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Summary Table

Category	Detail
Risk Factors	Elderly patients, chronic alcoholism, anticoagulant use, cerebral atrophy, infants (non-accidental trauma/shaken baby syndrome), coagulopathy, prior falls
Etiology	Rupture of bridging veins traversing the subdural space between the dura mater and arachnoid membrane
Patient Presentation	Acute: rapid neurological decline after trauma. Chronic: gradual cognitive decline, headache, gait disturbance weeks to months after minor or forgotten trauma
Classic Physical Exam	Altered mental status, contralateral hemiparesis, ipsilateral pupil dilation (uncal herniation), papilledema in chronic cases
Key Diagnostic Results	Non-contrast CT head: crescent-shaped hyperdense (acute) or hypodense (chronic) collection that crosses suture lines but does not cross the midline falx
Management	Acute with midline shift or neurological decline: surgical evacuation (craniotomy or burr hole). Chronic/small: observation with serial imaging. Reverse anticoagulation if applicable
Keywords	"Crescent-shaped," "crosses suture lines," "bridging veins," "elderly on anticoagulants," "fluctuating mental status," "lucid interval (less common than epidural)," "shaken baby"

1. Pathophysiology

The subdural space lies between the inner surface of the dura mater and the outer surface of the arachnoid membrane. Bridging veins travel through this potential space as they drain from the cerebral cortex toward the dural venous sinuses. These veins are vulnerable to shearing forces during rapid acceleration-deceleration of the head.

When the brain moves relative to the fixed dural sinuses, these bridging veins stretch and tear, producing venous bleeding into the subdural space. Because the source is venous (low pressure), the hemorrhage accumulates more slowly compared to arterial bleeding. This is a critical distinction from an epidural hematoma, which arises from arterial rupture (typically the middle meningeal artery) and progresses rapidly.

Cerebral atrophy is the key predisposing factor. In elderly patients and chronic alcoholics, the brain parenchyma shrinks, which causes the bridging veins to stretch over a longer distance across a widened subdural space. This increased tension makes even trivial head trauma sufficient to cause rupture. This explains why the inciting event is often so minor that the patient does not even recall it, a finding that is classically tested in chronic subdural hematoma vignettes.

In the chronic form, the initial small hemorrhage becomes encapsulated by a neomembrane rich in fragile capillaries. These immature vessels are prone to re-bleeding, which causes the hematoma to gradually enlarge over weeks to months. Osmotic forces also draw fluid into the collection, further expanding its volume. This slow expansion explains why chronic subdural hematoma patients present with progressive, fluctuating neurological symptoms rather than a sudden catastrophic event.

In infants, the mechanism is similar but the clinical context differs. Shaking produces rotational acceleration-deceleration forces that tear bridging veins. The combination of bilateral subdural hematomas and retinal hemorrhages in an infant is highly suggestive of non-accidental trauma (shaken baby syndrome) and is a frequently tested association.

2. Classification and Clinical Manifestation

Classification	Timeframe	CT Appearance	Typical Patient	Clinical Presentation
Acute SDH	< 72 hours	Hyperdense (bright white), crescent-shaped	Young adult with significant trauma, or elderly on anticoagulants with moderate trauma	Rapid decline in consciousness, contralateral motor deficits, signs of herniation (ipsilateral fixed dilated pupil, Cushing triad)
Subacute SDH	3 days to 3 weeks	Isodense (same density as brain parenchyma)	Intermediate presentation	Worsening headache, subtle personality changes, mild focal deficits. This variant is the hardest to detect on CT because it blends with adjacent brain tissue
Chronic SDH	> 3 weeks	Hypodense (dark), crescent-shaped, sometimes with fluid-fluid levels indicating re-bleeding	Elderly patient, alcoholic, anticoagulated patient	Gradual onset headache, cognitive decline mimicking dementia, gait instability, fluctuating level of consciousness, urinary incontinence

An important clinical nuance: the subacute phase is a diagnostic pitfall. Because the hematoma density on CT matches the brain parenchyma, it can be easily missed on non-contrast CT. In this window, contrast-enhanced CT or MRI becomes more valuable for detection. Exam writers may construct vignettes where a patient has worsening symptoms with a "normal" CT to test whether you recognize this entity.

3. Diagnostic Workup

Test	Role	Key Findings
Non-contrast CT of the head	Best initial test	Crescent-shaped extra-axial collection that crosses suture lines, midline shift, compression of ipsilateral ventricle
MRI Brain	Most accurate test (particularly for subacute/chronic and posterior fossa SDH)	Superior detection of isodense collections, better delineation of hematoma age and associated parenchymal injury
CT Angiography	Adjunct if vascular injury is suspected	Rules out underlying vascular malformation or active arterial extravasation
Coagulation studies (PT/INR, PTT, platelet count)	Essential in anticoagulated or coagulopathic patients	Guides urgency of reversal therapy
Complete blood count	Baseline assessment	Thrombocytopenia may be a contributing or complicating factor
Skeletal survey and retinal exam	Mandatory in infants with SDH	Evaluates for non-accidental trauma: multiple fractures at various stages of healing, retinal hemorrhages

Non-contrast CT of the head is the best initial test and is sufficient for diagnosis in the majority of clinical scenarios. The hallmark finding is a crescent-shaped collection that conforms to the brain surface, crosses suture lines, but does not cross the midline (because the falx cerebri acts as a barrier). This morphology directly reflects the anatomy: the subdural space is a potential space overlying the cerebral convexity, unrestricted by dural attachments at suture lines.

Contrast this with an epidural hematoma, which appears as a biconvex (lens-shaped) collection that does NOT cross suture lines because the dura is tightly adherent to the inner table of the skull at these points. This morphological distinction is one of the most commonly tested imaging concepts.

MRI is the most accurate test, particularly useful in three situations: (1) subacute hematomas that are isodense on CT, (2) posterior fossa subdural collections that are poorly visualized on CT due to beam-hardening artifact, and (3) determining the age of a hematoma when non-accidental trauma is suspected. On MRI, the signal characteristics of hemoglobin degradation products allow you to estimate the chronological age of the blood collection.

In any patient on anticoagulants, coagulation studies must be obtained immediately because reversal of anticoagulation is a time-sensitive intervention that directly affects outcomes.

4. Management and Treatment

Clinical Scenario	Intervention	Details
Acute SDH with midline shift > 5 mm or neurological deterioration	Surgical evacuation via craniotomy	Decompressive craniotomy with hematoma evacuation is the standard surgical approach for large acute collections
Acute SDH, small, no midline shift, neurologically stable	Observation with serial CT	Repeat imaging at 6 to 8 hours, then at 24 hours, with close neurological monitoring in an ICU setting
Chronic SDH, symptomatic or large	Burr hole drainage	One or two burr holes with closed-system subdural drain placement for 24 to 48 hours post-operatively
Chronic SDH, small, minimally symptomatic	Conservative management	Serial imaging every 2 to 4 weeks, monitor for expansion or clinical worsening
Anticoagulant-related SDH	Immediate reversal	Warfarin: IV vitamin K 10 mg + 4-factor PCC (prothrombin complex concentrate). Direct oral anticoagulants: idarucizumab for dabigatran, andexanet alfa for factor Xa inhibitors. Target INR < 1.4 before surgery
Elevated ICP management	Medical temporization	Head of bed elevation to 30 degrees, IV mannitol (1 to 1.5 g/kg bolus) or hypertonic saline (23.4%, 30 mL bolus), controlled hyperventilation targeting $PaCO_2$ of 30 to 35 mmHg as a bridge to surgery
Seizure prophylaxis	Consider in acute SDH	Levetiracetam 500 to 1000 mg IV or phenytoin loading dose 15 to 20 mg/kg IV for 7 days in the acute setting
Infant with suspected non-accidental trauma	Medical and legal response	Stabilize, report to child protective services, full skeletal survey, ophthalmologic exam for retinal hemorrhages

Acute stabilization follows standard trauma principles: secure the airway, ensure hemodynamic stability, and rapidly assess for signs of herniation (unilateral fixed and dilated pupil, contralateral hemiparesis, Cushing triad of hypertension, bradycardia, and irregular respirations). If herniation is imminent, temporize with osmotic therapy while preparing for emergent surgical evacuation.

The "next best step" logic tested on exams typically follows this sequence: (1) ABCs and hemodynamic stabilization, (2) non-contrast CT head, (3) reverse anticoagulation if applicable, (4) neurosurgical consultation, (5) surgical evacuation if indicated. A common tested scenario is an anticoagulated elderly patient with a large acute SDH: the next best step before surgery is reversal of anticoagulation.

For chronic SDH, burr hole drainage is preferred over craniotomy because chronic collections are liquefied and drain easily through a small opening. Recurrence rates after burr hole drainage range from 10% to 30%, and a subdural drain left in place for 24 to 48 hours significantly reduces recurrence.

An important contraindication to remember: lumbar puncture is absolutely contraindicated in any patient with suspected intracranial mass effect or midline shift, as it may precipitate transtentorial herniation.

5. Differential Diagnosis and Distractors

Differential	Why It Is Similar	Key Discriminator
Epidural Hematoma	Both are traumatic extra-axial hemorrhages causing mass effect and neurological decline	Epidural: biconvex (lens-shaped) on CT, does NOT cross suture lines, classically caused by middle meningeal artery rupture, associated with temporal bone fracture. Classic "lucid interval" is more strongly associated with epidural than subdural
Subarachnoid Hemorrhage	Both can present with sudden headache and altered consciousness after trauma or spontaneously	SAH: blood in the subarachnoid space (cisterns and sulci), "thunderclap headache," often from ruptured berry aneurysm, CT shows hyperdensity in the basal cisterns, NOT a crescent over the convexity
Normal Pressure Hydrocephalus	Chronic SDH in the elderly can mimic NPH with the triad of dementia, gait disturbance, and urinary incontinence	NPH: ventriculomegaly out of proportion to sulcal atrophy on imaging, no extra-axial collection. Chronic SDH shows the crescent-shaped hypodense collection. Lumbar puncture with large-volume tap improves symptoms in NPH
Dementia (Alzheimer, vascular)	Chronic SDH can present as progressive cognitive decline in the elderly, mimicking neurodegenerative disease	Chronic SDH is a reversible cause of cognitive decline. Imaging reveals the extra-axial collection. Always image elderly patients with new or worsening cognitive symptoms to exclude this treatable entity
Brain Metastasis	Can cause progressive headache, focal neurological deficits, and mass effect	Metastasis: ring-enhancing lesion(s) on contrast CT or MRI, located at the gray-white junction, history of known primary malignancy. No crescent-shaped extra-axial fluid collection
Cerebral Venous Sinus Thrombosis	Can cause headache, altered consciousness, and hemorrhagic infarction	CVST: often in young women, hypercoagulable states, headache with papilledema, hemorrhagic infarction that does NOT respect arterial territories, diagnosed by CT or MR venography showing thrombus in the dural sinuses

6. Traps and High-Yield Pearls

The most common way students miss subdural hematoma questions is by confusing its CT morphology with that of an epidural hematoma. The mnemonic to remember: subdural is crescent-shaped and crosses suture lines; epidural is biconvex and does not cross suture lines. This distinction is rooted in anatomy and is tested repeatedly.

A second major trap is the chronic SDH in the elderly patient who presents with dementia-like symptoms and no recalled trauma. Students frequently select Alzheimer disease or normal pressure hydrocephalus without recognizing that the vignette includes a subtle clue (anticoagulant use, a history of falls, or progressive gait instability with fluctuating cognition) pointing to a subdural collection. The teaching point being tested is that chronic SDH is a reversible cause of cognitive decline, and missing it means missing a treatable condition.

The subacute phase is another testing trap. The hematoma becomes isodense to brain parenchyma between roughly days 3 and 21, making it nearly invisible on non-contrast CT. If a vignette describes worsening neurological symptoms with a "normal CT," consider subacute SDH and recognize that MRI is the appropriate next step.

In pediatric vignettes, the tested competency is recognizing non-accidental trauma. The combination of bilateral subdural hematomas, retinal hemorrhages, and an inconsistent caregiver history should immediately raise suspicion for shaken baby syndrome. The next step is not just medical management but also mandatory reporting and a full skeletal survey.

Finally, remember the anticoagulation reversal question. When an anticoagulated patient presents with an acute SDH requiring surgery, the next best step is reversal of anticoagulation before proceeding to the operating room. The tested concept is that you cannot safely operate on a coagulopathic patient, and knowing the appropriate reversal agent for each anticoagulant (vitamin K plus PCC for warfarin, idarucizumab for dabigatran, andexanet alfa for rivaroxaban or apixaban) is essential.

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