• Acceleration-deceleration brain injury leading to tearing of bridging vessels. Blood collects in potential space between dura mater and arachnoid mater.
• Acute: ≤14 days; more severe than chronic; often associated with parenchymal brain injury; diagnosis with CT scan and treated with surgery
• Chronic: >2 weeks; often seemingly trivial injury in older patients, e.g., fall from standing; good prognosis after surgery; recurrence is common.

Predominant age (1):

• Acute: Predominant age group 30–50 years
• Chronic: Predominant age group >50 years

Incidence

• Acute: 1–2 per 100,000 per year; male > female; acute subdural hematoma is present in 5–22% of episodes of severe head trauma.
• Chronic: 3.4 per 100,000 <65 years; 8–58 per 100,000 ≥65 years (prevalence is increasing with aging population)

Prevalence As many as 1 in 2,000 patients from general population may have incidental subdural hemorrhage seen on MRI (2).

• Acute: Severe head trauma (e.g., MVAs, falls, child abuse)
• Chronic: Cerebral atrophy, chronic alcoholism, dementia, falls, minor trauma, epilepsy, coagulopathy/anticoagulation therapy, antiplatelet drugs, low ICP, hemodialysis, child abuse (1)

ALERT
Cerebral atrophy due to age or alcoholism predisposes to subdural hematomas. Never forget nonaccidental trauma in children!

• Acute: Seatbelt, bicycle/motorcycle helmet, construction hard hat
• Chronic: Alcoholism treatment, PT, epilepsy treatment, cautious prescription of anticoagulation therapy, shunts in hydrocephalus patients

• Acute:
  • Bleed (e.g., from tearing of bridging veins; most common cause), injury to small cortical arteries (rare), low CSF pressure causing reduced brain buoyancy
  • Fibroblasts proliferate, wall off hematoma, and migrate into hematoma.
  • Bleeding usually is tamponaded by compression due to increasing ICP or direct compression by the clot itself
  • Phagocytes cause liquefaction of the hematoma, which eventually resolves if the bleed is slow enough. If unable to reabsorb, it becomes chronic.
• Chronic:
  • Hematoma not entirely reabsorbed; alternatively, recurrent bleeding at least as fast as body's resorption rate. The membrane encapsulating the clot may calcify over time (1).

• Acute:
  • Young adult: 56% MVAs; 12% falls
  • Elderly: 22% MVAs; 56% falls
  • Trauma from high-velocity acceleration-deceleration brain injury
  • Blunt trauma often leads to subdural bleeds on coup side of brain as opposed to epidural bleeds, which tend to occur on contre-coup side.
• Chronic:
  • Geriatric patients: Trivial head injury (e.g., fall from standing)
  • Children: May be caused by nonaccidental head trauma, unrecognized/unreported trauma, or, rarely, birth trauma

ALERT
Both acute and chronic subdural bleeds can occur without physical impact!

• Acute: Brain contusion or subarachnoid hemorrhage (most common); epidural hematoma, diffuse axonal injury, facial fractures; cervical spine injury (1)
• Chronic: Subdural hygroma (extracranial collection of CSF), seizure disorder, coagulopathy, CSF shunt, birth trauma, child abuse, metastatic carcinoma (rare)