What differentiates the occurrence of a subarachnoid hemorrhage (SAH) from a subdural hemorrhage (SDH) after a fall?

Subarachnoid vs. Subdural Hemorrhage After a Fall: Anatomical and Pathophysiological Differences

The primary difference between subarachnoid and subdural hemorrhages after a fall is the anatomical location and mechanism of vessel disruption, with subarachnoid hemorrhages typically resulting from arterial bleeding into the cerebrospinal fluid space, while subdural hemorrhages occur from tearing of bridging veins between the brain surface and dural sinuses. 1

Anatomical Differences

Subarachnoid Hemorrhage (SAH)

• Located in the subarachnoid space between the arachnoid membrane and pia mater
• This space normally contains cerebrospinal fluid (CSF)
• Blood mixes directly with CSF
• Most commonly affects the basal cisterns and cerebral sulci

Subdural Hemorrhage (SDH)

• Located between the dura mater and arachnoid membrane
• Creates a collection of blood that can exert mass effect on the brain
• Often appears as a crescent-shaped collection following the curve of the skull

Mechanism of Injury After Falls

Subarachnoid Hemorrhage

• Traumatic SAH typically results from:
  • Contusion of small vessels on the brain surface 1
  • Tearing of small subarachnoid vessels during brain movement against the skull
  • Extension of contusion bleeding into subarachnoid space 2
  • Less commonly, traumatic rupture of an existing aneurysm

Subdural Hemorrhage

• Primarily caused by:
  • Tearing of bridging veins that cross from the brain surface to the dural sinuses 3
  • Acceleration-deceleration forces causing shearing of these veins
  • Angular rotation of the head causing strain on bridging veins

Clinical Presentation Differences

Subarachnoid Hemorrhage

• Classic presentation is "thunderclap headache" - described as "worst headache of my life" 1
• Headache occurs in approximately 74% of patients
• Nausea/vomiting in 77%
• Loss of consciousness in 53%
• Nuchal rigidity in 35%
• Seizures may occur in up to 20% of patients 1

Subdural Hemorrhage

• More variable presentation depending on size and rate of bleeding
• Often presents with:
  • Headache (typically less severe than SAH)
  • Progressive neurological deterioration
  • Altered mental status
  • Focal neurological deficits
  • May have a lucid interval before deterioration

Risk Factors Influencing Type of Hemorrhage

More Likely to Get Subarachnoid Hemorrhage

• Pre-existing aneurysms (though these cause spontaneous rather than traumatic SAH)
• Falls with direct impact to the head
• Younger patients with less brain atrophy

More Likely to Get Subdural Hemorrhage

• Elderly patients (brain atrophy increases strain on bridging veins)
• Anticoagulation therapy 4
• Chronic alcohol use (causes brain atrophy)
• Falls with rotational acceleration-deceleration

Diagnostic Approach

For Both Types

• Non-contrast CT scan is the initial diagnostic test of choice 1
• CT sensitivity for SAH is 98-100% in first 12 hours, declining to 93% at 24 hours 1

Differentiating Features on Imaging

• SAH: Blood in the subarachnoid spaces, cisterns, and sulci
• SDH: Crescent-shaped collection of blood along the inner table of the skull

Management Implications

Subarachnoid Hemorrhage

• Requires evaluation for underlying aneurysm with CTA or DSA
• Management focuses on preventing rebleeding and vasospasm
• Nimodipine is indicated for aneurysmal SAH 1

Subdural Hemorrhage

• Management depends on size and neurological status
• May require surgical evacuation if causing significant mass effect
• Acute SDH with neurological deterioration requires urgent decompression 1

Prognosis Differences

Subarachnoid Hemorrhage

• Severity of initial bleed is the most useful indicator of outcome 1
• Risk of rebleeding is highest in first 24 hours (3-4%) 1
• Carries risk of delayed cerebral ischemia from vasospasm

Subdural Hemorrhage

• Prognosis depends on initial neurological status, size, and promptness of intervention
• Acute SDH has higher mortality than SAH when severe

Common Pitfalls in Diagnosis

1. Misdiagnosing SAH as a simple headache (occurs in up to 12% of cases) 1
2. Failing to obtain a non-contrast CT scan in patients with sudden severe headache
3. Not recognizing sentinel bleeds before catastrophic rupture
4. Overlooking the possibility of aneurysmal rupture in patients with SDH (rare but documented) 3, 4
5. Not performing lumbar puncture when CT is negative but clinical suspicion for SAH is high

Remember that while these distinctions are generally reliable, there can be overlap, with some patients experiencing both types of hemorrhage simultaneously after severe trauma. The pattern of bleeding should guide the diagnostic workup and management approach.