A stroke is a medical event where the blood supply to a part of the brain is severely reduced or completely cut off, causing brain cells to die from a lack of oxygen and nutrients. While a blow to the head is not a common outcome of most head injuries, a traumatic head injury significantly increases the risk of a stroke. This risk occurs through two distinct biological pathways: one involving a blockage of blood flow (ischemic stroke) and the other involving bleeding (hemorrhagic stroke). The connection between trauma and a stroke is not always immediate, with some mechanisms leading to symptoms hours or even days after the initial impact.
Vascular Damage Leading to Ischemic Stroke
Head or neck trauma can trigger an ischemic stroke, which occurs when a blood vessel supplying the brain becomes blocked, by causing mechanical damage to the arteries. This blockage mechanism is often initiated by a specific injury called arterial dissection, which involves a tear in the inner lining of a major blood vessel. These vessels are typically the carotid arteries in the neck or the vertebral arteries that run through the bones of the spine.
The force of a sudden jolt, even from seemingly minor incidents like whiplash or a quick, forceful head turn, can stretch and tear the delicate internal layer of an artery. Once this tear occurs, blood can pool between the artery’s layers, or the body’s clotting response is activated at the site of the injury. This process leads to the formation of a blood clot on the damaged vessel wall.
This newly formed clot can then grow large enough to entirely block the vessel, immediately stopping blood flow to the brain tissue it supplies. Alternatively, a fragment of the clot can break off and travel upstream toward the brain, a process known as embolism. This traveling fragment then lodges in a smaller artery within the brain, causing a sudden and severe reduction of blood flow to that area.
A particular characteristic of a stroke caused by arterial dissection is the potential for delayed symptoms. Because it takes time for the clot to form and then break loose, the neurological symptoms of the stroke may not appear until hours, days, or even weeks after the initial traumatic event. Research indicates that the risk of ischemic stroke is highest in the first few months following a traumatic brain injury.
Direct Bleeding Causing Hemorrhagic Stroke
The second pathway by which a head injury can cause a stroke is through direct bleeding, resulting in a hemorrhagic stroke. This type of event occurs when the blunt force of the impact causes a blood vessel to rupture, spilling blood into the brain tissue or the surrounding spaces. Unlike the delayed clotting process seen in ischemic stroke, this mechanism is often an immediate consequence of the physical trauma.
One common result is an intracerebral hemorrhage, where blood leaks directly into the brain tissue itself. The presence of this pooled blood is toxic to the brain cells and immediately increases pressure within the skull. This rise in intracranial pressure compresses the surrounding brain structures, leading to rapid neurological decline and damage.
Another type of traumatic bleeding is a subarachnoid hemorrhage, where the rupture occurs in the blood vessels that lie between the surface of the brain and the protective membranes covering it. Additionally, a subdural hematoma, which is a collection of blood typically from torn veins beneath the outermost brain layer, can exert immense pressure on the brain tissue. These traumatic hemorrhages produce the same damaging effect of blood accumulation and pressure on the brain.
The severity of the impact is often directly related to the risk of this type of bleeding, as a forceful blow is required to tear the vessels. The resulting brain damage is a direct consequence of the physical disruption and the tissue compression caused by the expanding volume of blood.
Urgent Signs of Stroke After Head Injury
Recognizing that a head injury has progressed to a stroke, regardless of the underlying mechanism, requires immediate action because every minute counts in preserving brain function. The onset of stroke symptoms after trauma demands an emergency response, even if the original injury seemed minor and the person initially appeared fine. A reliable method for the public to quickly identify the most common neurological signs is the FAST acronym:
- Face drooping: Check if one side of the person’s face is numb or droops when they try to smile.
- Arm weakness: Ask the person to raise both arms to see if one arm drifts downward.
- Speech difficulty: Look for slurred, strange, or incomprehensible speech.
- Time to call 911 immediately, as this is a medical emergency.
Additional signs that may accompany a stroke after a head injury include a sudden, severe headache, problems with balance or coordination, and abrupt vision loss. If any of these symptoms appear following a head injury, especially within the first few days or weeks, immediate medical evaluation is necessary to confirm the diagnosis and begin treatment.