A concussion is a temporary neurological impairment resulting from a blow to the head or body. This type of injury is categorized as a functional injury, meaning it alters how the brain works rather than causing visible, large-scale physical damage. Because a CT scan is a structural imaging tool, the images of a concussed brain typically appear completely normal, even when the patient is experiencing significant symptoms.
Why CT Scans Miss Concussions
A CT scan, or Computed Tomography scan, uses X-rays to create cross-sectional images of the brain and skull. The technology detects physical changes in tissue density, making it excellent at highlighting solid structures like bone and major fluid accumulations like blood. This focus on physical structure explains why it is not suited for diagnosing a concussion.
A concussion involves a complex cascade of events at the cellular level that disrupts normal brain metabolism and communication. The injury causes a rapid stretching of neurons, leading to an uncontrolled release of neurotransmitters and a temporary energy crisis within the brain cells. These microscopic changes are invisible to the resolution of a standard CT scan.
The CT scan looks for gross structural pathology, such as a tear in brain tissue or a pool of blood. Since a concussion rarely involves this type of physical injury, the scan returns a negative result in most patients. A normal CT scan does not mean the brain is uninjured; it simply means there is no major structural damage that the scanner can detect.
The Critical Role of CT Scans in Head Injury
Despite their inability to diagnose a concussion, CT scans play an important role immediately following head trauma. The primary purpose of ordering a CT scan in an emergency setting is to rule out life-threatening conditions that require immediate intervention. The scan acts as a rapid triage tool, helping medical professionals differentiate between a functional injury and a structural emergency.
The CT scan detects acute, severe structural damage, specifically looking for intracranial hemorrhage (bleeding on the brain) and skull fractures. Intracranial hemorrhages, such as epidural or subdural hematomas, involve blood accumulating and putting pressure on the brain tissue. This condition is a neurosurgical emergency, and the CT scan’s speed and accuracy in detecting these blood collections is crucial.
A CT scan also identifies skull fractures, which can indicate a higher risk of underlying brain injury or complications. For a patient presenting with severe headache, persistent vomiting, or loss of consciousness, the CT scan quickly provides information needed to determine if emergency surgery is necessary. Its use is justified to ensure the patient is not facing a severe medical event, even if the scan is negative for structural damage.
How Concussions Are Actually Diagnosed
Since imaging tests are not used to diagnose a concussion, the standard of care relies on a thorough clinical assessment. Concussion is a clinical diagnosis, based on the patient’s symptoms, medical history, and a physical examination. The process begins with a healthcare professional taking a detailed account of the injury mechanism and the patient’s subsequent symptoms, such as headache, dizziness, nausea, and confusion.
A neurological examination assesses the patient’s balance, coordination, reflexes, and cranial nerve function. Cognitive testing evaluates concentration, memory, and information recall. Clinicians often use standardized symptom assessment tools, such as the Post-Concussion Symptom Scale (PCSS), where the patient rates symptom severity.
Specific tests like the Vestibular and Oculomotor Screening (VOMS) may be used to evaluate the balance and eye-movement systems, which are frequently affected. Advanced imaging (fMRI or DTI) can show subtle cellular changes, but these are primarily reserved for research or persistent symptoms, and are not standard acute diagnostic tools. The diagnosis is made by correlating the patient’s reported functional deficits with the objective findings from the physical and cognitive examinations.