06 The spinal cord: tracts, levels and syndromes

Of all the structures in the nervous system, the spinal cord is the most obliging. It is a tube of long tracts wrapped around a column of gray matter, built the same way at every level. Three of those tracts are enough to predict almost every cord syndrome you will meet. Once you know where each of them crosses, working out a cord syndrome comes close to arithmetic.

Draw all three on a single cross-section and the syndromes can be read straight off the drawing (Fisch 2012, pp. 116-117).

Putting the tracts together

Within the cord, a one-sided lesion produces weakness and loss of position sense on the same side, below the lesion, and loss of pain and temperature on the opposite side, starting a little lower down. The pain and temperature loss usually starts one or two segments below the lesion (Brazis 2011, p. 106), and the simulator in this module draws it two segments down.

A pattern that splits the modalities between the two sides, with weakness on one side and pain loss on the other, comes close to identifying the cord on its own. Nowhere else in the nervous system do the motor and pain pathways run on opposite sides of a single small structure.

In a child

The tracts are the same at every age, but the corticospinal tract is still myelinating in infancy, which is why an upgoing toe is normal in the first year (Utah PediNeuroLogic Exam; see Module 2). In a toddler a sensory level is hard to map by questioning, so the motor signs, the reflexes and the bladder carry more of the weight.

Choose a level and a lesion pattern in the simulator above. The body map is computed from the three tracts, the anterior horn and the entering roots, so you can check each syndrome below for yourself.

Complete transverse lesion. The child is weak on both sides below the lesion, with lower motor neuron signs at the level. All sensation below the level is lost, and so is bladder and bowel control. In a slowly developing myelopathy, urgency is the usual bladder symptom and retention comes later. After an acute lesion the bladder is atonic at first, so retention can come early (Brazis 2011, p. 106).

Hemisection (Brown-Séquard). Weakness and loss of position sense appear below the lesion on its own side, and loss of pain and temperature appears on the opposite side, starting one or two segments down (Brazis 2011, p. 106). Lesions outside the cord, including stab wounds, are the usual causes (Brazis 2011, p. 106).

Anterior cord. The anterior spinal artery supplies the front of the cord. An infarct takes the corticospinal and spinothalamic tracts and the anterior horns and spares the dorsal columns. The child cannot move the legs or feel a pin, yet still feels the tuning fork.

Central cord (syrinx). A cavity in the center of the cord first cuts the pain fibers that cross in front of it. Pain and temperature are lost over just the segments involved, often across the shoulders and arms like a cape. Sensation above and below stays normal and touch is preserved, so the loss is dissociated and suspended. As the cavity reaches the anterior horns, the hands waste. Syringomyelia is often associated with Chiari type I and II and Dandy-Walker malformations (Brazis 2011, p. 106), so a syrinx in a child should send you to look at the back of the skull.

Posterior columns. Vibration and position sense are lost below the lesion, and strength and pain sensation are kept. The child sways or falls with the eyes closed and walks on a broad base, watching the feet.

Inside or outside the cord?

Lesions outside the cord tend to cause early radicular and vertebral pain and early upper motor neuron signs. Lesions inside the cord tend to cause deep, poorly localized pain, prominent lower motor neuron signs, late upper motor neuron signs, and early sphincter trouble when they lie low (Brazis 2011, p. 114, Table 5.11).

At the bottom of the cord the anatomy changes. The cord tapers into the conus medullaris, and below it the canal holds only the bundle of lumbar and sacral roots called the cauda equina. A conus lesion is a cord lesion, and a cauda lesion is a root lesion. The two sit a few centimeters apart, produce overlapping pictures, and can be hard to tell apart (Brazis 2011, p. 118).

Conus

Conus lesions cause early sphincter dysfunction. The bladder becomes autonomous, with no voluntary start to urination, retained urine and absent bladder sensation, and constipation is common. There may be a symmetric saddle anesthesia, and pain is uncommon (Brazis 2011, p. 117).

Cauda equina

Cauda equina lesions compress the lumbosacral roots below the L3 vertebra and cause early radicular pain, often one-sided or asymmetric and worse with straining (Brazis 2011, p. 118). The weakness is lower motor neuron, follows root distributions and comes with lost reflexes. In any patient with absent ankle reflexes, test sensation over the buttocks, because a cauda equina lesion may spare sensation elsewhere (Morris 2012, p. 29).

A common teaching shorthand says the conus gives upper motor neuron signs and the cauda lower motor neuron signs. Conus lesions often damage the nearby roots as well, so expect a mixture. The more reliable conus clue is early, symmetric sphincter failure with little pain.

The tethered cord

In a child the most important lesion here is a cord held too low and stretched as the spine grows. A tethered cord may present with a combination of neurologic, urologic, orthopedic and skin findings. These include numb feet, wasting of one calf or thigh, bladder and bowel trouble, foot deformities such as a high arch or clubfoot, and scoliosis. The cutaneous markers of spinal dysraphism are a midline hair tuft, a midline birthmark, a dermal sinus, excess hair or a fatty lump over the low back (Brazis 2011, p. 118). Many cases are associated with spina bifida occulta or a previous myelomeningocele repair (Brazis 2011, p. 118).

At the bedside

Undress the child enough to see the lower back, and look at the feet. A new high arch on one side, new wetting in a child who was dry, and a hairy patch at the base of the spine belong together.

Delay costs more in the spinal cord than almost anywhere else in the nervous system. The cord has little room to swell, a blood supply with weak spots, and no capacity to regrow lost tracts. A cord that is compressed or inflamed can go from weak to paralyzed in hours. Some causes can be treated only while function remains, so here the value of the localizing exam lies mainly in its speed.

Common causes

Pitfalls

When to image

Back pain together with any new weakness, sensory level or bladder change in a child calls for urgent MRI of the whole spine. The exam tells the radiologist where to look first. Write the level you predict on the request and check it against the scan, which is how localization skill is built.