PICA (posterior cerebellar artery syndrome). 

AKA "lateral medullary syndrome" (LMS), or "Wallenberg's syndrome"

Timothy C. Hain, MD Page last modified: April 11, 2022 •See also: Brainstem StrokesAICA StrokeSCA Stroke

The PICAstroke syndrome is also known as "lateral medullary syndrome", or "Wallenberg's syndrome", after Wallenberg's description in 1895. It is the most common brainstem stroke. It is typified by vertigo, ipsilateral hemiataxia, dysarthria, ptosis and miosis. Most patients with this stroke recover very well and often resume their previous activities (Nelles et al, 1998). There are rare exceptions.

Patients with LMS almost always have a Horner's syndrome (acutely unilateral ptosis, miosis and facial anhidrosis), but partial Wallenberg's are more the rule than the exception. In addition, there are a lot of important structures supplied by PICA, and depending where the damage lies, there may be other findings. Kim (2003) provided a table including 18 symptoms (or categories) of symptoms. (whew). These were divided up into the upper group (rostral) and the more common lower group (caudal).

Wallenberg's/Lateral Medullary syndrome (abstracted in part from Kim, 2003)
Symptom category Symptoms Damaged area of brainstem Frequency (from Kim, 2003)
Dizziness Nystagmus, vomiting, vertigo Vestibular nuclei/connections Common in all groups
Eyelid Droopy eyelid, smaller pupil Sympathetic connections descending from hypothalmus into cervical cord. Common in all groups
Speech Dysarthria, Swallowing difficulty Nucleus ambiguous Common in upper group
Unsteadiness Ipsilateral hemiataxia Inferior cerebellar peduncle About half
Numb body Contralateral loss pain/temperature Spinothalamic tract Uncommon in both groups
Numb face Ipsilateral loss pain/temperature Trigeminal nucleus and tract Uncommon in both groups
Weakness Same side weakness (Opalski) Corticospinal tract Rare
Weakness Opposite side weakness (Babinski-Nageotte) Corticospinal tract Rare

LMS generally does not include hearing symptoms however. Hearing symptoms are generally from strokes in the territory of he next higher artery in the brainstem, the anterior inferior cerebellar artery or AICA.

What causes Wallenberg's/Lateral Medullary syndrome ?

cerebral blood vessals Central vestibular pathways
PICA is usually a branch of the vertebral artery. It both supplies the medulla as well as the inferior cerebellum. Central vestibular pathways, drawn from back of brainstem with cerebellum removed. SVN,LVN,MVN are the superior,lateral and medial vestibular nuclei. The "VN" is the inferior vestibular nucleus, more commonly labeled as either DVN or IVN. The vestibular nuclei are dorsal (i.e. close to the cerebellum). Source of figure: what-when-how neuroscience. See also this similar figure:

Usually LMS/Wallenberg's is caused by loss of blood flow in the posterior inferior cerebellar artery (PICA). PICA may arise from the vertebral artery (the usual case), or as a separate branch of the basilar artery. Because of the far more common origin from the vertebral artery, most "PICA" syndrome strokes actually are due to vertebral artery occlusion (Kim 2003). Cardiac embolism causes only 5% of these strokes, while dissection causes 15% (Kim, 2003).

PICA is also the most common site of occlusion from propagating thrombus or embolism caused by injury to the third section of the vertebral artery, and Wallenberg's syndrome is the most common stroke caused by chiropractic manipulation (Caplan, 1986).

The medulla is the portion of the brainstem just below the pons (recognized by cerebellar peduncles), and the first cervical spinal nerve (C1). It is about 3 cm in length and 2cm in greatest diameter, and is a big target. As the diagram illustrates above, the vestibular nucleus which extends from lower pons into medulla, is also a big target because of it's length. Thus there is a lot of room for variation in the areas of damage. Also in the medulla (besides the vestibular nucleus) is the spinal tract of V (accounting for facial numbness), the nucleus ambiguus (accounting for swallowing problems), the spinothalamic tract (contralateral numbness), and the lower portion of the inferior cerebellar peduncle as well as spinocerebellar connecting tracts (ipsilateral clumsiness). Quite far away from the vestibular nuclei, in the front (i.e. top below), is the corticospinal tract (pyramidal tract below). The nomenclature of these nuclei is complex and there are short pithy acronyms for a very large number of structures, as needed to succinctly label diagrams and sections. .

Utricular connections (that might cause a skew, which is part of the "HINTS" test for brainstem stroke) are in the rostral (top) part of the medulla, roughly at the border with the pons.

Figure 1 from Kim (2003)
Figure 1 from Kim, 2003
Figure 1 from Kim (2003) showing various locations of medullary lesions on MRI found in his series of Wallenberg/LMS. Back of medulla is at the bottom of axial (horizontal plane) slices.

Kim (2003) divided 130 MRIs of LMS/Wallenbergs into into 10 subcategories -- upper (rostral) vs lower (middle and caudal), and horizontally between "typical", ventral, large, lateral and dorsal. (i.e 2*5). Whew.

Damage to the lateral medulla, perhaps due to tumor, can cause a similar set of symptoms as the vascular version of lateral medullary syndrome. (Auff and Vass, 1981) However, reports of nonvascular causes of lateral medullary syndrome are exceedingly rare. Hanyu et al (1990) reviewed 10 cases of tumors, and reported "the clinical features of this syndrome are characterized by gradual development and steady progression of symptoms, non-classical or atypical symptomatology, numerous additional symptoms and signs depending on the site and size of tumors, and poor prognosis. " There are almost no reports of MS causing LMS. Perhaps this is because MS tends to affect structures close to CSF pathways (i.e. medial, not lateral).

Radiology of LMS/Wallenbergs

Diagnosis is generally via MRI of the brain in a person with suspicious symptoms. For example, vertigo and nystagmus lacking a positive HIT test, and with a skew deviation.

medullary infarct wallenberg
Medullary infarct from left vertebral artery dissection (L dorsolateral medulla) associated with "contrapulsion". This is the "axial" view, roughly horizontal, showing the cerebellum and brainstem. Another image of a posterior inferior cerebellar artery (PICA) territory stroke of the left inferior cerebellum following a vertebral artery dissection. The damage is shown on the lower right side of picture

According to Sacco et al, (1993), "Cerebellar infarcts only infrequently accompany lateral medullary syndrome, suggesting that most of the posterior inferior cerebellar artery territory is spared, despite the high frequency of vertebral artery occlusion."

Right lateral medullary syndrome
Diffusion image of Wallenberg
Acute diffusion image of patient described below. There is low blood flow on the right medulla (dark area on left side above)

DWI MRI (diffusion) is often positive in Wallenbergs (Kitis et al, 2004). Diffusion is abnormal more acutely than is standard MRI or CT scan.

CT scans of the brain may show large cerebellar infarcts, but may not be able to visualize a small stroke in the brainstem. For example, Wang et al (2018) reported a case ofwhere the initial CT/DWI was negative, but it was positive one day later.

CT-angiography with 3D reconstruction has gotten good enough in recent years to be helpful too, and because it is fast and the scanner often resides in the emergency department, it is a favored technique in the Emergency department, although resolution is not nearly as good as MRI. CT angiography also can imperil the patient due to the iodine needed for the test, and of course, just the radiation.

Oculomotor findings/tests:

Brazis (1992) discussed ocular motor abnormalities in Wallenberg/LMS and stated that "These abnormalities include signs of dysfunction of ocular alignment (skew deviation, ocular tilt reaction, and environmental tilt), various types of nystagmus, smooth pursuit and gaze-holding abnormalities (eye deviation, ipsipulsion or lateropulsion, and impaired contralateral pursuit), and saccadic abnormalities (ipsipulsion and torsipulsion). " Of all of these signs, the skew deviation and the frequent lack of a positive HIT test are probably the most specific. See this page for the implementation of this logic in the HINTs test.

The vertigo in Wallenberg's/LMS is generally attributed to damage to the vestibular nucleus, and one or both of the vestibular nuclei that relay to the eyes -- MVN or SVN. Against this logical thought is that Kim (2003) reported that vertigo was equally common in caudal (lower) infarctions, which is could be quite far away from MVN or SVN.

There also may be saccadic dysmetria (overshoot), saccadic pulsion (pulling of the eye during vertical saccades toward the side of lesion -- called ipsipulsion). Rarely, the eyes go towards the opposite side of the lesion (called contrapulsion). (Kaski et al, 2012). Because the eyes can go either way, ipsi or contra, the localizing significance of pulsion is not very strong. This is thought to be a cerebellar sign.

Rarer oculomotor signs include torsional nystagmus and skew deviation (see below). The torsional nystagmus in Wallenberg generally beats away from the side of lesion (Morrow and Sharpe, 1988) , following the general rule for torsion when the vertical canals are inhibited. This is attributed to damage to the upper part of the vestibular nucleus, which processes input from the posterior and anterior canals. A variant of the nystagmus is "hemi-seesaw", where the eyes both have a rotatory movement (i.e. around the front back axis), as well as one eye depresses while the other elevates, generally accompanied by some torsion as well, and there is a jerk back to center. This results in a complex and difficult to describe eye movement.

While we have observed rebound nystagmus in several Wallenberg/LMS patients, there is no literature about this. We would guess that it is common in patients with gaze evoked nystagmus. Gaze-nystagmus might be due to damage to the NPH (which is very medial in the medulla, adjacent to the MVN), damage to the cerebellum, or perhaps from the gaze-evoked nystagmus that commonly accompanies vestibular lesions even when they are peripheral.

Kim et al (1994) reported that "MRI results showed that the lesions located in the rostral part of the medulla were usually diagonal band-shaped and were associated with more severe dysphagia, hoarseness, and the presence of facial paresis, whereas the caudal lesions, situated usually in the lateral surface of the medulla, appeared to correlate with more marked vertigo, nystagmus, and gait ataxia. Nausea/vomiting and Horner sign were common regardless of the lesion location, and lesions extending ventromedially correlated with facial sensory change on the contralateral side of the lesion."

Regarding the skew deviation, (i.e. vertical offsets between the eyes) the lower eye is generally on the side of the infarct. This is attributed to damage to tracts between the utricle and the oculomotor nucleus. The circuits start at the top of the medulla and thus are not a very large target. Skew deviation is generally not permanent as it is supranuclear. A static ocular counterroll would be expected with a skew caused by utricular disturbance.

This recording documents pulsion, which is found both in LMS and in the SCA teritory stroke. For vertical saccades upward or downward, eyes always move to the right. We know it is not due to the head being tilted, because in that case, the eyes would not always go the same direction.

Rarely head-shaking nystagmus is tested in LMS and may show unusual patterns. Below is head-shaking causing a reversal in the spontaneous.

Right lateral medullary syndrome
HSN in wallenberg
Reversal of spontaneous nystagmus from LB to RB on head-shaking in Wallenberg syndrome.

Vestibular/auditory tests:

While one would expect that VEMP testing would be abnormal in LMS/Wallenbergs, so far, little has been written on this. Presumably it also would depend on whether descending (vestibulospinal) connections are interrupted. Tseng and Young (2010) reported on VEMP testing in 5 patients, and reported "MRI demonstrated infarction at the ponto-medullary junction in one patient and upper medulla in one patient. Both patients revealed caloric areflexia and normal VEMPs. In contrast, another three patients with infarction at the middle inferior olive level, all displayed abnormal (including absent or delayed) VEMPs, and one patient showed caloric areflexia. Topographical correlations of lateral medullary infarction with caloric and VEMP tests reveal that caloric areflexia is possibly linked with rostrally located infarction, while absent or delayed VEMPs relate to caudally located infarction. "

As of 2022, there was nothing written about rotatory chair testing or VHIT testing in Wallenbergs/LMS.

Caloric tests can be positive -- but this is not universal. (Butler et al, 2006). Presumably HIT and VHIT tests are also sometimes positive.

Many clinical papers observe gaze evoked nystagmus in patients with LMS/Wallenberg. (e.g. Benoit et al, 1987)

ABR testing is not often abnormal in persons with LMS/Wallenbergs. While ABRs are often abnormal in the central Horner's syndrome (Faught and Oh, 1985),as the lesion in Wallenberg syndrome is usually below the auditory connections, Horner's due to Wallenberg's are not generally associated with abnormal ABR.

Case of Wallenberg's syndrome:

A man in his 50's developed dizziness after a brainstem stroke, experienced 2 months ago. The stroke was due to a cardiac arrhythmia, and at one point the vertebral artery on the right was blocked. When he was seen in the office, he was in a wheelchair, and his speech was hoarse. His right pupil was smaller but both reacted. There was no palatal myoclonus. Cerebellar examination was normal in the limbs.

There was a strong left-beating gaze-evoked nystagmus, and also there was a right-torsional beating torsional nystagmus. Positional testing was abnormal due to the ongoing nystagmus, but there was also a brief upbeating nystagmus. Headshaking provoked a very strong reversal of nystagmus. Vestibular testing elicited normal VHIT, and rotatory chair testing had a normal gain/time-constant, but there was strong spontaneous nystagmus.

On seeing him about 5 months later, he was walking without aid. He had a stronger right-torsional nystgmus, that became downbeating on lateral gaze, and became much larger after gaze-holding to the side and returning to the center (i.e. a rebound effect). There was still no palatal myoclonus. Cerebellar examination continued to be nearly normal, although a little better on the left (his nondominant side). The pupils remained smaller on the right.

Comment: This is a nearly classic LMN (Wallenberg) syndrome from the general features, with a persistent torsional nystagmus, beating in a paradoxical (RT) direction. According to Morrow and Sharpe (1988) and the logic that this should be a paretic nystagmus from damage to the SVN, a right sided infarct should have resulted in a torsional nystagmus with the top part of the eye beating to the left (i.e. left torsional). Nevertheless, we have encountered this in other well documented cases, and perhaps it is not all that rare.

He also has a paradoxical direction of head-shaking nystagmus. There are strong influences of both gaze and gravity, but relatively little limb dysmetria.



We thank Dr. Dusan Pavlocic for helping correct a reference on this page.