What is the next step in the workup for a patient presenting with symptoms such as headache, confusion, or focal neurological deficits, who has a negative head CT?

Workup for Obtunded Patient with Negative Head CT

For an obtunded patient with a negative head CT, proceed immediately with MRI brain with diffusion-weighted imaging to detect subtle lesions that CT cannot visualize, particularly traumatic axonal injury, small cortical contusions, and acute ischemic stroke. 1

Clinical Context Determines Next Steps

The workup depends critically on whether this is traumatic versus non-traumatic obtundation:

For Traumatic Brain Injury with Obtundation

MRI brain is the second-line study when persistent neurological deficits (such as obtundation) remain unexplained after negative head CT. 1 The American College of Radiology guidelines specifically address this scenario:

• MRI is more sensitive than CT for:

  • Traumatic axonal injury/diffuse axonal injury (DAI), particularly small white matter lesions 1
  • Small cortical contusions adjacent to calvarium or skull base 1
  • Small subdural hematomas near the skull base 1
  • Acute ischemic stroke (infarct core) when using diffusion-weighted imaging 1

• Key caveat: While MRI detects more lesions (especially shearing injuries) that may explain poor Glasgow Coma Scale scores, one study found these additional findings did not change acute management, though they provided prognostic value 1

• Repeat head CT is also appropriate if neurologic deterioration occurs, regardless of whether initial imaging was negative 1

For Non-Traumatic Obtundation (Suspected Stroke/Vascular)

If the clinical presentation suggests stroke with atypical symptoms (confusion, altered mental status) but CT is negative, MRI should be performed within 24 hours. 2

• High-risk features warranting MRI include: 2

  • Age >60 years (mean age of MRI-positive patients was 74.1 years)
  • History of prior stroke or TIA
  • Hypertension
  • Diabetes
  • Hyperlipidemia
  • Anticoagulation use

• Diagnostic yield: 11.5% of patients with atypical neurologic symptoms and negative CT had acute-to-subacute infarcts on subsequent MRI 2

For Suspected Subarachnoid Hemorrhage (SAH)

If obtundation developed with sudden-onset severe headache and CT is negative, lumbar puncture for xanthochromia is mandatory when performed >6 hours from symptom onset. 1

• CT sensitivity decreases with time: 98.7% within 6 hours, but significantly lower thereafter 1

• LP timing is critical: Must be performed >6 hours after ictus for xanthochromia evaluation 1

• Combined strategy: Negative CT plus negative LP has 100% sensitivity for ruling out SAH in prospective studies 3

• If SAH is confirmed, proceed immediately to CTA or catheter angiography to identify aneurysm source, as diffuse basal cistern SAH patterns require vascular imaging regardless of initial CTA results 1

For Suspected Infection (Meningitis/Encephalitis/Subdural Empyema)

MRI with gadolinium contrast is essential if subdural empyema or other intracranial infection is suspected, as CT can be nondiagnostic. 4

• Clinical red flags requiring MRI: 4
  • Fever with deteriorating neurological function
  • Focal neurological deficits in septic patient
  • CT nondiagnostic but high clinical suspicion

Common Pitfalls to Avoid

• Do not discharge an obtunded patient based solely on negative CT - persistent altered mental status (GCS <15) mandates admission with documented half-hourly observations until GCS 15 is achieved 1

• Do not delay MRI if clinical suspicion is high - the negative predictive value of CT alone is insufficient when neurological deficits persist 1

• Do not perform LP before ruling out mass effect - though in this case CT is already negative, ensuring no contraindication to LP is critical 1

• Do not assume delayed hemorrhage is impossible - while rare (<0.5% incidence), delayed intracranial hemorrhage can occur after negative initial CT, particularly in patients >65 years, on anticoagulation, or with subfrontal/temporal contusions 1

Algorithmic Approach

1. Reassess clinical context: Trauma vs. non-trauma, sudden vs. gradual onset, fever present? 1, 2

2. If trauma: Order MRI brain with DWI to detect DAI, small contusions, or ischemia 1

3. If suspected stroke/TIA: Order MRI brain with DWI, especially if age >60 or vascular risk factors present 2

4. If suspected SAH: Perform LP if >6 hours from onset; if <6 hours, consider repeat CT or proceed to LP based on clinical suspicion 1

5. If suspected infection: Order MRI with gadolinium contrast 4

6. Admit for observation with serial neurological examinations regardless of imaging, as obtundation (GCS <15) is an absolute indication for admission 1