07 The brainstem: crossed signs and the rule of four

Most lesions in the nervous system keep their effects on one side of the body. A stroke in the internal capsule weakens the opposite face, arm and leg together, and a spinal cord lesion leaves the face alone entirely. The brainstem is different. Its cranial nerve nuclei sit beside long tracts that have not yet finished their journey. The corticospinal tract crosses only at the bottom of the medulla, so through most of the brainstem it still serves the opposite body. A single small lesion can therefore catch a cranial nerve on its own side and the limbs on the other, a pattern called an alternating hemiplegia. Crossed signs, with a cranial nerve affected on one side and the body on the other, point to the brainstem (Brazis 2011, p. 12).

Side and level

The cranial nerve finding gives you the side, because a cranial nerve nucleus or fascicle serves its own side of the head. Its number gives you the level, because the nuclei are stacked in order, with the third at the midbrain, the sixth and seventh in the pons and the twelfth in the medulla. The limb signs tell you how far toward the front or the middle the lesion reaches. Facial sensation follows the same logic. A brainstem lesion numbs pinprick on the face of its own side, because the descending trigeminal tract has not yet crossed, and it often numbs the body on the other side as well (Pearl 2014, p. 47).

At the bedside

When a weak arm and leg arrive with a cranial nerve finding, first settle which side the cranial nerve is on. If the face and limbs are weak on the same side, the lesion lies above the facial nucleus, usually in the hemisphere, though the cerebral peduncle or upper pons can do the same (Brazis 2011, p. 11). If they are on opposite sides, the lesion is in the brainstem, and the cranial nerve gives the level. A crossed pattern excludes every other level, provided the findings began together, and the first module explains why exclusion is the most useful thing a finding can do.

The reasoning is the same in children, but tumors, demyelination and inflammation are more common causes than the vascular syndromes that fill adult textbooks.

The brainstem can look like a maze of nuclei and tracts with nineteenth-century names. The rule of four reduces it to a few groups of four (Gates 2005; Gates 2011).

Four medial structures, all beginning with M

Four lateral structures, all beginning with S

Four cranial nerves at each level

The medulla holds 9 to 12 and the pons 5 to 8. Of the four above the pons, 3 and 4 sit in the midbrain. The motor nuclei whose numbers do not divide evenly into 12 (5, 7, 9 and 11) lie laterally, and so do the eighth nerve in the pons and the tenth in the medulla. A medial lesion names its level by 3, 6 or 12, and a lateral lesion by 5, 7 and 8 in the pons or 9 and 10 in the medulla. The figure above lays the rule out as a map. Tap a box to see the findings it produces, or switch to Find the lesion and work backward from a set of findings.

Exceptions to the rule

Real lesions, especially tumors in children, ignore a tidy line between medial and lateral, so use the rule to decide what to look for.

A name records who described a patient and says nothing about where the lesion is. Each classic brainstem syndrome is also a worked example of the rule of four, though. Read each one as a lesion with a level and a side, as in the table below (Pearl 2014, pp. 109-112; Brazis 2011, pp. 393, 397).

Weber and Benedikt syndromes

Weber syndrome is the medial midbrain in its simplest form, with the third-nerve fascicle and the corticospinal tract lying side by side in the peduncle. The third-nerve palsy includes the pupil, because the parasympathetic fibers travel with the nerve (Brazis 2011, p. 397). A lesion further back, in the tegmentum, also takes in the red nucleus. The cerebellar outflow passes through the red nucleus on its way to the thalamus, so the opposite limbs shake as well (Pearl 2014, p. 110).

Lateral medullary syndrome

The lateral medullary syndrome contains every one of the S structures. The spinal trigeminal tract accounts for numbness of the face on the same side, and the syndrome adds ataxia and a Horner syndrome on that side, with numbness of the body on the opposite side. The nucleus ambiguus brings hoarseness and a poor swallow, and the vestibular nuclei bring vertigo, nausea and vomiting. There is no weakness, because the corticospinal tract runs medially and a purely lateral lesion misses it (Pearl 2014, p. 111).

In children

Classic vascular syndromes are uncommon in childhood. A child with a pontine tumor or a demyelinating plaque more often shows part of a row in the table than the whole of it. Knowing the full pattern lets you recognize the part and predict which neighboring signs to look for next.

The circuits that move the eyes run from the frontal eye fields down the whole length of the brainstem to the vestibular nuclei at the pontomedullary junction. Eye movements are therefore one of the most sensitive tests of the brainstem you have. Three eye findings are worth knowing well enough to recognize at a glance.

Internuclear ophthalmoplegia

The medial longitudinal fasciculus, one of the four medial structures, links the sixth nucleus on one side to the third nucleus on the other so that both eyes turn together. When it is cut and the patient looks away from the lesion, the eye on the side of the lesion fails to adduct while the abducting eye jerks with nystagmus (Pearl 2014, p. 45). The sign localizes precisely, because the eye that fails to adduct is on the side of the lesion.

Gaze deviation

A destructive lesion of one frontal eye field leaves the other unopposed, so the eyes deviate toward a hemispheric lesion and away from the hemiparesis. A pontine lesion destroys the horizontal gaze center beside the sixth nucleus, so the eyes deviate away from the lesion and toward the hemiparesis (Pearl 2014, pp. 43, 148). A seizure reverses the hemispheric rule, because it excites the eye field and drives the eyes away from the focus (Pearl 2014, p. 9). The doll's-eye maneuver separates a hemispheric cause from a pontine one. It can overcome a cortical gaze preference but cannot overcome a pontine gaze palsy, because the gaze center itself is gone (Pearl 2014, p. 45).

Central or peripheral nystagmus

A few bedside rules separate the two (Pearl 2014, pp. 120-121).

Antiseizure drugs can also produce a nystagmus that looks central (Pearl 2014, p. 121), which matters in children being treated for epilepsy.

In a child

Children rarely complain of double vision. They close one eye, tilt the head or turn the face instead, so watch the head posture before you test the eyes, and test pursuit with a toy rather than a finger.

Brainstem localization in adults is taught through strokes. In children it is taught through tumors, malformations and inflammation, which grow, infiltrate and compress without respecting any arterial territory.

The posterior fossa

Primary posterior fossa tumors are common in children, and two of the commonest are medulloblastoma and the cystic cerebellar astrocytoma (Pearl 2014, p. 63). Medulloblastoma accounts for about 20% of pediatric brain tumors. It tends to begin in the caudal vermis and gives a staggering gait and truncal imbalance with little limb ataxia at first (Brazis 2011, p. 412). As it grows it blocks the fourth ventricle, and the picture becomes one of raised intracranial pressure, with morning headache, vomiting and papilledema.

A new sixth-nerve palsy

The sixth nerve takes a long course across the base of the skull, which leaves it vulnerable to raised pressure anywhere. An abducens palsy can therefore be an early sign of a tumor, hydrocephalus or idiopathic intracranial hypertension rather than of a lesion in the nerve (Pearl 2014, p. 43). This is the classic false localizing sign. To decide whether to believe it, ask what keeps the palsy company.

Infiltrating pontine tumors and the Chiari malformation

A tumor that infiltrates the pons does not stay medial or lateral. It gathers cranial nerves and long tracts as it grows. In a Chiari malformation the cerebellar tonsils descend through the foramen magnum and crowd the lower brainstem, and a syrinx may form in the cord below. Look for lower cranial nerve signs and for the cape of lost pain sensation described in the spinal cord module.

What to examine

When a child presents with any brainstem sign, examine the optic discs, every cranial nerve including the palate and tongue, the gait and tandem walk, and the reflexes and plantar responses on both sides, which together set the urgency of the scan.