A 72-year-old woman with atrial fibrillation presents with acute onset vertigo, nystagmus, and ipsilateral facial numbness. Suspicion for posterior circulation stroke is high. Non-contrast CT head is normal. Which investigation is most appropriate to confirm acute brainstem/cerebellar infarction?

Explanation

## Investigation of Choice: MRI with DWI for Posterior Circulation Stroke ### Why MRI-DWI is Superior for Posterior Circulation Strokes **Key Point:** MRI with DWI and FLAIR sequences is the gold standard for detecting acute brainstem and cerebellar infarcts. These lesions are frequently **missed on CT** due to beam hardening artifact from the skull base, making MRI essential for posterior circulation stroke confirmation. **High-Yield:** Posterior circulation strokes (vertebrobasilar territory) account for 10–15% of all ischemic strokes. They present with: - Vertigo, nystagmus, ataxia (cerebellar signs) - Cranial nerve deficits (facial numbness, diplopia) - Crossed syndromes (ipsilateral CN deficit + contralateral motor/sensory loss) DWI-MRI detects these lesions with >95% sensitivity, even when CT is normal. ### Clinical Context: Why CT Fails in Posterior Circulation ```mermaid flowchart TD A[Acute Posterior Circulation Stroke Suspected]:::outcome A --> B[Non-contrast CT Head]:::action B --> C{Lesion Visible?}:::decision C -->|No - Beam Hardening Artifact| D[High Risk of Missed Diagnosis]:::urgent C -->|Yes - Large Infarct| E[Proceed to MRI/CTA]:::action D --> F[MRI-DWI Required]:::action E --> F F --> G[DWI Hyperintensity Confirms Acute Ischemia]:::outcome G --> H[Initiate Reperfusion Therapy if Within Window]:::action ``` **Clinical Pearl:** Skull base artifacts on CT make small brainstem infarcts invisible. MRI-DWI overcomes this limitation and is mandatory when posterior circulation stroke is clinically suspected but CT is normal. ### Comparison of Investigations for Posterior Circulation Stroke | Investigation | Sensitivity for Brainstem/Cerebellar Infarct | Clinical Role | Limitations | |---|---|---|---| | **MRI-DWI** | >95% in acute phase | Gold standard; detects small lesions | Requires MRI availability; longer acquisition time | | Conventional Angiography (DSA) | N/A for ischemia | Invasive; reserved for thrombectomy | Risk of procedural stroke; not diagnostic | | Carotid Duplex | N/A for posterior circulation | Assesses carotid disease (anterior circulation) | Irrelevant for vertebrobasilar pathology | | TEE | N/A for ischemia detection | Identifies cardioembolic source (e.g., AF thrombus) | Does not confirm tissue-level infarction | ### Why FLAIR is Included FLAIR (Fluid-Attenuated Inversion Recovery) suppresses cerebrospinal fluid (CSF) signal, improving visualization of: - Subarachnoid hemorrhage (if hemorrhagic conversion occurs) - Subacute infarcts (days 3–7 post-stroke) - Brainstem edema Together, **DWI + FLAIR** provide comprehensive assessment of acute and subacute ischemia in the posterior fossa. ### Thrombolytic Eligibility This patient is within the potential thrombolysis window (time of symptom onset to presentation). MRI-DWI confirmation is essential before administering IV alteplase or considering mechanical thrombectomy via vertebral artery access. [cite:Harrison 21e Ch 296]