Lateral Medullary Syndrome (Wallenberg Syndrome)

Definition

Lateral medullary syndrome is one of the most recognizable vascular syndromes in neurology. It results from infarction of a wedge-shaped area of the lateral medulla, posterior to the inferior olivary nucleus. It was described by Adolf Wallenberg in 1895 and is sometimes called Wallenberg syndrome or PICA syndrome.

Etiology and Pathogenesis

Despite the classic attribution to posterior inferior cerebellar artery (PICA) occlusion, careful studies show that most cases are caused by vertebral artery occlusion - either by atherothrombosis or dissection. True isolated PICA occlusion accounts for fewer cases.

In a series of 130 consecutive patients with pure lateral medullary infarction:

Large vessel atherosclerotic steno-occlusive disease: 50%
Arterial dissection: 15% (more common with caudal lesions)
Small vessel disease: 13%
Cardioembolism: 5%

Less common causes include cocaine abuse, medullary neoplasms (usually metastases), demyelinating disease, vascular malformation hemorrhage, neck manipulation, and trauma.

Localization in Clinical Neurology, 8e, p. 921
Adams and Victor's Principles of Neurology, 12th Ed., p. 816

Anatomy of the Lesion

The lesion involves a wedge of lateral medullary tegmentum that contains multiple structures simultaneously:

Structure Damaged	Clinical Deficit
Spinal nucleus and tract of CN V	Ipsilateral facial hypalgesia and thermoanesthesia
Spinothalamic tract	Contralateral trunk/limb hypalgesia and thermoanesthesia
Nucleus ambiguus	Ipsilateral palatal, pharyngeal, and vocal cord paralysis
Descending sympathetic fibers	Ipsilateral Horner syndrome (ptosis, miosis, anhidrosis)
Vestibular nuclei	Vertigo, nausea, vomiting, nystagmus
Inferior cerebellar peduncle / restiform body	Ipsilateral cerebellar ataxia, lateropulsion
Nucleus tractus solitarius (rostral/lateral zone)	Loss of taste
Nucleus tractus solitarius (caudal/medial zone)	Autonomic dysfunction, BP lability
Dorsolateral reticular formation	Hiccups (singultus)
Utricular nucleus	Vertical diplopia, ocular tilt reaction

Classic Clinical Features

The "Crossed" Sensory Pattern (Pathognomonic)

Ipsilateral face: loss of pain and temperature (CN V spinal nucleus/tract)
Contralateral body: loss of pain and temperature (spinothalamic tract)
Spared: vibration and proprioception (medial lemniscus is in medial medulla - unaffected)

Motor System

No hemiparesis in classic lateral medullary syndrome - the corticospinal tract (pyramids) runs in the medial medulla and is spared
Ipsilateral gait ataxia and tendency to fall/topple toward the side of the lesion (lateropulsion)

The Diagnostic Triad

The clinical triad that best identifies lateral medullary infarction:

Horner syndrome + ipsilateral ataxia + contralateral hypoalgesia

Full Feature List

Ipsilateral facial sensory loss (pain/temperature) - CN V spinal tract
Contralateral body sensory loss (pain/temperature) - spinothalamic tract
Dysphagia and hoarseness (with ipsilateral palatal/vocal cord weakness) - nucleus ambiguus
Ipsilateral Horner syndrome (miosis, ptosis, decreased sweating) - descending sympathetics
Vertigo, nausea, vomiting - vestibular nuclei
Ipsilateral limb/gait ataxia and lateropulsion - inferior cerebellar peduncle
Nystagmus (direction-changing; almost invariable)
Hiccups - dorsolateral reticular formation
Diminished gag reflex ipsilaterally
Loss of taste (from nucleus tractus solitarius involvement)

Adams and Victor's Principles of Neurology, 12th Ed., p. 816
Localization in Clinical Neurology, 8e, p. 921

Ocular Motor Abnormalities

Eye signs are varied and diagnostically useful:

Nystagmus: nearly invariable; often direction-changing (gaze-evoked)
Skew deviation: hypotropia on the side of the lesion
Lateropulsion of saccades: hypometric toward the lesion side, hypermetric away
Ocular tilt reaction: involves head tilt, skew deviation, and ocular counter-rolling
Fragment of INO (internuclear ophthalmoplegia) occasionally seen

Lesion Location Correlations

Rostral lateral medullary lesions: more severe dysphagia, hoarseness, facial paresis
Caudal lateral medullary lesions: more prominent vertigo, nystagmus, gait ataxia; also correlate with vertebral dissection
Ventromedial extension: may cause contralateral facial sensory changes (unusual variant)

Rare Manifestations

Wild arm ataxia (lateral cuneate nucleus involvement)
Central pain with allodynia
Contralateral hyperhidrosis with ipsilateral anhidrosis
Inability to sneeze (sneezing center in ventromedial descending trigeminal tract)
Paroxysmal sneezing
Autonomic dysfunction: tachycardia, BP lability, respiratory failure
Ondine's curse (failure of automatic breathing) - nucleus ambiguus + adjacent reticular formation
Transient urinary retention
Ipsilateral hemiparesis if lesion extends to level of pyramidal decussation (this variant = Opalski syndrome)
Pure sensory stroke (very caudal lesion)

Localization in Clinical Neurology, 8e, pp. 923-924

Opalski Syndrome

A rare variant in which the lesion extends caudally below the pyramidal decussation, adding ipsilateral spastic hemiplegia to the classic lateral medullary features. Because the corticospinal fibers have already crossed at this level, the hemiparesis is ipsilateral (not contralateral). A 2024 systematic review (PMID: 39096581) reviewed all published cases of Opalski syndrome.

Imaging

DWI-MRI is the investigation of choice and shows the lateral medullary infarct early
The infarct may not appear on T2-weighted imaging in the first 24 hours but is visible by day 5
MRA (or CT angiography) shows vertebral artery flow loss in dissection
T1-weighted axial MRI through the vertebral arteries shows a bright intramural clot (crescent sign) in vertebral dissection

Prognosis

Prognosis is generally good because the motor system (pyramids) is usually spared
Most patients recover substantially in the first days to weeks
Dysphagia can be prolonged and require nasogastric feeding
A subset of patients experience sudden death from respiratory or cardiac arrest even after initial good recovery - likely from autonomic dysfunction involving the nucleus tractus solitarius

Treatment

Acute ischemic stroke management: IV tPA if within window and no contraindications
Vertebral dissection (common in young patients, especially post neck manipulation): antiplatelet therapy (aspirin); anticoagulation may be considered in selected cases
Dysphagia management: speech therapy, modified diet, NG tube if severe
Vestibular rehabilitation for persistent vertigo/ataxia

Mnemonic: "避 Lateral Medulla"

A quick clinical summary: D-VANS-H (Dysphagia, Vertigo, Ataxia [ipsilateral], Nystagmus, Sensory loss [crossed], Horner [ipsilateral])

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