaging (DTI), reflecting ongoing axonal reorganization and myelination changes.

Major Causes (Mechanism-Based)

• Blunt head trauma: Motor vehicle accidents (MVAs) represent the leading cause of TBI in working-age adults; falls predominate in elderly and young children. Sports-related TBI accounts for ~10% of high school and collegiate athletic injuries, with highest incidence in American football, ice hockey, rugby, and soccer. Interpersonal violence contributes significantly in certain populations.
• Acceleration-deceleration injuries: Rapid head movement without direct contact (e.g., whiplash, "shaken baby syndrome") causes diffuse axonal injury through rotational forces. Coup-contrecoup mechanisms produce injury both at the impact site and diametrically opposite.

Intrinsic Risk Factors for Adverse Outcomes

• Age extremes: Pediatric patients (especially <5 years) demonstrate heightened vulnerability to diffuse injury and prolonged recovery; elderly individuals exhibit reduced neuroplasticity and increased risk of delayed epidural hematomas. Female sex is associated with higher concussion rates in sex-matched sports and potentially worse symptom trajectories.
• Prior concussion history: Cumulative TBI exposure demonstrates a dose-dependent relationship with persistent postconcussive symptoms and possibly earlier-onset neurodegenerative disease. Return to play before symptom resolution substantially increases recurrent injury risk.
• Genetic polymorphisms: Apolipoprotein E4 (APOE4) genotype predicts worse cognitive outcomes; catechol-O-methyltransferase (COMT) val158met polymorphisms influence dopaminergic dysfunction severity.
• Comorbid psychiatric/neurologic conditions: Preexisting depression, anxiety, or attention-deficit/hyperactivity disorder (ADHD) is associated with prolonged symptom recovery and higher PCS prevalence.

Acute Phase (Minutes to Hours Post-Injury)

• Cardinal symptoms: Alteration of consciousness (ranging from absent to brief loss of consciousness [LOC]), confusion, disorientation, and amnesia. Post-traumatic amnesia (PTA) includes both retrograde amnesia (events before injury) and anterograde amnesia (events after injury); PTA duration correlates with injury severity.
• Neurologic symptoms: Headache (most common, 85%), dizziness/vertigo, visual disturbances (photophobia, blurred vision), and auditory symptoms (tinnitus). Nausea and vomiting occur in 50% of cases, often presaging more severe injury. Some patients report "seeing stars" or transient loss of vision.
• Cognitive dysfunction: Slowed mental processing, difficulty concentrating, memory impairment, and difficulty with multitasking. Patients may demonstrate slowness of speech (bradyphemia) or slurred speech if accompanying cerebellar involvement.

Physical Examination Findings

• Neurologic deficits: Usually minimal in isolated concussion; however, careful examination may reveal balance impairment (positive Romberg test, abnormal tandem gait), oculomotor dysfunction (saccadic pursuit abnormalities, convergence insufficiency), and pupillary asymmetry if epidural hematoma is developing. Nystagmus suggests vestibular or brainstem involvement. Nuchial rigidity warrants consideration of subarachnoid hemorrhage (SAH).
• Behavioral changes: Irritability, emotional lability, and mood disturbance (depression, anxiety). Severe injury may include decorticate or decerebrate posturing (indicating brainstem dysfunction).

Subacute/Chronic Phase (Days to Weeks)

• Post-concussive syndrome (PCS): Persistent constellation of symptoms including headache, cognitive impairment ("brain fog"), sleep disturbance, mood changes, and sensory sensitivity. Typically resolves within 3 months in adults; pediatric cases may persist longer.
• Sleep-wake disorders: Initial insomnia and fragmented sleep, paradoxically followed by hypersomnia in some cases due to altered circadian regulation and melatonin dysfunction.

Clinical Assessment Tools

• Glasgow Coma Scale (GCS): Standard severity grading; mild TBI (concussion) defined as GCS 13-15 with brief or no LOC. Eye opening (E), verbal response (V), and motor response (M) components scored 3-15, with lower scores indicating greater severity. GCS ≤8 requires intubation for airway protection.
• Standard Assessment of Concussion (SAC): Sideline assessment tool evaluating orientation, immediate memory, concentration, and delayed memory; used to quantify cognitive deficits and monitor recovery.
• ImPACT (Immediate Post-Concussion Assessment and Cognitive Testing): Computerized neuropsychological battery assessing verbal and visual memory, processing speed, and reaction time; useful for objective post-injury documentation in athletes.

Imaging Modality Selection and Interpretation

• CT head (non-contrast): Gold standard in acute setting for detecting acute intracranial complications (epidural hematoma, subdural hematoma, subarachnoid hemorrhage, cerebral contusion, diffuse axonal injury). CT is insensitive for pure concussion in absence of structural injury. Typical protocol uses 5mm slice thickness. CT is indicated for any patient with GCS <15, focal neurologic deficits, vomiting, severe headache, or high-risk mechanism (pedestrian struck by vehicle, falls >3-5 feet, assault, ejection from vehicle).
• Canadian Head CT Rule: Clinical decision support tool indicating when CT is necessary in patients with minor head injury (GCS 15): HIGH-RISK factors (age >65, dangerous mechanism, anticoagulation use) warrant CT; MEDIUM-RISK factors (loss of consciousness, amnesia, vomiting) necessitate CT if no low-risk features. This rule has 99%+ sensitivity for serious injury while reducing unnecessary imaging.
• Advanced MRI techniques: Conventional MRI appears normal in pure concussion but is superior for detecting subtle white matter lesions. Diffusion tensor imaging (DTI) quantifies axonal integrity via fractional anisotropy (FA) and mean diffusivity (MD); decreased FA in corpus callosum and internal capsule correlates with symptomatic recovery timeline. Functional MRI (fMRI) demonstrates altered activation patterns in working memory networks that normalize with recovery. MR spectroscopy reveals altered N-acetylaspartate/choline ratios reflecting mitochondrial dysfunction. These are research tools not routine clinical diagnostics.

Laboratory Testing

• Blood biomarkers: Plasma phosphorylated tau (p-tau181, p-tau217), phosphorylated neurofilament light chain (p-NfL), and glial fibrillary acidic protein (GFAP) are emerging biomarkers with potential prognostic value. tau protein (total and phosphorylated) in cerebrospinal fluid (CSF) is elevated acutely but requires lumbar puncture (contraindicated in acute setting). These are still primarily research tools with variable clinical utility.
• Serum S100B protein: Sensitive marker of brain injury but lacks specificity for concussion; more commonly used in European protocols for acute TBI triage.

Diagnostic Criteria

• Concussion Definition (Consensus Statement on Concussion in Sport—5th International Conference, Berlin 2016): "A traumatic brain injury induced by biomechanical forces with sudden onset of short-lived impairment of neurologic function that resolves spontaneously." Not all impacts result in concussion; must demonstrate symptoms of impaired neurologic function acutely. Absence of abnormality on structural imaging (CT/MRI) is typical and does not exclude concussion diagnosis.
• PECARN (Pediatric Emergency Care Applied Research Network) Criteria: For children <18 years with minor blunt head trauma (GCS 15), rules identify low-risk patients who do not require CT: absence of GCS 15, severe mechanism, vomiting, severe headache, altered behavior, loss of consciousness, and post-traumatic seizure indicates CT not needed.

Acute Management (Emergency Department/First 24 Hours)

• Physical and cognitive rest (Activity Restriction): This is the cornerstone of acute concussion management. Complete rest (physical inactivity, cognitive restriction from school/work, screen avoidance) for 24-48 hours reduces symptom burden and accelerates recovery. Cognitive rest includes limiting reading, texting, video games, and concentration-demanding tasks. Excessive rest beyond 48 hours may impair recovery by preventing neuroplasticity-promoting activity.
• Symptom monitoring: Serial neurologic assessments document baseline status and detect deterioration suggesting evolving complications (epidural hematoma, diffuse axonal injury with increased intracranial pressure [ICP]). Red flag symptoms warranting emergent CT/MRI: sudden severe headache, progressive confusion, vomiting, focal neurologic deficits, seizure, altered consciousness.
• Pharmacologic symptom management:
• Acetaminophen or NSAIDs: First-line for headache management; avoid morphine as it masks deterioration and elevates ICP. Limit opioid use to severe pain unresponsive to non-opioid agents (contradictory to historical practice).
• Sleep aid consideration: Melatonin (3-10 mg at bedtime) or low-dose doxepin (10-25 mg) may benefit acute sleep disruption; benzodiazepines are generally avoided due to cognitive effects.

Subacute Management (Days 1-2 Weeks)

• Graduated return-to-activity protocol: Progressive, stepwise return to physical and cognitive activity, advancing only when asymptomatic at current level. Standard protocol includes: (1) rest, (2) light aerobic activity, (3) sport-specific training, (4) non-contact training drills, (5) full-contact practice, (6) return to competition. Each step lasts ≥24 hours; regression occurs if symptoms recur. This structured approach reduces symptom burden and prevents recurrent injury from premature return to play.
• Vestibular/oculomotor rehabilitation: Patients with persistent balance impairment or visual dysfunction benefit from supervised physical therapy targeting gaze stability, balance, and proprioception. Vestibular rehabilitation therapy (VRT) specifically addresses dizziness and gait disturbance through canalith repositioning and habituation exercises.
• Psychological support: Early intervention for mood disturbance, anxiety, or depression accelerates cognitive recovery. Cognitive-behavioral therapy (CBT) demonstrates efficacy for post-concussive headaches and anxiety; anxiolytics (SSRIs preferred over benzodiazepines) manage persistent anxiety.
• Pharmacologic management of persistent symptoms:
• Cognitive dysfunction/brain fog: Amantadine (100-200 mg BID), a NMDA antagonist, may improve dopaminergic function and cognition in patients with persistent cognitive impairment; evidence is moderate. Stimulants (methylphenidate 5-20 mg BID or modafinil 100-200 mg daily) are reserved for ADHD-like symptoms unresponsive to structured rehabilitation.
• Post-traumatic headaches: Topiramate (25-100 mg daily) or propranolol (40-120 mg daily) are prophylactic agents with evidence in TBI populations. Tricyclic antidepressants (amitriptyline 10-100 mg nightly) address concurrent sleep disturbance and mood symptoms.
• Sleep disturbance: Continued melatonin, doxepin, or trazodone (25-100 mg nightly); avoid benzodiazepines and anticholinergic agents due to cognitive effects.
• Mood disturbance: SSRIs (sertraline 50-200 mg, paroxetine 20-60 mg, or citalopram 20-40 mg daily) are first-line for depression and anxiety. Avoid tricyclics in patients with persistent concussion symptoms due to anticholinergic effects worsening cognition.

Non-Pharmacological Measures

• Cognitive rehabilitation: Structured program addressing attention, memory, executive function, and processing speed; beneficial for persistent cognitive impairment. Computerized cognitive training shows modest benefit; real-world task practice appears superior.
• Aerobic exercise: Progressive aerobic conditioning (once symptom-free at rest) promotes neuroplasticity and accelerates recovery. Subthreshold exercise protocols tailored to individual tolerance optimize outcomes.
• Sleep hygiene optimization: Regular sleep schedule, cool/dark sleeping environment, caffeine avoidance, and screen time restriction in evening hours.
• Nutritional support: High-protein diet, omega-3 supplementation (DHA/EPA), and micronutrient repletion (magnesium, B vitamins) may support neuronal repair; evidence is emerging.

Monitoring and Follow-Up

• Outpatient reassessment at 2-4 weeks: Reassess for persistent symptoms requiring escalation of therapy; neuropsychological testing indicated if symptoms persist beyond 4 weeks. Return-to-work/school clearance should be gradual and symptom-contingent.
• Neuroimaging follow-up: Repeat imaging (CT or MRI) is not indicated unless symptoms worsen or red flags emerge; imaging should not delay clinical decision-making.

Acute Intracranial Complications

• Epidural hematoma: Arterial bleeding (typically middle meningeal artery) between dura and skull, presenting with classic "lucid interval" (brief unconsciousness, improvement, then sudden

• No same-day return to play: Any athlete with suspected concussion is removed from play and not returned the same day — "when in doubt, sit them out" (American Academy of Neurology 2013 sport concussion guideline; Consensus Statement on Concussion in Sport). The single best next step in a stem describing an athlete with headache and confusion after a hit is removal from play and serial sideline assessment, not imaging.
• Second impact syndrome: A second head injury before the first has resolved can cause catastrophic diffuse cerebral edema from loss of cerebral autoregulation, classically in an adolescent athlete. This is the mechanistic reason behind the graduated return-to-play protocol.
• Hematoma discrimination: Biconvex/lentiform hemorrhage that does not cross sutures = epidural, middle meningeal artery, lucid interval; crescentic hemorrhage that crosses sutures but not the midline = subdural, bridging veins, favored by age-related atrophy, alcohol use, and anticoagulation.
• Normal CT does not exclude concussion: Concussion is a clinical diagnosis. CT is obtained to exclude a surgical lesion, guided by the Canadian CT Head Rule in adults and PECARN in children — not to confirm concussion. MRI is not the acute first-line study; it is the test of choice for suspected diffuse axonal injury (punctate lesions at the gray–white junction, corpus callosum, dorsolateral midbrain).
• Blown pupil = uncal herniation: An ipsilateral fixed dilated pupil with contralateral hemiparesis reflects CN III compression. The Cushing reflex (hypertension, bradycardia, irregular respirations) signals critically raised ICP; head elevation, osmotic therapy, and neurosurgical consultation take priority over further imaging.
• GCS ≤8 → intubate; GCS 13–15 defines mild TBI. Post-traumatic amnesia duration tracks with severity better than loss of consciousness does.
• Basilar skull fracture: Raccoon eyes, Battle sign, hemotympanum, CSF rhinorrhea/otorrhea — avoid blind nasogastric or nasotracheal tube placement.
• Distractors to avoid: Glucocorticoids are harmful in TBI (CRASH trial) and should never be selected; prophylactic hyperventilation is not used routinely (Brain Trauma Foundation severe TBI guidelines); antiseizure prophylaxis after severe TBI reduces only early seizures and is not continued long-term; prolonged strict rest beyond the first day or two delays recovery.