09 The hemispheres: lobes, proportions and the visual pathway

In the cord and the brainstem you localize by wiring, because tracts cross at known levels and nuclei sit beside them. The hemispheres call for different reasoning. Much of what we know about where functions live there was first learned from strokes and other lesions, true "experiments of Nature" (Brazis 2011, p. 493). At the bedside you read the map by noticing which abilities have gone missing.

Signs that point to the cortex

A cord or brainstem lesion can weaken a hand, but only a hemisphere lesion can take away the ability to name that hand, to notice it, or to recognize a coin placed in it. Cortical signs include these (Pearl 2014, p. 108).

Seizures and gaze preference also belong here. A seizure arises in the cortex, and a focal seizure is often the most precise localizing sign a child gives. A gaze preference comes from the frontal eye fields, each of which drives the eyes toward the opposite side. A destructive lesion lets the eyes drift toward the lesion, and a seizure drives them away (Pearl 2014, pp. 9-10).

Hemiparesis and its neighboring signs

When you find a hemiparesis, look for aphasia, hemisensory loss, homonymous hemianopia, unawareness of the deficit and a history of focal seizures (Brazis 2011, p. 11). Any one of them places the lesion above the brainstem, usually in the cortex or the white matter beneath it. A pure motor hemiparesis with none of them points deeper, to the internal capsule or the brainstem (Brazis 2011, p. 12).

In a child

Young children cannot report most of these signs, so watch for them. In a toddler neglect looks like ignoring the toys on one side, apraxia like failing to copy a simple gesture, and aphasia like losing words that were there last month.

The motor strip lies like a figure upside down over the surface of the hemisphere. The face sits low on the lateral convexity, with the arm and hand above it, and the leg folds over the top onto the medial surface between the two hemispheres. The blood supply matches, with the middle cerebral artery feeding the lateral convexity, which holds the face, arm and tongue, and the anterior cerebral artery feeding the medial surface, which holds the leg (Pearl 2014, p. 7).

Comparing face, arm and leg

The proportions of a weakness therefore tell you where the lesion is. Grade the face, arm and leg separately and compare them with the table (Pearl 2014, pp. 7, 9-10, 108-109; Brazis 2011, p. 11).

In the cortex the fibers for face, arm and leg spread over a large surface, so a lesion usually catches one part more than another. In the internal capsule the same fibers are bundled into a small space, so one small lesion can take all of them equally (Pearl 2014, p. 7).

The face tells you the level

Upper motor neuron facial weakness usually spares the forehead, because the upper face receives cortical input from both sides. When a lower facial droop is on the side of a hemiparesis, the lesion is generally above the upper pons, most often in the cortex, the corona radiata or the internal capsule (Brazis 2011, p. 11). If the lower face is spared and the tongue deviates toward the strong side, consider a functional weakness (Brazis 2011, p. 12).

Tumors, seizures and children

Tumors do not respect vascular territories, so their deficits are less tidy and build up over time (Pearl 2014, p. 10). Most childhood hemispheric lesions are not strokes, so check the proportions against the rest of the story. A focal motor seizure can show the same map in motion. Twitching that spreads from the face to the hand and then to the leg is following the order of the motor strip (Pearl 2014, p. 10).

The visual pathway localizes better than any other system in the hemispheres. It runs from the front of the brain to the back, and damage to each segment leaves its own pattern in the visual field, and three anatomical facts explain most of those patterns.

Three anatomical facts

Defects by site

Try each site in the figure above and watch which quadrants go dark.

Occipital lesions

Near the calcarine cortex the fibers from the two eyes gather into tight bundles, so a lesion there affects both eyes alike, a congruous defect. The macular area also has a dual blood supply, from the posterior and middle cerebral arteries, so a posterior cerebral infarct often spares central vision. The more congruous and the more macula-sparing the defect, the more confidently you can place the lesion in the occipital lobe (Pearl 2014, pp. 27-28).

Testing a child

Test each eye separately, and make sure the child's eyes stay on your face. Counting fingers in each quadrant is the best bedside method. Wiggling fingers is easier for children but less sensitive (Pearl 2014, p. 26). In younger children a small toy brought in from the side, or a blink to a threatening hand, can stand in. Presenting stimuli on both sides at once looks for extinction, a sign of neglect (Pearl 2014, p. 26). Children rarely report a field defect themselves. The first hint is often bumping into door frames on one side, or missing the start of each line when reading.

In the nineteenth century both William Little and Sigmund Freud wrote on the origins of cerebral palsy (George 1992). Cerebral palsy is a lasting disorder of movement and posture caused by injury to the developing brain, before or around birth or in early infancy. Two patterns of early injury are common enough that every child neurologist should recognize them from the examination alone. In both, the shape of the deficit reflects where the injury fell.

Early hand preference and perinatal stroke

A stroke around the time of birth often goes unnoticed in the newborn period, and the first sign may be a baby who reaches with only one hand. Hand preference before the first birthday is abnormal and points to weakness of the other hand, because true handedness develops only after twelve months (Utah PediNeuroLogic Exam). The child later shows a hemiplegia, with the arm and hand more affected than the leg, a fisted hand, and a leg that circumducts when walking (DeMyer, p. 340). Those are the proportions of the lateral cortex and the middle cerebral territory.

Spastic diplegia after premature birth

The premature brain is vulnerable in a different place. Its characteristic injury is periventricular leukomalacia, damage to the white matter around the lateral ventricles, often with injury to neurons and axons as well (Volpe 2009). The fibers for the legs pass closest to the ventricles on their way down from the medial cortex, so they are caught first. The result is spastic diplegia, with severe spasticity in the legs, little in the arms, and little or no trouble with speech or swallowing. Double hemiplegia is the reverse, with the arms weaker than the legs and with pseudobulbar palsy (DeMyer, p. 340). Children with spastic diplegia walk on their toes, scissor their legs or walk in a crouch.

Deep gray matter injury

Injury to the basal ganglia and thalamus around birth, classically from hypoxia, produces a third pattern, in which slow writhing movements are mixed with spasticity (DeMyer, p. 340). Its anatomy is covered with the loops in Module 8.

At the bedside

When an infant shows an early hand preference, examine the neglected hand for tone, reflexes and fisting, and ask about the pregnancy and delivery. When a child born early walks on the toes with stiff legs and good hands, the history and the proportions point to the periventricular white matter before any imaging does.