A subdural hematoma (SDH) is bleeding inside the skull where blood collects between the brain’s outer covering (dura mater) and the brain itself. This accumulation creates a mass within the rigid skull, increasing pressure on the brain tissue. The term “midline shift” indicates that this pressure has become severe enough to push the brain’s central structures across their normal vertical axis. This displacement signifies a medical emergency that requires immediate intervention and guides urgent treatment strategies.
The Mechanics of Subdural Hematoma and Midline Shift
The formation of a subdural hematoma begins with the tearing of bridging veins, which traverse the space between the brain’s surface and the dura mater. These veins are vulnerable to rupture during rapid head acceleration or deceleration, causing the brain to move relative to the skull. Once torn, blood pools in the subdural space, creating an expanding mass.
The rate of formation classifies the hematoma: acute SDH appears immediately, subacute SDH develops over days to weeks, and chronic SDH evolves over weeks to months. Acute SDH is most frequently associated with a rapid midline shift because blood accumulates quickly, exerting high pressure. This mass effect forces the brain to compensate by displacing cerebrospinal fluid and blood, but the brain tissue itself begins to shift when compensation fails.
The brain’s central dividing structure, the falx cerebri, normally separates the two hemispheres. When the hematoma’s mass effect becomes overwhelming, it pushes brain structures, such as the ventricles, laterally across this central plane. This physical displacement distorts the brainstem and other deep structures, leading to widespread neurological dysfunction.
Identifying Symptoms of Increased Intracranial Pressure
Symptoms arise from the pressure of the hematoma. A severe, unrelenting headache is a common initial complaint as internal pressure mounts. This is often accompanied by forceful vomiting not preceded by nausea, a classic sign of increased intracranial pressure.
As pressure rises, the patient’s level of consciousness progressively declines, moving from confusion toward stupor and eventually coma. Anisocoria, where pupils are unequal or fail to react to light, is a concerning sign. This indicates the brainstem, which controls these reflexes, is being compressed and distorted by the shifting brain mass.
The severity of the midline displacement correlates strongly with observable neurological deficits. A patient with a significant shift may exhibit focal neurological signs, such as weakness or paralysis on one side of the body. The appearance of these symptoms signals a neurological emergency, as the brain can no longer accommodate the growing hematoma.
Imaging and Quantifying Midline Displacement
Diagnosis relies on rapid neuroimaging, with a Computerized Tomography (CT) scan being the primary diagnostic tool. The CT scan is fast, readily available, and clearly visualizes the blood collection as a crescent-shaped layer conforming to the skull’s inner surface. The scan also provides a precise measurement of the displacement of the brain’s central structures.
The midline shift (MLS) is quantified in millimeters by measuring the perpendicular distance from a central landmark, such as the septum pellucidum, to the theoretical midline of the skull. This measurement directly indicates the mass effect and the degree of brain compression. It is a definitive factor in surgical decision-making.
Consensus guidelines suggest that a midline shift exceeding 5 millimeters is a strong indication for immediate surgical intervention. This threshold signifies a level of mass effect that carries an increased risk of brain herniation and permanent damage. The visible displacement confirms the severity of the internal pressure, driving the need for urgent decompression.
Immediate and Surgical Management
Initial management focuses on immediate stabilization to minimize ongoing brain injury. Medical interventions aim to temporarily reduce intracranial pressure (ICP) while preparing for definitive surgery. This includes ensuring proper airway management and ventilation, and maintaining systolic blood pressure between 100 to 160 mmHg to ensure adequate cerebral blood flow.
To rapidly decrease brain swelling, osmotic agents like Mannitol or hypertonic saline are administered intravenously. These medications draw excess fluid out of the brain tissue, providing a temporary volume reduction before the operation. The definitive treatment for a severe acute SDH with a significant midline shift is the surgical evacuation of the hematoma.
The most common procedure is a craniotomy, which involves removing a large section of the skull bone to access the hematoma. The surgeon opens the dura mater and removes the blood clot, immediately relieving the compressive mass effect. For smaller, less acute hematomas, a less invasive burr hole trephination may be used to drain the fluid collection, allowing displaced brain structures to return to their normal position.
Prognosis and Neurological Recovery
Prognosis is influenced by several factors present upon the patient’s arrival. Age is a significant determinant, as older individuals often have less physiological reserve to tolerate the injury and surgery. The initial score on the Glasgow Coma Scale (GCS), which measures the patient’s level of consciousness, is a reliable predictor of the final outcome.
Radiological factors, particularly the magnitude of the midline shift, play a substantial role in predicting recovery. Patients with shifts less than 5 millimeters generally have a better chance of a favorable outcome than those exceeding 10 millimeters. The degree of shift reflects the extent of underlying brain injury, which may not be reversible even after successful clot removal.
Neurological recovery is often a long process, requiring extensive post-operative rehabilitation, including physical, occupational, and speech therapy. Long-term deficits may include motor weakness, cognitive impairment, or behavioral changes. The most important factor determining a positive outcome is the speed of diagnosis and the promptness of surgical decompression, as untreated severe midline shift increases the risk of irreversible brain damage.