Brainstem Syndromes Mnemonic: Rule of 4 & CN Nuclei

The highest-yield brainstem mnemonic is the Rule of 4. Four structures near the midline start with M (Motor pathway, Medial lemniscus, MLF, Motor nucleus of CN III, IV, VI or XII), four lateral structures start with S (Spinocerebellar, Spinothalamic, Sensory nucleus of V, Sympathetic), and the cranial nerves stack four per level: I–IV above the pons (only III and IV in the midbrain), V–VIII in the pons, IX–XII in the medulla. Use the cranial nerve to find the level and the M's or S's to decide medial versus lateral; an ipsilateral cranial nerve deficit with contralateral body signs points to the brainstem.

Brainstem stroke syndromes at a glance

Most named brainstem syndromes pair a cranial nerve sign (which sets the level) with one or more long-tract signs (which set medial versus lateral); Parinaud, a supranuclear vertical gaze palsy without long-tract signs, is the exception.

The Rule of 4 of the brainstem

Neurologist Peter Gates published this shortcut in 2005 as four rules:

1. Four M's run down the midline (medial brainstem).
2. Four S's sit to the side (lateral brainstem).
3. Four cranial nerves per level: medulla IX–XII, pons V–VIII, above the pons I–IV (only III and IV are in the midbrain).
4. The midline motor nuclei are the numbers that divide evenly into 12, excluding 1 and 2: III, IV, VI, XII. The other brainstem nerves (V, VII, VIII, IX, X, XI) are lateral.

Why the sides split: in the brainstem, the long tracts serving the body all represent the opposite side (the corticospinal tract crosses below, in the lower medulla; the medial lemniscus and spinothalamic tract have already crossed), while cranial nerve nuclei, the MLF, spinocerebellar fibers and sympathetics serve the same side. Hence crossed findings. One exception: trochlear fibers cross before they exit, so a CN IV nucleus lesion weakens the opposite superior oblique.

Brainstem nuclei mnemonic: cranial nerves by level

Two caveats explain classic stroke findings: the trigeminal nuclei span the whole brainstem (down to the upper cervical cord), so a lateral medullary stroke still numbs the ipsilateral face, and the vestibular nuclei extend into the medulla, so it also causes vertigo. CN I and II are forebrain structures, not brainstem nuclei.

Cranial nerve nuclei and columns mnemonic: motor medial, sensory lateral

In the embryo the sulcus limitans separates the basal plate (motor) from the alar plate (sensory). In the brainstem the motor nuclei end up nearer the midline and the sensory nuclei more lateral and posterior, so cranial nerve nuclei line up in functional columns.

The sulcus limitans divides the three motor (efferent) columns from the three sensory (afferent) columns. Hook: eye and tongue muscles, arch muscles, glands, then guts and taste, face, ears. The nucleus ambiguus sits in the lateral part of the rostral medulla, which is why it is hit in lateral (PICA or vertebral) strokes while XII stays medial.

Midbrain syndromes mnemonic: Weber, Claude, Benedikt, Parinaud

The ventral and tegmental midbrain syndromes all start with an ipsilateral CN III palsy; the second finding names the syndrome.

• Weber = Weakness: CN III fascicles plus the cerebral peduncle, giving contralateral hemiparesis that includes the lower face and tongue.
• Claude = Clumsy: CN III fascicles plus dentatorubrothalamic fibers near the red nucleus, giving contralateral ataxia.
• Benedikt = Both: CN III plus red nucleus, substantia nigra and peduncle, giving contralateral hemiparesis together with ataxia, tremor (Holmes tremor) or choreoathetosis.
• Parinaud = can't look uP: a dorsal midbrain (pretectal) lesion, classically compression by a pineal tumor in a young patient, while vascular causes dominate in middle and older age. Upgaze palsy, convergence–retraction nystagmus, pupils that react to near but not light, and eyelid retraction (Collier sign). This one is a vertical gaze palsy, not a CN III palsy.

Pontine syndromes: abduction, the face and gaze

The pons holds CN VI, the facial nucleus, the horizontal gaze machinery (abducens nucleus, paramedian pontine reticular formation, MLF) and corticospinal fibers in its base.

• Raymond: ipsilateral CN VI palsy with contralateral hemiparesis, and no ipsilateral (LMN) facial palsy; a contralateral central facial paresis may accompany the hemiparesis.
• Millard-Gubler: Raymond's findings plus an ipsilateral facial (CN VII) palsy that involves the forehead, because the facial fascicle is hit within the pons.
• Foville: Millard-Gubler plus a conjugate horizontal gaze palsy toward the side of the lesion, because the abducens nucleus also drives the opposite medial rectus through the MLF.
• Lateral pontine (AICA): ipsilateral facial paralysis and hearing loss (the AICA gives off the labyrinthine artery), ipsilateral Horner syndrome, facial pain and temperature loss, and limb ataxia, with contralateral body pain and temperature loss. When the infarct also reaches the corticospinal fibers (described as Marie-Foix syndrome), contralateral hemiparesis is added.
• Locked-in syndrome: a ventral pontine infarct, usually from basilar artery occlusion, destroys both corticospinal and corticobulbar tracts. Expect quadriplegia, bilateral facial paralysis and horizontal gaze palsy with preserved consciousness, vertical eye movements and blinking.

INO and the one-and-a-half syndrome

An MLF lesion causes internuclear ophthalmoplegia: on looking away from the lesion, the eye on the side of the lesion fails to adduct and the abducting eye shows nystagmus, while convergence can be preserved. Stroke (usually unilateral, older patient) and multiple sclerosis (often bilateral, young adult) each account for about a third of cases. Add the PPRF on the same side and you get the one-and-a-half syndrome: no horizontal movement remains except abduction of the opposite eye.

Medullary syndrome mnemonic: Wallenberg vs Dejerine

Wallenberg (lateral medullary) is the most tested brainstem stroke. About 80% of cases come from vertebral artery occlusion, and vertebral artery dissection is the commonest cause in younger patients (neck trauma or manipulation, connective tissue disease). PICA remains the classic exam answer; a young patient with neck pain after manipulation points to dissection.

Mnemonic: "Don't pick a horse that can't eat": PICA, hoarse, can't eat (dysphagia). Prognosis is generally better than for most stroke syndromes; gait instability is the commonest residual problem.

Dejerine (medial medullary) accounts for only about 1% of cerebral infarctions: paramedian branches of the anterior spinal artery or vertebral artery, usually atherothrombotic. The triad is the tongue deviating toward the lesion (CN XII), contralateral hemiparesis that spares the face (pyramid) and contralateral loss of vibration and proprioception (medial lemniscus), although only a minority show all three. Babinski-Nageotte combines Wallenberg features with contralateral hemiparesis.

Localize any brainstem lesion in four steps

1. Confirm it is the brainstem. Crossed findings (cranial nerve on one side, body on the other) mean brainstem; face and body weak on the same side, with no crossed cranial nerve sign, usually point above the brainstem (cortex or internal capsule), although a ventral pontine lacune can do the same.
2. Find the level from the cranial nerve. III or IV = midbrain; V, VI, VII or VIII = pons; IX, X or XII = medulla.
3. Decide medial or lateral. Weakness, proprioception loss, INO or a midline motor nerve (III, IV, VI, XII) = medial. Ataxia, crossed pain/temperature loss, Horner or a lateral nerve (V, VII, VIII, IX, X) = lateral. Exception: in the midbrain, contralateral ataxia with a CN III palsy (Claude, Benedikt) comes from the paramedian red nucleus region, so it is a medial (PCA paramedian) syndrome.
4. Name the artery. Medial = paramedian branches (PCA, basilar, anterior spinal or vertebral); lateral = circumferential branches (AICA in the pons, PICA or vertebral in the medulla).

For cortical, subcortical and lacunar territories, use stroke localization by artery; for the posterior circulation as a whole, see anterior vs posterior circulation stroke.

Brainstem reflexes mnemonic: which reflex tests which level

Hook: the nerve pairs climb as you descend (2→3, 5→7, 9→10), so pupil, blink and gag test midbrain, pons and medulla. Loss of all brainstem reflexes, together with apnea, is part of the brain death examination.

Acute brainstem stroke on Step 2 CK

• How it presents: vertebrobasilar ischemia causes roughly 20% of strokes and TIAs, and the pons is the commonest brainstem site. "Top of the basilar" embolism causes vertical gaze palsy, pupillary changes, hypersomnolence and hallucinations with little motor or sensory loss.
• Imaging: noncontrast CT first to exclude hemorrhage, accepting that CT is often normal early; MRI with diffusion-weighted imaging is highly sensitive in the hyperacute phase, although early DWI can miss small brainstem infarcts. CTA or MRA of the head and neck looks for vertebrobasilar occlusion, stenosis or dissection.
• Thrombolysis: IV alteplase or tenecteplase within 4.5 hours of last known well, if eligible; the 2026 AHA/ASA guideline adds that IV thrombolysis may be reasonable at 4.5–9 hours (or within 9 hours of the midpoint of sleep in wake-up stroke) when automated perfusion imaging shows salvageable penumbra.
• Thrombectomy: the 2026 AHA/ASA acute ischemic stroke guideline recommends endovascular thrombectomy within 24 hours for basilar artery occlusion in patients with a prestroke modified Rankin Scale score of 0–1, an NIHSS of 10 or more and a posterior circulation ASPECTS of 6 or more. Untreated basilar occlusion carries a poor outcome in most cohorts.

Exam traps and look-alikes

• Which way does it deviate? Tongue toward a CN XII lesion; uvula away from a CN X lesion; jaw toward a CN V motor lesion.
• UMN vs LMN face. A facial nucleus or nerve lesion in the pons paralyzes the whole ipsilateral face, forehead included. A corticobulbar lesion (cortex, internal capsule, or the peduncle in Weber) weakens only the contralateral lower face.
• Nerve vs nucleus for CN VI. One eye that can't abduct = CN VI nerve or fascicle. Neither eye can look toward the lesion = abducens nucleus or PPRF (horizontal gaze palsy).
• PICA vs AICA. Both give crossed pain/temperature loss, Horner and ataxia; hoarseness and dysphagia mean medulla (PICA or vertebral), facial paralysis and hearing loss mean pons (AICA).

Worked examples

Vignette 1. A 64-year-old man with diabetes wakes with weakness of his left arm and leg. His face is symmetric. On protrusion, his tongue deviates to the right. Vibration and joint-position sense are lost in the left limbs, but pinprick is normal everywhere. Where is the lesion?

Answer: Right medial medullary syndrome, from occlusion of paramedian branches of the right anterior spinal or vertebral artery.

Reasoning: CN XII (a "divides into 12" motor nucleus) puts the lesion in the medulla. Two more M's (pyramid → left weakness, medial lemniscus → left vibration loss) with no S's make it medial, and the tongue points toward the lesion.

Vignette 2. A 41-year-old woman develops neck pain two days after a chiropractic adjustment, then sudden vertigo, hoarseness and hiccups. Her right pupil is small with mild ptosis, pinprick is reduced on the right face and the left arm and leg, and she falls to the right. Strength is normal. What is the most likely cause?

Answer: Right vertebral artery dissection causing a right lateral medullary (Wallenberg) infarct.

Reasoning: Hoarseness (nucleus ambiguus), hiccups, ipsilateral Horner, crossed pain/temperature loss and ipsilateral ataxia without weakness are lateral medullary findings; in a young patient after neck manipulation, dissection is the most likely mechanism.

Key takeaways

• Rule of 4: four M's medial, four S's lateral, four cranial nerves per level; midline motor nuclei are III, IV, VI, XII.
• Level first, then side: the cranial nerve sets the level; M versus S findings decide medial versus lateral; then name the artery.
• Midbrain: Weber weak, Claude clumsy, Benedikt both, Parinaud can't look up.
• Pons: Raymond (VI + motor), Millard-Gubler (VI + VII + motor), Foville (adds a gaze palsy), AICA (face and hearing), locked-in (only the eyes move).
• Medulla: Wallenberg = hoarseness, dysphagia, crossed pain/temperature loss and Horner without weakness; Dejerine = tongue toward the lesion plus contralateral weakness.
• Reflex levels: pupil = midbrain, corneal = pons, gag = medulla.

Browse the rest of the neurology topic hub for spinal cord, cerebellar and cortical localization.

Sources

• Gates P. The rule of 4 of the brainstem: a simplified method for understanding brainstem anatomy and brainstem vascular syndromes for the non-neurologist. Intern Med J. 2005 (PubMed)
• Brainstem Stroke — StatPearls, NCBI Bookshelf
• Vertebrobasilar Stroke — StatPearls, NCBI Bookshelf
• Pontine Infarction (archived chapter) — StatPearls, NCBI Bookshelf
• Wallenberg Syndrome (archived chapter) — StatPearls, NCBI Bookshelf
• Medial Medullary Syndrome — StatPearls, NCBI Bookshelf
• Benedikt Syndrome — StatPearls, NCBI Bookshelf
• Parinaud Syndrome — StatPearls, NCBI Bookshelf
• Internuclear Ophthalmoplegia — StatPearls, NCBI Bookshelf
• Prabhakaran S, et al. 2026 Guideline for the Early Management of Patients With Acute Ischemic Stroke (AHA/ASA). Stroke. 2026 (PubMed)