Does a patient with suspected severe encephalopathy and non-reactive pupils require a stat (immediate) MRI?

Immediate MRI for Suspected Severe Encephalopathy with Non-Reactive Pupils

Yes, MRI should be performed as soon as possible—ideally within 24-48 hours but urgently in this clinical scenario given the severe presentation with non-reactive pupils, which suggests critical brain dysfunction requiring immediate diagnostic evaluation. 1, 2

Rationale for Urgent Neuroimaging

Non-reactive pupils in the context of encephalopathy represent a neurological emergency signaling potential brainstem involvement, raised intracranial pressure, or severe diffuse brain injury. 1 This clinical finding demands immediate imaging to:

• Exclude life-threatening structural lesions such as large infarctions, hemorrhage, cerebral edema, or mass effect that require urgent neurosurgical intervention 1
• Identify treatable causes including herpes simplex encephalitis (HSE), autoimmune encephalitis, or other infectious/inflammatory processes where early treatment dramatically improves outcomes 1, 2
• Guide critical management decisions regarding intracranial pressure management, anticoagulation, and need for intensive care interventions 1

MRI as the Preferred Modality

MRI without contrast is the optimal imaging study for encephalopathy, offering superior sensitivity (approximately 90% within 48 hours) compared to CT (only 25% sensitivity for early changes). 2, 3, 4

Key MRI sequences to obtain urgently:

• Diffusion-weighted imaging (DWI) to detect cytotoxic edema, early ischemia, and HSE changes in the temporal lobes and cingulate gyrus 2, 3
• FLAIR sequences to identify vasogenic edema, inflammatory changes, and subtle parenchymal abnormalities 4, 5
• Gradient echo/susceptibility-weighted imaging to detect hemorrhage, microbleeds, or superficial siderosis 1
• T2-weighted sequences for comprehensive evaluation of posterior fossa structures, which CT visualizes poorly 1

Critical Timing Considerations

The British Neurological Association guidelines explicitly state that MRI should be performed as soon as possible in all patients with suspected encephalitis, with results available within 24-48 hours of admission. 1, 2

However, in your scenario with non-reactive pupils:

• Immediate imaging is warranted because this represents potential herniation risk or severe brainstem dysfunction 1
• CT may be performed first if MRI is not immediately available or if the patient is too unstable to transport, as CT can rapidly exclude hemorrhage, mass effect, or hydrocephalus requiring emergent intervention 1, 2
• Do not delay empiric treatment while awaiting imaging—if HSE is suspected, intravenous aciclovir (10 mg/kg three times daily) should be started within 6 hours of admission 2

Important Clinical Caveats

Contraindications to immediate imaging include:

• Hemodynamic instability requiring ongoing resuscitation 1
• Inability to protect airway without intubation first 1
• Active seizures requiring stabilization before transport 1

Common pitfalls to avoid:

• Never delay empiric aciclovir for HSE while waiting for MRI results, as untreated HSE has devastating mortality and morbidity 2
• Do not assume normal early imaging excludes serious pathology—MRI can be normal in the first 24-48 hours of HSE or other encephalopathies 2, 3
• Recognize that non-reactive pupils may indicate need for immediate neurosurgical consultation before or concurrent with imaging 1
• Consider that multiple concurrent etiologies may be present (e.g., metabolic derangement plus structural lesion), requiring comprehensive evaluation 3, 4

Practical Algorithm

1. Stabilize airway, breathing, circulation and assess for herniation signs 1
2. Start empiric aciclovir immediately if any suspicion of viral encephalitis 2
3. Obtain STAT MRI brain with DWI, FLAIR, T2, and gradient echo sequences 2, 3
4. If MRI unavailable or patient too unstable, perform urgent non-contrast head CT to exclude hemorrhage/mass effect 1, 2
5. Arrange ICU admission for close neurological monitoring and management of raised intracranial pressure 1
6. Perform lumbar puncture after imaging excludes mass effect, for CSF PCR and analysis 1, 2

The presence of non-reactive pupils elevates this from routine encephalopathy workup to a true neurological emergency requiring immediate imaging and intensive care management. 1