Difference between subdural haemorrhage and subarachnoid haemorrhage
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subdural haemorrhage vs subarachnoid haemorrhage differences clinical features
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Subdural Haemorrhage vs Subarachnoid Haemorrhage
Understanding the difference starts with anatomy. The brain is wrapped in three meningeal layers - from outside in: dura mater, arachnoid mater, pia mater. Where blood collects defines the type of haemorrhage.
1. Location (Most Fundamental Difference)
| Feature | Subdural Haemorrhage (SDH) | Subarachnoid Haemorrhage (SAH) |
|---|---|---|
| Space | Between dura mater and arachnoid mater | Between arachnoid mater and pia mater |
| Normal space? | Potential space (not naturally present) | Subarachnoid space - contains CSF |
| Spread | Creeps over cerebral convexity, can extend to falx and tentorium | Fills basal cisterns and sulci; follows CSF pathways |
2. Aetiology and Source of Bleeding
Subdural Haemorrhage
- Almost always venous bleeding - from rupture of cortical bridging veins that extend from the cortical surface to the dural venous sinuses
- Usually traumatic (head injury, falls, acceleration-deceleration), even minor trauma in the elderly
- Risk factors: old age, cerebral atrophy (stretches bridging veins), anticoagulants, alcoholism, coagulopathy, shaken baby syndrome
- Occasionally spontaneous from a ruptured aneurysm (especially posterior communicating artery) or dural arteriovenous fistula
Subarachnoid Haemorrhage
- Usually arterial bleeding
- Spontaneous SAH: ~80% from rupture of a saccular (berry) aneurysm at arterial branch points in the circle of Willis; ~10% from non-aneurysmal perimesencephalic SAH (venous origin, excellent prognosis); remaining from AVMs, mycotic aneurysms, cocaine abuse, dissection
- Traumatic SAH is the most common cause overall, but is managed differently (conservatively, no vasospasm)
- Risk factors for aneurysmal SAH: hypertension, smoking, female sex, family history, polycystic kidney disease, Ehlers-Danlos, Marfan syndrome
"Spontaneous SAH is most commonly due to a vascular abnormality, with a ruptured aneurysm accounting for approximately 80% of SAH." - Bailey & Love's Surgery, 28th Ed.
3. Clinical Presentation
| Feature | SDH | SAH |
|---|---|---|
| Headache | Gradual, progressive | Sudden "thunderclap" headache - worst of life, often during exertion |
| Onset | Hours to weeks (depending on type) | Instantaneous ("like a bat hitting the head") |
| LOC | Gradual deterioration, confusion, drowsiness | ~50% brief unresponsiveness at ictus |
| Vomiting | Can occur | Very common (~70%) |
| Meningism | Absent (no blood in CSF space) | Present - neck stiffness, photophobia (develops over hours) |
| Seizure | Can occur | ~10% at ictus |
| Focal deficits | Common (mass effect on adjacent cortex) | May be present if poor grade; 3rd nerve palsy with PComm aneurysm |
| Prodromal symptoms | Personality change (chronic SDH) | "Sentinel headache" (aneurysm growth/minor leak) |
| Fundoscopy | Usually normal | Subhyaloid haemorrhages; Terson's syndrome (vitreous + SAH) in 15-20% |
The classic SDH presentation is a gradually worsening course after head trauma. The classic SAH presentation is a sudden, explosive headache that is qualitatively unlike any previous headache.
4. Classification (SDH specific)
SDH is uniquely classified by timing:
- Acute (<3 days): typically after high-energy trauma; urgent evacuation by craniotomy
- Subacute (3-21 days)
- Chronic (>21 days): common in elderly with cerebral atrophy; osmotic expansion of a degrading clot over weeks; burr hole drainage preferred
5. CT Imaging Appearances
SDH on CT:
- Crescentic (concave-convex) hyperdensity conforming to brain surface - no anatomical constraint because the subdural space is open
- Can extend along falx, tentorium, and floor of cranial fossae
- Acute SDH: hyperdense (white)
- Subacute SDH: isodense (hard to see, may need MRI)
- Chronic SDH: hypodense (dark), may have fluid levels or mixed density if re-bled
- May NOT have associated skull fracture (unlike extradural haematoma)
SAH on CT:
- Hyperdense blood in sulci, basal cisterns, Sylvian fissures - follows the CSF spaces
- CT sensitivity is 98% within 12 hours of ictus, falling to <75% after 24 hours
- If CT is negative but clinical suspicion high: lumbar puncture (xanthochromia) or CT angiography
6. Complications
| Complication | SDH | SAH |
|---|---|---|
| Raised ICP / mass effect | Yes - main concern | Yes, from hydrocephalus |
| Vasospasm | Not typical | Major complication (days 4-14); causes delayed ischaemic deficit |
| Hydrocephalus | Uncommon | Common (blood blocks CSF reabsorption) |
| Rebleeding | Possible (chronic SDH re-expands) | High risk if aneurysm not secured (up to 30% within 30 days) |
| Herniation | Yes, from expanding haematoma | Yes, from raised ICP |
7. Management
SDH:
- Reverse anticoagulation
- Acute SDH: urgent craniotomy
- Chronic SDH: burr hole drainage (can be done under LA even in elderly); corticosteroids for small/asymptomatic bleeds
- Conservative management for small bleeds with headache only
SAH:
- Immediate CT; if positive - CT angiography or formal digital subtraction angiography to identify aneurysm
- Aneurysm secured by neurosurgical clipping or endovascular coiling
- Nimodipine (calcium channel blocker) to reduce vasospasm
- Treat hydrocephalus (external ventricular drain if needed)
- Mortality approaches 50%; post-ictus dependency ~30%
- About one-third of SAHs are missed at initial presentation - a significant diagnostic pitfall
Summary Table
| Feature | SDH | SAH |
|---|---|---|
| Space | Dura-arachnoid | Arachnoid-pia |
| Bleeding source | Bridging veins (venous) | Aneurysm (arterial) |
| Cause | Trauma, falls | Aneurysm rupture, trauma |
| Headache | Gradual/chronic | Thunderclap |
| Meningism | No | Yes |
| CT shape | Crescent | Fills sulci/cisterns |
| Vasospasm | No | Yes |
| Treatment | Burr hole / craniotomy | Coiling / clipping + nimodipine |
| Prognosis | Variable (chronic better) | ~50% mortality |
Sources: Grainger & Allison's Diagnostic Radiology; Bailey & Love's Short Practice of Surgery, 28th Ed.
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