Brainstem lesions and clinical correlations

Brainstem Lesions - The Rule of 4s

A mnemonic 📌 for localizing brainstem lesions based on cranial nerves and long tracts.

• 4 Midline Structures (M)

  • Motor pathway (Corticospinal tract)
  • Medial Lemniscus
  • Medial Longitudinal Fasciculus (MLF)
  • Motor nuclei (CN III, IV, VI, XII)

• 4 Side/Lateral Structures (S)

  • Spinocerebellar pathway
  • Spinothalamic pathway
  • Sensory nucleus of CN V
  • Sympathetic pathway

• 4 CNs per Level

  • Midbrain: III, IV
  • Pons: V, VI, VII, VIII
  • Medulla: IX, X, XI, XII

⭐ Wallenberg Syndrome (Lateral Medullary Syndrome) is caused by PICA occlusion. It affects the lateral 'S' structures (e.g., spinothalamic tract, sympathetics) but spares the medial 'M' structures (e.g., motor pathway), so patients have no contralateral weakness.

Midbrain Syndromes - Weber's & More

• Weber's Syndrome (Ventral Midbrain)

  • Structures: Crus cerebri (corticospinal tract) + CN III fascicles.
  • Deficits: Ipsilateral CN III palsy (ptosis, mydriasis, "down and out" eye) + contralateral spastic hemiparesis.

• Benedikt's Syndrome (Tegmentum)

  • Structures: Red nucleus + CN III fascicles.
  • Deficits: Ipsilateral CN III palsy + contralateral ataxia and intention tremor.

• Parinaud's Syndrome (Dorsal Midbrain)

  • Structures: Superior colliculus & pretectal area.
  • Deficits: Impaired vertical gaze (especially upgaze), pupillary light-near dissociation, convergence-retraction nystagmus.

⭐ Exam Favorite: Parinaud syndrome is a classic presentation for a pinealoma compressing the dorsal midbrain.

Pontine Syndromes - Locked-In Lookouts

• Locked-In Syndrome:

  • Lesion: Bilateral ventral pons (basilar artery thrombosis).
  • Tracts: Corticospinal & corticobulbar → quadriplegia & anarthria.
  • Sparing: Reticular activating system (consciousness) & vertical gaze centers. Patient is awake, paralyzed; communicates via eye movements.
  • 📌 Mnemonic: "Locked-in, but looking up."

• Medial Pontine Syndrome (Basilar a. branches):

  • Contralateral hemiparesis (Corticospinal tract).
  • Contralateral ↓ proprioception/vibration (Medial lemniscus).
  • Ipsilateral CN VI palsy (Abducens nucleus).

• Lateral Pontine Syndrome (AICA):

  • Ipsilateral ataxia (Cerebellar peduncles).
  • Ipsilateral facial paralysis (CN VII nucleus).

⭐ AICA stroke often presents with ipsilateral facial paralysis, ipsilateral hearing loss, and vertigo/nystagmus, a classic triad for lateral pontine syndrome.

Medullary Syndromes - Wallenberg's World

• Vascular Event: Ischemic stroke in the territory of the Posterior Inferior Cerebellar Artery (PICA).
• **Key Deficits (Ipsilateral):
  • Face: Loss of pain and temperature sensation (Spinal Trigeminal Nucleus).
  • Coordination: Ataxia, dysmetria, dysdiadochokinesia (Inferior Cerebellar Peduncle).
  • Pharynx/Larynx: Hoarseness, dysphagia, diminished gag reflex (Nucleus Ambiguus).
  • Autonomic: Horner's Syndrome (ptosis, miosis, anhidrosis).
• **Key Deficits (Contralateral):
  • Body: Loss of pain and temperature sensation (Spinothalamic Tract).
• Vestibular Signs: Vertigo, nystagmus, vomiting (Vestibular Nuclei).

⭐ Exam Favorite: Note the absence of contralateral hemiparesis. The corticospinal (pyramidal) tract is located medially and is spared in this syndrome.

• Medial brainstem lesions affect motor pathways (corticospinal tract) and midline cranial nerve nuclei (III, IV, VI, XII).
• Lateral brainstem lesions involve the spinothalamic tract, sympathetic tract, and cerebellar pathways.
• Wallenberg syndrome (Lateral Medulla, PICA) causes ipsilateral facial and contralateral body sensory loss, plus Horner's syndrome.
• Weber's syndrome (Medial Midbrain) combines an ipsilateral CN III palsy with contralateral hemiparesis.
• Medial Medullary syndrome (ASA) presents with an ipsilateral CN XII palsy and contralateral hemiparesis/proprioception loss.
• Locked-in syndrome from a ventral pons lesion spares consciousness and vertical eye movements.