Lateral Medullary Syndrome: How a Wallenberg Stroke Crosses the Body

What it is and why it matters

Lateral medullary syndrome — eponymously Wallenberg syndrome, described by Adolf Wallenberg in 1895 — is a posterior-circulation ischemic stroke of the lateral wedge of the medulla oblongata. It is the most frequently encountered and most classically taught brainstem stroke syndrome.

It matters clinically for three reasons:

• It is easily missed. There is usually no limb weakness and no facial droop, so it fails the FAST screen. Patients present with vertigo, imbalance, and swallowing trouble and are mistaken for peripheral vertigo, gastroenteritis, or intoxication.
• The cause is often a dissection. Vertebral artery dissection is the leading etiology in younger patients (under ~45), sometimes after neck trauma, chiropractic manipulation, or trivial hyperextension.
• Dysphagia is dangerous. Nucleus ambiguus involvement produces a weak swallow and high aspiration risk — the practical driver of morbidity.

Recognizing the crossed sensory pattern and Horner sign at the bedside routes the patient to urgent posterior-fossa imaging rather than a benign-vertigo discharge.

Mechanism: why the deficits cross the body

The lateral medulla is a crowded neighborhood of tracts and nuclei. A single ischemic wedge knocks out several at once, and the crossed pattern arises from where each pathway has (or hasn't) decussated:

• Spinothalamic tract — carries pain/temperature from the contralateral body. Second-order fibers cross in the spinal cord at their entry level, so by the medulla they represent the opposite side. Lesion here = contralateral body pain/temp loss.
• Spinal trigeminal nucleus and tract (CN V) — carries pain/temperature from the ipsilateral face; these fibers have not yet crossed. Lesion = ipsilateral facial pain/temp loss.
• Nucleus ambiguus (CN IX, X) → ipsilateral palatal/pharyngeal/laryngeal weakness (dysphagia, hoarseness, absent gag).
• Vestibular nuclei → vertigo, nystagmus, nausea, vomiting.
• Inferior cerebellar peduncle / spinocerebellar fibers → ipsilateral limb ataxia.
• Descending sympathetic fibers → ipsilateral Horner syndrome.

The medial lemniscus and corticospinal pyramids sit medially and are spared — which is exactly why vibration/proprioception and strength are preserved.

Clinical presentation and classic signs

The classic bedside gestalt is a patient with sudden vertigo and imbalance who is not weak. Cardinal features cluster into a memorable set:

• Crossed (dissociated) sensory loss: ipsilateral face, contralateral body loss of pain and temperature — the pathognomonic sign.
• Ipsilateral Horner syndrome: ptosis, miosis, and facial anhidrosis from interrupted descending sympathetics.
• Bulbar signs (nucleus ambiguus): dysphagia, hoarse/nasal voice, absent gag reflex — high aspiration risk.
• Vestibulocerebellar signs: vertigo, nystagmus, nausea/vomiting, ipsilateral limb ataxia and a tendency to fall or veer toward the side of the lesion (lateropulsion / ocular lateropulsion).
• Intractable hiccups (singultus) — a curious but well-recognized feature.

Critically, there is no hemiparesis and no facial motor weakness — the face is numb, not droopy. Skew deviation and a bias in ocular saccades toward the lesion may be seen. This preservation of strength distinguishes it from most anterior-circulation strokes and is why FAST-based screens miss it.

Diagnosis: imaging and the HINTS pitfall

Diagnosis is clinical localization confirmed by imaging. Key steps:

• MRI with diffusion-weighted imaging (DWI) is the test of choice — it shows the acute lateral medullary infarct. Note the caveat: DWI is falsely negative in up to ~15–20% of posterior-fossa/brainstem strokes in the first 24–48 hours, so a negative early MRI does not exclude the diagnosis; repeat if suspicion is high.
• Non-contrast CT is insensitive here (bone artifact in the posterior fossa) but is done first to exclude hemorrhage.
• CTA or MRA of the vertebrobasilar system looks for vertebral artery occlusion or dissection (crescent sign, tapered lumen).
• The HINTS exam (Head Impulse, Nystagmus, Test of Skew) helps at the bedside: a central pattern — normal head impulse, direction-changing nystagmus, or positive skew — points to stroke over peripheral vertigo, and HINTS outperforms early MRI-DWI for acute vestibular strokes.

A young patient with neck pain preceding the deficit should raise dissection; obtain vessel imaging and screen for stroke risk factors (hypertension, atrial fibrillation, smoking).

Management and complications

Management is that of an acute ischemic stroke plus posterior-fossa vigilance:

• IV thrombolysis (alteplase/tenecteplase) within the window if criteria are met — it works by activating plasminogen to lyse the occluding thrombus. Isolated small medullary infarcts may score low on the NIHSS yet still qualify given the disabling deficits.
• Antithrombotics: antiplatelet therapy for atherothrombotic disease; for confirmed vertebral artery dissection, either antiplatelet or anticoagulation is used to prevent artery-to-artery embolism (trials show broad equivalence).
• Aspiration prevention is the highest-yield step: strict NPO until a formal swallow evaluation, because nucleus ambiguus dysfunction makes aspiration pneumonia the dominant complication.
• Monitoring for extension: a lesion creeping medially or a large PICA-territory cerebellar infarct can cause posterior-fossa edema, obstructive hydrocephalus, and brainstem compression — a neurosurgical emergency that may need decompression.

Other issues include central hypoventilation (Ondine's curse if the respiratory network is hit) and refractory hiccups. Prognosis is generally favorable, with most patients regaining swallowing and independent function.

Mimics, pitfalls, and significance

The great trap is calling Wallenberg a peripheral problem. Distinguishing it sharpens neurologic reasoning:

• Vestibular neuritis / peripheral vertigo: a HINTS exam that is peripheral (abnormal head impulse, unidirectional nystagmus, no skew) argues against stroke; a central HINTS argues for it. Any accompanying dysphagia, Horner, or crossed sensory sign should make you think medulla, not inner ear.
• Medial medullary (Dejerine) syndrome: instead of crossed pain/temp, it gives contralateral limb weakness, contralateral proprioception loss, and ipsilateral tongue deviation (CN XII) — the medial counterpart (see table).
• Lateral pontine syndrome (AICA): adds ipsilateral facial motor weakness (CN VII) and hearing loss (CN VIII) — features absent in Wallenberg.
• Vertebral dissection vs. atherothrombosis: a neck-pain-preceding-vertigo history in a young patient flips the differential toward dissection.

The enduring teaching value is the crossed brainstem lesion: ipsilateral cranial-nerve/face signs plus contralateral body signs pin the lesion to the brainstem and let you localize precisely without waiting for the scan.

Lateral (Wallenberg) vs. medial (Dejerine) medullary syndrome — localizing the deficit

Feature	Lateral medullary (Wallenberg)	Medial medullary (Dejerine)
Vessel	Vertebral artery / PICA	Anterior spinal artery / vertebral artery
Motor	No limb weakness (pyramids spared)	Contralateral arm/leg weakness (pyramid)
Sensation	Crossed: ipsilateral face, contralateral body pain/temp	Contralateral body loss of proprioception/vibration (medial lemniscus)
Cranial nerves	IX/X (dysphagia, hoarseness), V spinal nucleus, vestibular	CN XII — ipsilateral tongue deviation toward lesion
Autonomic/cerebellar	Ipsilateral Horner, ipsilateral ataxia, vertigo, hiccups	Usually absent
Tongue	Normal	Deviates toward the lesion (ipsilateral hemiatrophy)