08 The loops: cerebellum, basal ganglia and gait

Neither the cerebellum nor the basal ganglia sends fibers to the anterior horn cells, so neither moves anything on its own. Both work in loops that take in what the cortex intends and what the body reports and send their output back through the thalamus to the motor cortex that will act on it. The cerebellum is "the great modulator of the nervous system" (Pearl 2014, p. 116), and the phrase fits the basal ganglia too. The cerebellar loop tunes the accuracy and timing of movement. The basal ganglia loop sets how much movement is allowed and how fast it runs.

Test strength before coordination

Because these loops shape movement without powering it, their disease leaves strength intact. A child with cerebellar disease is clumsy, overshoots and walks on a broad base, yet pushes against your hand normally. A child with basal ganglia disease has too little movement or too much of the wrong kind, but no paralysis. Prove that a limb is strong before you call its movement ataxic or dystonic, because a weak arm also misses the target and a weak child also falls. That is why the first fork comes before any question about coordination.

Cerebellar and basal ganglia signs

Tremor is a practical way to tell the two apart. A cerebellar tremor appears on action and worsens near the target, and a tremor at rest belongs to the basal ganglia.

In a child

Clumsiness is one of the commonest reasons a child is referred. Most clumsy children have no cerebellar or basal ganglia lesion, and many have developmental coordination disorder. The first question is whether the clumsiness is simply normal for the child's age. The toddler's broad-based, jerky, falling gait is normal at the age when walking begins (DeMyer, p. 338).

A child who could walk yesterday and staggers today is one of the commonest urgent problems in child neurology. The level is usually the cerebellum or its connections. The acute tempo narrows the mechanisms to a short list, which you work through in order of danger and of likelihood.

The common causes

The first examination

Start with the level of consciousness, and ask what medicines and substances are in the house. Look at the eyes for nystagmus and opsoclonus, and at the optic discs for papilledema. Check reflexes, position sense and strength to make sure the ataxia is truly cerebellar. A child who is drowsy or getting worse, or who has papilledema, focal signs or a severe headache, needs urgent imaging. An alert child with isolated truncal ataxia after a viral illness and an otherwise normal exam can often be watched closely.

Post-infectious ataxia is common enough that it is tempting to stop as soon as it seems to fit. Ask each of these questions anyway, because that is how the ingestion, the tumor and the neuroblastoma are found.

The cerebellum governs how accurately a movement is made. The basal ganglia govern how much movement occurs and which movement is chosen. Their disorders fall along a single axis, from hypokinetic disorders, with too little movement, to hyperkinetic disorders, with movements that should not be there (Pearl 2014, p. 116).

Hypokinetic and hyperkinetic disorders

Describing the movement

There is no weakness to measure and often no change in the reflexes, so the movement itself has to show where the lesion is. Chorea, dystonia and a tremor at rest place the lesion in the subcortex and deep gray matter, the level the localizer assigns them to. Describe the movement precisely before you give it a name. Note what part moves, whether it is brief or sustained, random or patterned, at rest or on action, and whether it can be suppressed.

Sydenham chorea

The classic childhood example is Sydenham chorea, a post-streptococcal disorder that is a hallmark of acute rheumatic fever (Pearl 2014, p. 116). A school-aged child becomes fidgety and clumsy, drops things, cannot keep the tongue protruded and grips with a squeeze that waxes and wanes (the milkmaid grip), and behavior and mood often change too. The localization is the basal ganglia, and the diagnosis obliges you to look for the rest of rheumatic fever, including an echocardiogram, because carditis can be silent.

Other childhood patterns

Several other childhood patterns point to the same level but to very different mechanisms. They include dystonia that worsens through the day, as in dopa-responsive dystonia, dyskinetic cerebral palsy after injury to the deep gray matter around birth, and movement disorders that arrive with regression. They share a level, so the tempo of the illness has to separate them.

If you could watch a child do only one thing, choose walking. Gait is an excellent screen of the whole motor system (Pearl 2014, p. 114). Lesions at each level of the neuraxis impair it in a characteristic way, so a gait suggests both where the lesion is and what it probably is (DeMyer, p. 338). Working from muscle up to cortex (Pearl 2014, pp. 114-117) turns gait into a tour of the levels.

Gait at each level

Pick a gait in the figure above to see where it points.

The developing gait

The newborn steps reflexly when held upright, and this gives way to cruising along the furniture. Around the first birthday the toddler walks freely on a broad base, with short, jerky steps and frequent falls. The mature heel-toe stride with swinging arms comes later (DeMyer, p. 338). A broad-based, falling gait is normal at thirteen months and abnormal at five years.

Toe walking

Toe walking does not localize on its own. It can be the first sign of Duchenne dystrophy or of spastic diplegia, and it is seen in autism. In a child who runs, skips and hops normally, it may be an idiosyncratic, sometimes familial pattern with no disease at all (DeMyer, p. 338). The rest of the examination decides which of these it is.

Examining gait

Ask the child to walk, to run, to walk on the heels and on the toes, and to walk heel to toe. Heel and toe walking test the muscles of the sciatic territory as well as balance (DeMyer, p. 338). Young children cooperate better when the examination becomes a race or a game.