Weber syndrome

**Anatomy/Pathophysiology:** Infarct of the ventral midbrain involving the oculomotor nerve fascicles (CN III) as they exit through the cerebral peduncle (crus cerebri) and the corticospinal/corticobulbar fibers within the peduncle. Vascular supply: paramedian/penetrating branches of the posterior cerebral artery (P1 segment) and the basilar artery tip (interpeduncular branches).

**Clinical findings:** - Ipsilateral (to lesion) CN III palsy: "down and out" eye (unopposed lateral rectus [CN VI] and superior oblique [CN IV]), ptosis (levator palpebrae), mydriasis with loss of pupillary light reflex and accommodation (parasympathetic fibers affected, as they lie superficially). - Contralateral hemiparesis/hemiplegia of the arm and leg (corticospinal tract above the pyramidal decussation), often with contralateral lower facial and tongue weakness (corticobulbar fibers to CN VII and XII, UMN pattern) with tongue deviating away from the lesion. - Mnemonic: "Weber = Wide (dilated) pupil + Weakness contralateral." Lesion side is indicated by the cranial nerve; the body weakness is on the opposite side.

**Related midbrain syndromes:** - Benedikt syndrome: CN III palsy + contralateral involuntary movements (tremor, chorea, ataxia) due to red nucleus involvement (tegmentum). - Claude syndrome: CN III palsy + contralateral ataxia (red nucleus/dentatorubrothalamic fibers; dorsal-medial midbrain). - Parinaud syndrome: dorsal midbrain (pineal tumor) with vertical gaze palsy, light-near dissociation, convergence-retraction nystagmus, lid retraction (Collier sign). - Nothnagel: CN III palsy + ipsilateral cerebellar ataxia (superior cerebellar peduncle). - Other brainstem "alternating" (crossed) syndromes: Medial medullary (Dejerine) – CN XII; Lateral medullary (Wallenberg) – PICA; Millard-Gubler/Foville – pons, CN VI/VII.

**Diagnosis/Management:** MRI (DWI) brain, MRA/CTA of the posterior circulation. Treat as ischemic stroke: IV thrombolysis (alteplase/tenecteplase) within window, thrombectomy for basilar occlusion if eligible, antiplatelets, statin, BP/risk-factor control, and rehabilitation. Other causes include hemorrhage, tumors, aneurysm compression, demyelination, and (rarely) infection.

**Boards pearl:** A pupil-involving CN III palsy plus contralateral hemiparesis localizes to the midbrain (ventral). In contrast, ischemic (microvascular, e.g., diabetic) CN III palsy in the periphery typically spares the pupil, whereas compressive lesions (e.g., posterior communicating artery aneurysm) involve the pupil early. Uncal herniation can also produce a dilated pupil with contralateral hemiparesis (or Kernohan notch ipsilateral).