A key principle for localizing brainstem strokes is identifying crossed findings: ipsilateral cranial nerve (CN) deficits with contralateral motor/sensory loss. The "Rule of 4s" can be a helpful mnemonic.

Midbrain Syndromes

• Vascular Supply: Primarily posterior cerebral artery (PCA) and its penetrating branches.

1. Medial Midbrain Syndrome (Weber Syndrome)

• Artery: Paramedian branches of PCA.
• Structures Hit:
  • CN III (Oculomotor) fibers: Ipsilateral CN III palsy ("down and out" eye, ptosis, mydriasis).
  • Cerebral Peduncle (Corticospinal & Corticobulbar tracts): Contralateral hemiparesis/hemiplegia (face, arm, leg).
• Presentation: Ipsilateral CN III palsy + Contralateral weakness.

2. Paramedian Midbrain Syndrome (Benedikt Syndrome)

• Artery: Paramedian branches of PCA.
• Structures Hit:
  • CN III (Oculomotor) nucleus/fibers: Ipsilateral CN III palsy.
  • Red Nucleus: Contralateral ataxia, tremor, or involuntary choreoathetotic movements.
• Presentation: Ipsilateral CN III palsy + Contralateral involuntary movements/ataxia.

3. Dorsal Midbrain Syndrome (Parinaud Syndrome)

• Etiology: Often due to compression (e.g., pinealoma, hydrocephalus) rather than stroke, but can be caused by PCA territory infarct.
• Structures Hit: Superior colliculus, pretectal area.
• Presentation (Classic Triad):
  • Upward Gaze Palsy: Inability to look up is the hallmark.
  • Pupillary Light-Near Dissociation: Pupils accommodate but do not react to light (Pseudo-Argyll Robertson pupil).
  • Convergence-Retraction Nystagmus: Eyes pull in and retract on attempted upgaze.
  • Associated Sign: Collier's sign (eyelid retraction).

Pontine Syndromes

• Vascular Supply: Basilar artery (paramedian branches) and Anterior Inferior Cerebellar Artery (AICA).

1. Medial Pontine Syndrome

• Artery: Paramedian branches of the basilar artery.
• Structures Hit:
  • Corticospinal Tract: Contralateral hemiparesis.
  • Medial Lemniscus: Contralateral loss of proprioception/vibration.
  • CN VI (Abducens) nucleus: Ipsilateral CN VI palsy (inability to abduct eye), leading to horizontal gaze palsy.
• Presentation: Contralateral weakness/sensation loss + Ipsilateral CN VI palsy.

2. Lateral Pontine Syndrome

• Artery: Anterior Inferior Cerebellar Artery (AICA).
• Mnemonic: "Facial droop means AICA is pooped."
• Structures Hit:
  • Vestibular Nuclei (CN VIII): Vertigo, nystagmus, N/V.
  • Facial Nucleus (CN VII): Ipsilateral facial paralysis (LMN), loss of taste, decreased salivation/lacrimation.
  • Spinal Trigeminal Nucleus (CN V): Ipsilateral loss of pain/temp on the face.
  • Spinothalamic Tract: Contralateral loss of pain/temp on the body.
  • Sympathetic Fibers: Ipsilateral Horner's syndrome.
  • Cerebellar Peduncles: Ipsilateral ataxia.
  • Cochlear Nuclei: Ipsilateral sensorineural hearing loss (AICA supplies the labyrinthine artery).
• Key Differentiator: AICA strokes cause hearing loss and facial paralysis, unlike PICA strokes.

Medullary Syndromes

• Vascular Supply: Vertebral artery, Anterior Spinal Artery (ASA), and Posterior Inferior Cerebellar Artery (PICA).

1. Medial Medullary Syndrome (Dejerine Syndrome)

• Artery: Paramedian branches of the vertebral artery or anterior spinal artery (ASA).
• Structures Hit (Mnemonic: 4 Medial "M"s):
  • Corticospinal Tract (Motor pathway): Contralateral hemiparesis (spares the face).
  • Medial Lemniscus: Contralateral loss of proprioception/vibration.
  • CN XII (Hypoglossal) nucleus: Ipsilateral tongue deviation (tongue points toward the lesion).
  • (Medial Longitudinal Fasciculus - MLF): Sometimes involved, causing internuclear ophthalmoplegia.
• Presentation: Contralateral weakness + Contralateral proprioception loss + Ipsilateral tongue deviation.

2. Lateral Medullary Syndrome (Wallenberg Syndrome)

• Artery: Posterior Inferior Cerebellar Artery (PICA) is classic, but vertebral artery occlusion is more common.
• Mnemonic: "Don't PICA horse that can't chew" (hoarseness, dysphagia).
• Structures Hit:
  • Vestibular Nuclei (CN VIII): Vertigo, nystagmus, N/V.
  • Inferior Cerebellar Peduncle: Ipsilateral ataxia, dysmetria.
  • Spinal Trigeminal Nucleus (CN V): Ipsilateral loss of pain/temp on the face.
  • Spinothalamic Tract: Contralateral loss of pain/temp on the body.
  • Nucleus Ambiguus (CN IX, X): Ipsilateral dysphagia, hoarseness, decreased gag reflex.
  • Sympathetic Fibers: Ipsilateral Horner's syndrome (ptosis, miosis, anhidrosis).
• Key Features: No significant weakness. Vertigo, nystagmus, and crossed sensory findings are prominent. Intractable hiccups are common.