brainstem · equilibrium

🧠 Vestibular & Brainstem · Clinical Core

High‑yield principles for equilibrium, eye movements, and localisation

⚖️ Vestibular system

The vestibular apparatus (labyrinth) and its central pathways maintain posture, equilibrium, and gaze stabilisation during head movement. Three semicircular canals detect angular acceleration; the utricle and saccule sense linear acceleration and gravity.

Peripheral receptors
• Semicircular ducts → angular (rotational) movement
• Utricle & saccule → linear acceleration & head tilt

Central relay
• 4 vestibular nuclei (medulla/pons) receive CN VIII afferents
• Project to MLF → innervate CN III, IV, VI (conjugate gaze)
• Reciprocal connections with flocculonodular cerebellum

Vestibulo‑ocular reflex (VOR)

VOR keeps images stable on the retina during rapid head motion. The vestibular nuclei integrate semicircular canal signals and drive a compensatory eye movement in the opposite direction to head rotation. This reflex is a key brainstem function tested in comatose patients.

📌 High‑yield · The VOR pathway: vestibular nerve → vestibular nuclei → MLF → abducens / oculomotor nuclei → extraocular muscles. Lesions cause disconjugate gaze or nystagmus.

🌊 Caloric testing

Irrigation of the external auditory canal with water at different temperatures stimulates the horizontal semicircular canal. In an intact brainstem, this induces nystagmus with a predictable fast phase.

Warm water → endolymph rises → stimulates canal → slow phase to opposite side → fast phase toward the irrigated ear.
Cold water → endolymph sinks → inhibits canal → slow phase toward the irrigated ear → fast phase away.

Clinical pearl · Cold water mimics a vestibular lesion. In comatose patients, absence of caloric-induced eye movement indicates brainstem dysfunction.

Vertigo & Meniere disease

peripheral (labyrinth / nerve) – severe, episodic, with nausea, nystagmus, tinnitus; hearing loss often present.
central (brainstem / cerebellum) – milder, often chronic (>2–3 weeks), associated with other brainstem signs.
Meniere disease – endolymphatic hydrops; recurrent vertigo (minutes–hours), fluctuating hearing loss, aural fullness, tinnitus. Typically unilateral, middle‑aged.

👁️ Nystagmus & fast phase

Nystagmus is rhythmic oscillation with a slow (pathological) drift and a fast (cortical corrective) phase. The direction of nystagmus is named after the fast phase.

Vestibular nystagmus – e.g. left vestibular lesion: unopposed right vestibular nuclei drive eyes slowly to the left (slow phase). Cortex corrects with quick movement to the right → fast phase away from lesion.

🧪 Caloric rule (conceptual) · Warm water stimulates the canal → fast phase to the same side. Cold water inhibits → fast phase opposite. This reflects the underlying pathophysiology.

🎯 Horizontal conjugate gaze

For coordinated lateral gaze:

Rightward gaze → right abducens (CN VI) abducts right eye + left oculomotor (CN III) adducts left eye.
MLF interconnects abducens and oculomotor nuclei, enabling yoked movements.
Frontal eye field (Brodmann 8) → contralateral saccades via the PPRF (paramedian pontine reticular formation), which provides ipsilateral gaze signals.

Lesion location	Deficit	Key feature
Right abducens nerve (CN VI)	Right eye cannot abduct (look right)	Diplopia on right gaze
Right PPRF / abducens nucleus	Neither eye looks right (ipsilateral gaze palsy)	May have facial paralysis (fibres loop over nucleus)
Left MLF (internuclear ophthalmoplegia)	Left eye adduction failure on right gaze; right eye nystagmus	Convergence spared (differentiates from CN III palsy)
Left frontal eye field	Both eyes cannot look right; slow drift left	Contralateral gaze palsy; often with hemiparesis

📌 INO pearl · MLF lesion causes adduction failure ipsilateral to the lesion (e.g., right MLF → right eye adduction deficit on left gaze). Convergence remains intact because it uses separate pathways.

🧬 Brainstem syndromes

Rule of thumb: brainstem lesions typically affect ipsilateral cranial nerves and contralateral long tracts (except descending hypothalamic fibres → ipsilateral Horner).

🩸 Medial medullary syndrome

Vascular territory – anterior spinal artery / vertebral artery occlusion.

corticospinal → contralateral spastic hemiparesis
medial lemniscus → contralateral loss of proprioception / vibration / fine touch
hypoglossal (CN XII) → ipsilateral tongue weakness; deviation toward lesion on protrusion

🌀 Lateral medullary (Wallenberg) syndrome

Vascular territory – PICA (posterior inferior cerebellar artery) occlusion.

spinal trigeminal → ipsilateral facial pain/temperature loss
spinothalamic → contralateral body pain/temperature loss
vestibular nuclei → vertigo, nystagmus (fast phase away from lesion), nausea
nucleus ambiguus → dysphagia, dysarthria, loss of gag reflex
hypothalamic fibres → ipsilateral Horner syndrome (ptosis, miosis, anhidrosis)
Ipsilateral limb ataxia (inferior cerebellar peduncle)

Alternating sensory loss – face ipsilateral, body contralateral (pain/temperature).

🧩 Medial pontine (paramedian) syndrome

Vascular – paramedian branches of basilar artery.

corticospinal + medial lemniscus → contralateral hemiparesis and sensory loss
abducens (CN VI) → ipsilateral medial strabismus (failure to abduct)
facial (CN VII) if extension → ipsilateral lower motor neuron facial palsy

📡 Lateral pontine (AICA / SCA) syndrome

Vascular – AICA (caudal) or SCA (rostral).

facial nucleus / fibres → ipsilateral facial paralysis, loss of taste, reduced lacrimation, hyperacusis
vestibulocochlear → vertigo, nystagmus, ipsilateral hearing loss
spinal trigeminal → ipsilateral facial pain/temperature loss
spinothalamic → contralateral body pain/temperature
descending hypothalamic → ipsilateral Horner
Middle cerebellar peduncle → ipsilateral ataxia

🎧 Pontocerebellar angle syndrome

Typically from acoustic neuroma (vestibular schwannoma).

Slowly growing; compresses CN VII (facial) and CN VIII (vestibulocochlear); may affect CN V (trigeminal) if large.
Key – cranial nerve deficits without long tract signs → localises to the angle (outside brainstem).

🧠 Midbrain (Weber) syndrome

Vascular – branches of posterior cerebral artery (PCA).

CN III fibres → ipsilateral oculomotor palsy (ptosis, dilated pupil, "down and out" eye)
corticospinal + corticobulbar → contralateral spastic hemiparesis and lower face weakness (forehead spared)

⬆️ Parinaud (dorsal midbrain) syndrome

Often from pineal mass compressing the superior colliculi.

paralysis of upward gaze (vertical gaze palsy)
Pupillary light‑near dissociation, dilated pupils
Hydrocephalus (aqueduct compression)

Arterial supply of the brainstem

Vertebral · anterior spinal (medial medulla), PICA (lateral medulla)

Basilar · paramedian (medial pons), AICA (lateral pons), SCA (rostral pons)

PCA · branches to midbrain (Weber, Parinaud)

Labyrinthine · from AICA or basilar → inner ear

🌐 Reticular formation & arousal

Network of nuclei in the brainstem core that regulates arousal, muscle tone, respiration, and cardiovascular function.

Raphe nuclei – serotonergic (5‑HT) projections; modulate mood, aggression, non‑REM sleep.
Locus coeruleus – noradrenergic; cortical activation / arousal; activity decreases in REM sleep.
Periaqueductal grey – opioid‑rich; descending pain modulation via spinal dorsal horn.

🧠 Alzheimer's link · Degeneration of raphe and locus coeruleus neurons is seen in Alzheimer disease.

📘 USMLE Step 1 · Brainstem pearls

MLF lesion → INO (adduction failure, convergence spared).
Lateral medullary (Wallenberg) → ipsilateral facial pain/temp loss, contralateral body loss, Horner, dysphagia.
Weber (midbrain) → CN III palsy + contralateral hemiparesis.
Parinaud → upgaze palsy + pupillary light‑near dissociation.
Pontocerebellar angle → CN VII + VIII without long‑tract signs.