10 When the lesion is everywhere: coma and diffuse disease

Every other module in this curriculum asks the patient to do something, such as squeezing your fingers, following a target, walking across the room or reporting a pinprick. A child in coma can do none of these things, so you have to localize from the reflexes and from the level of consciousness itself.

The brainstem and both hemispheres

Wakefulness depends on the ascending reticular activating system in the brainstem working together with the cerebral hemispheres. In people the activating system lies in the paramedian tegmentum of the pons and midbrain (Brazis 2011, p. 604). Coma therefore has only two true addresses in the brain. One is a lesion in that small tegmental core, and the other is dysfunction of both hemispheres at once.

A lesion confined to one hemisphere does not abolish consciousness on its own. A child with a large one-sided stroke or tumor may be aphasic, hemiplegic and miserable, and still be awake. If that child becomes unresponsive, something has changed. The lesion may have grown or swollen enough to press on the brainstem, a second process may have appeared on the other side, or the child may be having seizures without convulsions. Unresponsiveness in a child with a one-sided lesion is a new localizing sign, and usually an emergency.

Describing responsiveness

Words such as lethargic, obtunded and stuporous sound precise, but they carry little of the information needed for localization. Describe responsiveness in plain words instead, recording the stimulus you used and exactly how the child responded (Brazis 2011, p. 603). The next examiner can compare their own findings with a note that reads "Opened her eyes to her name, looked at her mother, then drifted off without speaking." A note that reads "lethargic" gives them nothing to compare. In a child the useful stimuli include a parent's voice, a favorite toy or a feed.

The rest of this module applies the same habit to the brainstem reflexes, which keep working when the child cannot cooperate. It then turns to the diffuse diseases, where "everywhere" is a genuine answer.

The four signs that need no effort from the child are the pattern of breathing, the pupils, the eye movements and the motor response to pain. Each changes in a characteristic way from one level to the next (Pearl 2014, pp. 94-97). Set each sign in the figure above to see whether they agree.

Breathing. Breathing that waxes and wanes with pauses in between (Cheyne-Stokes) points to both hemispheres or the diencephalon. Sustained, fast, deep breathing, once acidosis and lung disease are excluded, points to the brainstem from the low midbrain to the middle of the pons. Long pauses at full inspiration (apneustic breathing) point to the pons. Chaotic, irregular breathing points to the medulla, where the respiratory rhythm is generated (Pearl 2014, pp. 94-95).

Pupils. Small pupils that still react fit diencephalic or metabolic depression (Brazis 2011, p. 608). A midbrain lesion interrupts the light reflex, so the pupils lose their reactivity. A pontine lesion cuts the descending sympathetic fibers and leaves the pupils pinpoint (Pearl 2014, p. 95), although they still react to light if you look closely (Brazis 2011, p. 608).

Eye movements. When you turn the head of a comatose child whose brainstem is intact, the eyes move the opposite way and keep pointing in the same direction in space, the doll's-eye movement (Pearl 2014, p. 96). Do not turn the head if the neck may be injured. Cold water in the ear is a stronger stimulus, and if both eyes turn toward the cold ear the brainstem from the upper medulla to the midbrain is working. In coma there is no corrective fast phase, so the eyes deviate slowly without nystagmus (Pearl 2014, p. 97).

Motor response. Flexion of the arms with extended legs places the lesion between the cortex and the upper midbrain. Extension of all four limbs places it between the midbrain and the pontomedullary junction, and flaccidity places it below that (Pearl 2014, p. 97).

Putting it together. If all four signs point to the same level, a single structural lesion there fits. If they disagree, the process is probably multifocal, and multifocal coma is most often metabolic. If they move downward over hours, think of herniation (Pearl 2014, p. 97). Children are scored with modified versions of the adult coma scales (Pearl 2014, p. 147), and the logic of finding the level is the same at every age.

At the bedside of a comatose child, the most useful early decision concerns the kind of problem more than its level. Either a structural lesion is damaging or compressing the brain, or the brain is being poisoned or starved. The two are managed differently, and a poisoned or starved brain can sometimes be helped within minutes.

Pupils and eye movements

In metabolic coma the pupils may still react when the eye movements are gone (Brazis 2011, p. 610). The pupillary light reflex is one of the most resistant reflexes in the brainstem, so a drug or a metabolic derangement can silence the eye movements while the small pupils still constrict to light. A structural brainstem lesion severe enough to abolish the eye movements usually changes the pupils as well, leaving them fixed in mid-position or dilated after midbrain damage, or pinpoint after pontine damage (Pearl 2014, p. 95). Small, briskly reactive pupils with absent eye movements point toward a toxic or metabolic cause.

Drugs and the pupils

Drugs can do almost anything to the pupils (Pearl 2014, p. 96). Anticholinergics and stimulants dilate them, and barbiturates, benzodiazepines and other sedatives constrict them. Opioids and clonidine can make them pinpoint, much like a pontine lesion. Most sedative overdoses do not abolish the light reflex (Brazis 2011, p. 608). A toddler who has found a grandparent's tablets can produce a coma examination that mimics almost any level.

Signs of a structural cause

Asymmetry favors a structural cause, whether it shows as unequal pupils, a gaze deviation, a hemiparesis or posturing on one side. So does a pattern that descends over hours. Symmetry, fluctuation and signs that disagree about the level point toward a toxic or metabolic cause (Pearl 2014, p. 97).

In a child

The metabolic list in children is long and urgent. It includes low glucose, ingestion, high ammonia, diabetic ketoacidosis, infection and non-convulsive seizures. Only an EEG can confirm non-convulsive seizures, so request one early (the EEG curriculum covers how to read it). Check the glucose at once, because low glucose can itself cause focal signs. Look for a chemical cause while you examine, and image urgently whenever the examination is asymmetric or worsening.

A growing mass in a closed skull pushes brain from one compartment into another. The signs then come from the place being squeezed, so the localization itself moves while you watch.

Uncal herniation and the pupil

An expanding mass in or beside one temporal lobe, such as a hematoma, can push the uncus over the edge of the tentorium and onto the third nerve and the midbrain (Pearl 2014, pp. 37, 93). The pupil-constricting fibers run on the outside of the third nerve, so external pressure dilates the pupil early, on the same side as the mass, before a progressive third-nerve palsy and falling consciousness.

The side of the hemiparesis is a less reliable guide. You would expect the mass to weaken the opposite side, because it presses on the corticospinal fibers on its own side. But the shifted midbrain can be pushed against the far edge of the tentorium, which notches the other cerebral peduncle (Kernohan's notch) and weakens the limbs on the same side as the mass. In uncal herniation the dilated pupil reliably shows the side of the lesion, and the hemiparesis may not (Pearl 2014, p. 37).

Other forms of herniation

The cingulate gyrus can slide under the falx and compress the anterior cerebral artery, which weakens the opposite leg. Swelling of both hemispheres pushes the diencephalon straight down and causes progressive dysfunction of both sides of the brainstem from the top. Cerebellar tonsils pushed through the foramen magnum compress the medulla, and breathing and circulation can then fail abruptly (Pearl 2014, pp. 92-93). A slowing pulse with a widening pulse pressure and a change in breathing is a late sign of raised pressure (Pearl 2014, p. 94), so do not wait for it.

Following the coma signs

The coma signs from section 2 are the best way to watch for herniation. Breathing may change from Cheyne-Stokes to sustained hyperventilation, the pupils from small and reactive to fixed, and posturing from flexion to extension. Treat a level that descends over hours as herniation unless something else clearly explains it (Pearl 2014, p. 97). To see the sequence, press Watch a herniation descend in the coma figure in section 2. Posterior fossa tumors are common in children, so downward herniation of the cerebellar tonsils is a particular concern, and a lumbar puncture in a child with a mass and signs of raised pressure can set it off.

Localization trains you to look for the single point where the findings meet, and sometimes there is no such point. The answer is then "everywhere", which narrows the differential as sharply as any single level on the map of levels.

Discrete lesions and system disease

Much of localization was learned from discrete lesions such as infarcts, tumors, abscesses and demyelinating plaques. Other diseases attack whole arrays of neurons that share a function (Brazis 2011, p. 4). Discrete lesions give signs that meet at one point. System diseases give signs that follow a system, a tract or a cell type instead of a place.

Diffuse, multifocal and system disease

Regression: localization in time

In a child, "everywhere" often shows itself as a loss of skills rather than a coma. Regression places the process in time as clearly as a sensory level places it in space, because it shows that the disease is active now. It also moves the differential toward neurodegenerative, genetic and metabolic causes, the epileptic encephalopathies and acquired encephalitis.

Once you have named the pattern as diffuse, multifocal or system, use the tempo of the illness to choose among the causes, with the grid that Module 11 builds.