Are interhemispheric bleed and subdural hematoma (SDH) the same?

Interhemispheric Subdural Hematoma vs. Subdural Hematoma: Differences and Relationship

No, interhemispheric bleed and subdural hematoma (SDH) are not the same, but rather interhemispheric subdural hematoma (IHSDH) is a specific subtype of subdural hematoma that occurs in the interhemispheric fissure between the cerebral hemispheres. 1, 2

Anatomical Differences and Classification

• Interhemispheric subdural hematoma (IHSDH) is a specific subtype of subdural hematoma that occurs in the space between the cerebral hemispheres along the falx cerebri 2
• Standard subdural hematomas (SDHs) typically form over the convexity of the cerebral hemispheres 1
• Both are classified under the broader category of intracranial hemorrhage, which includes any form of bleeding within the intracranial vault 1
• IHSDH may extend above the tentorium and onto the hemispheric convexity in some cases, creating a mixed presentation 3

Epidemiology and Frequency

• IHSDHs were historically considered rare, but recent evidence suggests they are more common than previously thought, representing approximately 35.5% of all acute traumatic subdural hematomas in one large study 2
• Of 1182 patients with acute traumatic SDHs in a Level I trauma center study, 420 patients (35.5%) had IHSDHs 2
• Only 3.8% of IHSDHs occur in isolation, with most cases occurring alongside other intracranial injuries 2

Etiology and Risk Factors

• IHSDHs are commonly associated with:
  • Head trauma, particularly occipital trauma due to the anatomy of bridging veins 4
  • Bleeding disorders or coagulopathy 5, 3
  • Anticoagulation therapy 1
• The pathophysiology typically involves:
  • Tearing of bridging veins that cross from the cerebral cortex to the superior sagittal sinus 3
  • Cortical contusions extending into the subdural space 3
  • In rare cases, arterial bleeding from pericallosal artery branches 3

Clinical Presentation

• IHSDHs may present with specific neurological abnormalities:
  • "Falx syndrome" - characterized by contralateral leg weakness greater than arm weakness (due to the anatomical location affecting the leg area of the motor cortex) 5, 4
  • Hemiparesis that affects the lower limb more than the upper limb 4
  • Seizures 6
  • Transient loss of consciousness (51.5% of large IHSDHs) 2
  • Post-traumatic amnesia (47.8% of large IHSDHs) 2
  • Motor weakness (37.9% of large IHSDHs) 2

Diagnostic Considerations

• Computed tomography (CT) is the primary diagnostic tool for both IHSDH and conventional SDH 1
• On imaging, IHSDH appears as a crescent-shaped hyperdensity in the interhemispheric fissure, while conventional SDH typically appears as a crescent-shaped collection over the cerebral convexity 2
• Accurate measurement of hemorrhage volume is critical for both prognosis and as an outcome measure 1

Management Approaches

• Treatment options for IHSDH include:
  • Conservative management for patients without disturbances of consciousness or with stable clinical conditions 5, 2
  • Surgical intervention (craniotomy and evacuation) for patients with progressive neurological deterioration 5
  • Twist drill craniostomy and drainage has been reported as an alternative to craniotomy in selected cases 6
• Most IHSDHs resolve spontaneously with conservative management, with resolution of associated symptoms 2
• Surgical management presents unique challenges due to proximity to the sagittal sinus and bridging veins 2

Prognosis and Outcomes

• Mortality for IHSDH has historically been high (approximately 35%), though this may be lower with modern management 3
• In a recent study, motor deficits directly related to IHSDH resolved in 4 out of 5 patients with conservative management 2
• Progression of IHSDH width occurred in only one patient in a series of 35 large IHSDHs 2
• Chronic interhemispheric subdural hematomas are rare since the natural history of these lesions is to spread out over the cerebral convexity 4

Clinical Pitfalls and Considerations

• IHSDHs may be overlooked or underdiagnosed if not specifically sought on imaging 2
• The risk of progressive bleeding is greater for all forms of intracranial hemorrhage when the baseline CT scan is performed early after injury and when coagulopathy is present 1
• Rapid reversal of anticoagulation is generally recommended for any patient with an intracranial hemorrhage, including IHSDH 1
• The decision between conservative and surgical management should be based on the patient's neurological status and progression rather than just the size of the hematoma 2