A concussion is a mild traumatic brain injury (mTBI) resulting from an immediate, temporary disturbance in how the brain functions, rather than structural damage like bleeding or swelling. Many people assume a concussion requires a direct strike to the head, but this is a common misconception. A concussion can occur without a visible “bump” or direct blow. The injury is caused by the forceful movement of the brain within the skull, which can be generated by an impact anywhere on the body.
The Mechanism of Injury Without Direct Impact
The brain is suspended in cerebrospinal fluid inside the skull, which provides a cushion during normal movement. A concussion occurs when an external force causes the head to accelerate, decelerate, or twist rapidly. This sudden, violent motion causes the soft brain tissue to slosh and rotate inside the rigid skull. This mechanism explains why whiplash from a car accident or a hard tackle to the body can result in a concussion even if the head never makes contact with anything.
The rapid movement generates shear and tensile forces that stretch and damage the long, delicate nerve fibers, called axons. This microscopic damage triggers a cascade of chemical and metabolic events within the brain cells. This functional disruption is the core of a concussion, leading to an alteration of neurological function.
In some cases, the brain’s sudden movement can cause it to strike the inner wall of the skull, a phenomenon known as a coup injury. The brain may then rebound and strike the opposite side of the skull, which is called a contrecoup injury. In non-impact injuries, the forces are often rotational, causing the brain to twist and shear nerve fibers throughout different regions, leading to widespread functional issues.
Recognizing Invisible Concussion Symptoms
Since there is no outward sign like a bruise or cut, recognizing a non-impact concussion relies entirely on identifying a cluster of symptoms, which may be delayed by hours or even days. Concussion symptoms are typically grouped into physical, cognitive, and emotional/sleep categories. Physical symptoms often include a headache, which is the most commonly reported symptom, along with dizziness, balance problems, or feeling unsteady.
Other common physical signs are nausea or vomiting, ringing in the ears (tinnitus), and heightened sensitivity to light and noise. These symptoms reflect the functional disturbance in areas controlling sensory processing and equilibrium. Cognitive symptoms involve changes in thinking, such as feeling “foggy,” being confused or disoriented, or having difficulty concentrating on tasks.
A person may experience amnesia (memory loss surrounding the injury event) or simply a slower reaction time. The third group includes emotional and sleep disturbances, which can manifest as increased irritability, anxiety, or sadness. Changes to sleep patterns, such as sleeping much more or much less than usual, or having trouble falling asleep, are also recognized signs of a concussion.
When to Seek Medical Evaluation
Any suspicion of a concussion, even without a direct head strike, warrants a medical evaluation because the injury is functional, not structural. Healthcare providers diagnose a concussion based on a detailed history of the injury, the reported symptoms, and a thorough neurological and cognitive examination. Standard imaging tests like CT or MRI scans are not used to diagnose a concussion itself, as they cannot detect the functional disturbance at the cellular level.
However, imaging is sometimes necessary to rule out a more severe structural brain injury, such as a skull fracture or bleeding in the brain. Immediate emergency care is required if certain “red flag” symptoms develop, indicating a potentially life-threatening complication. These urgent signs include a headache that worsens and does not go away, repeated vomiting, or an inability to wake up.
Other immediate warning signs are seizures, slurred speech, weakness or numbness in the limbs, or significant worsening of confusion or disorientation. If symptoms like dizziness, poor balance, or persistent cognitive issues do not resolve after 10 to 14 days, a follow-up with a healthcare professional is also necessary. A clinical diagnosis focuses on a neurological exam checking vision, balance, coordination, and reflexes, along with cognitive tests to assess memory and concentration.