04 The motor unit: horn cell, nerve, junction, muscle

Once the first fork has pointed to the motor unit, resist the temptation to write "neuromuscular" in the notes and hand the child over to the electromyographer. A lower motor neuron lesion can lie anywhere from the motor neuron through the roots, plexus and nerves to the junction and the muscle (Brazis 2011, p. 12). Module 5 takes the roots and plexus. This module takes the four addresses of the motor unit itself, the cell body in the anterior horn, the axon in the nerve, the junction where axon meets muscle, and the muscle. Each fails in a way that leaves its own fingerprint at the bedside.

The four sites compared

Two of the rows do most of the sorting. Sensation separates the nerve from the other three addresses, because the horn cell, the junction and the muscle are purely motor. Reflexes separate the horn cell and the nerve, which lose them early, from the junction and the muscle, which keep them until late.

Using the figure

The figure above is the localizer loaded with motor-unit cases. For the boy who climbs up his legs, horn cell and muscle stay tied, which tells you the next finding to look for is a fasciculating tongue. The child with high arches and numb toes leaves only the nerve standing, and the teenager whose lids droop by evening leaves the junction on top.

In a child

In an infant these signs are harder to elicit and the complaint is often hypotonia, but the same four addresses apply. Module 3 shows how to decide first whether a floppy infant's problem is central at all. Commit to an address before the electrodiagnostic study, because the test is far more useful when it has been asked a specific question.

In most myopathies the girdle muscles, which hold the hips and shoulders against gravity, fail first. The typical result is proximal, symmetric weakness without sensory loss. This is the child who cannot get up off the floor, climb the stairs or lift a heavy book onto a high shelf, but whose hands and feet still work well.

The classic example is a boy with Duchenne muscular dystrophy (Brazis 2011, p. 13). He has enlarged calves, and to stand he uses the Gowers maneuver, pushing up on his own thighs because his hip extensors are weak. He also has an exaggerated lumbar curve, a waddling, wide-based gait, and a habit of walking on his toes. Most of these signs follow from proximal weakness. The waddle comes from weak hip abductors that cannot hold the pelvis level. The lumbar curve comes from leaning the trunk back to keep the center of gravity behind the weak hips.

Patterns that fit no root or nerve

Roots and nerves supply muscles in fixed groups, so a root or nerve lesion weakens a predictable set of them. Muscle disease ignores those maps. In a dystrophy one muscle can be wasted while its neighbor, which shares the same root, is spared, and the same selective pattern appears in spinal muscular atrophy (Morris 2012, p. 17). When a pattern of weakness makes no sense in terms of roots or nerves, the muscle and the horn cell are the addresses to consider.

Reflexes and sensation

Sensation stays normal in muscle disease, and the reflexes are kept until the muscle can no longer contract well enough to produce one. A child who has lost the reflexes while the muscles are still fairly strong points away from the muscle and toward the horn cell or nerve.

At the bedside

Before any formal testing, watch the child rise from the floor. It is the most efficient test of hip strength there is, and a child will happily do it as part of a game. Look at the calves, at the shoulder blades for winging, and at the face. The Gowers maneuver tells you the hips are weak without naming a disease, since spinal muscular atrophy and inflammatory myopathy produce it as well. The individual diseases are covered in the site's Neuromuscular section.

A nerve cell body has to supply its entire axon with everything it needs, and the longest axons in the body are the ones that run to the feet. When a neuron is poisoned, starved or genetically weakened, it cannot keep the far end of a long axon going, and the axon dies back from its tip. In this dying-back process the largest neurons with the longest axons are affected first (Brazis 2011, p. 5). That idea accounts for the shape of most polyneuropathies.

Length dependence

Symptoms begin in the toes and climb from there. In a motor neuropathy the small foot muscles usually fail first, then the muscles supplied by the peroneal nerve, and then the calf. The intrinsic hand muscles become involved once everything below the knee is affected (Brazis 2011, p. 26). In the same way, symptoms in the hands are not usually seen until the legs are affected up to the knees (Pearl 2014, p. 76). The result is the glove-and-stocking pattern of sensory loss, and the ankle reflexes are lost first because their reflex arc is the longest.

Heel walking and toe walking

Because the nerve to the anterior compartment is longer, a neuropathy weakens dorsiflexion before plantar flexion. A patient who can walk on the heels but not on the toes probably has an intraspinal problem, such as spinal muscular atrophy or a conus or cauda equina lesion, rather than a polyneuropathy (Brazis 2011, p. 26).

Neuropathy in children

In a child, a slowly progressive, symmetric neuropathy that has been present since early life suggests an inherited cause. The most familiar is Charcot-Marie-Tooth disease, with its high arches, thin lower legs, absent ankle reflexes and steppage gait. Early wasting of the first dorsal interosseous in the hand and of the short toe extensor on the foot has localizing value for a neuropathy (Brazis 2011, p. 11). An acute, ascending, areflexic weakness that develops over days is Guillain-Barré syndrome, which damages the roots as well as the nerves, so the localizer keeps both alive.

The anterior horn cell lives inside the spinal cord, yet a child whose anterior horn cells are dying looks weak, floppy and areflexic, just like a patient with peripheral disease. We localize to the function that is lost, and here the lost function is the lower motor neuron, which happens to be housed in the cord.

Signs of anterior horn disease

Anterior horn disease combines weakness with wasting and fasciculations, lost reflexes and no sensory loss at all. Fasciculations are visible twitches of small groups of muscle fibers, and they may accompany lower motor neuron disease (Brazis 2011, p. 11). They arise when sick motor units fire on their own. In a weak infant, look at the tongue at rest inside the mouth, where fine, writhing fasciculations are the classic sign of spinal muscular atrophy. The tongue has to be watched at rest, because a protruded tongue often quivers in healthy children too.

Spinal muscular atrophy and acute flaccid myelitis

Spinal muscular atrophy is a genetic loss of anterior horn cells. The infant is alert, socially bright and floppy, with a bell-shaped chest from weak intercostal muscles, absent reflexes and tongue fasciculations. Early recognition matters because disease-modifying treatments now exist, and the site's Neuromuscular section covers them.

Acute flaccid myelitis is an acquired injury to the gray matter of the cord, often after a viral illness. The weakness comes on over hours to days and is often asymmetric. It may involve a single limb, with absent reflexes in the weak limb and little or no sensory loss. This is a lower motor neuron fingerprint with its address in the cord, which is why the work-up is an MRI of the spine, as Module 6 explains.

Fasciculations elsewhere

Brief, isolated fasciculations in a healthy person are common and usually harmless. Fasciculations also appear with chronic damage to a root or nerve, which is why the localizer keeps root and nerve alive when you select fasciculations. What clinches the horn cell is finding fasciculations, wasting, areflexia and normal sensation together in the same child.