What are the next steps for a patient presenting with nystagmus and left-sided weakness?

Immediate Neuroimaging Required for Suspected Posterior Circulation Stroke

This patient requires urgent MRI of the brain to evaluate for posterior circulation stroke affecting the brainstem or cerebellum, as the combination of nystagmus with unilateral weakness represents a central nervous system lesion until proven otherwise. 1, 2

Critical Red Flags Present

This presentation contains multiple features that mandate immediate neuroimaging:

• Nystagmus with focal neurological deficits (left-sided weakness) strongly suggests a central etiology rather than benign peripheral vestibular disease 1, 2
• The combination indicates possible involvement of the brainstem, cerebellum, or cervicomedullary junction 2, 3
• MRI is the preferred imaging modality as CT may inadequately visualize posterior fossa structures 2

Immediate Diagnostic Steps

Before or concurrent with imaging, perform systematic nystagmus characterization:

• Observe for spontaneous nystagmus in primary gaze without positional provocation—this suggests central pathology requiring neuroimaging 1, 4
• Document whether the nystagmus is horizontal, vertical, or torsional 4
• Look specifically for downbeat nystagmus, which strongly indicates cerebellar or cervicomedullary junction pathology 2, 5
• Check for direction-changing nystagmus in primary gaze, which is a central sign 1, 2

Central vs. Peripheral Differentiation

The following nystagmus patterns confirm central pathology and contraindicate benign positional vertigo diagnosis:

• Pure vertical nystagmus (upbeat or downbeat) in primary gaze 1
• Downbeat component without torsional features 1, 2
• Direction-changing nystagmus in primary gaze 1, 3
• Nystagmus present constantly without requiring positional provocation 1

Likely Anatomical Localization

Given left-sided weakness with nystagmus, consider:

• Left posterior inferior cerebellar artery (PICA) territory stroke affecting the lateral medulla and cerebellum 3
• This can produce horizontal direction-changing nystagmus with ipsilateral ataxia and contralateral weakness 3
• Vestibular nuclei involvement in the lower brainstem causes spontaneous nystagmus 3
• Concurrent damage to the nucleus prepositus hypoglossi or cerebellum affects gaze-holding, producing gaze-evoked nystagmus 3

Common Pitfalls to Avoid

• Do not assume this is benign paroxysmal positional vertigo (BPPV) despite nystagmus being present—the focal weakness excludes peripheral vestibular causes 1, 2
• Do not perform positional maneuvers (Dix-Hallpike or supine roll test) until stroke is excluded, as these are only appropriate for peripheral vestibular disorders 6, 1
• Do not delay imaging to complete vestibular function testing when focal neurological signs are present 2

Time-Sensitive Considerations

• If presenting within the thrombolysis window and stroke is confirmed, intravenous fibrinolysis should be considered as favorable outcomes have been reported in posterior circulation strokes with this presentation 3
• Obtain vascular imaging (MRA or CTA) to assess for vertebrobasilar insufficiency or dissection 2