Neuroscience Clerkship

COMMON SPINAL CORD SYNDROMES

Spinal cord disorders can extremely important to recognize for two important reasons:

• Many have treatable and reversible causes if detected early

• If not detected and treated, they can easily lead to permanent paralysis and disability

Relevant Spinal Cord Anatomy

The well understood arrangement of tracts and cell columns in the spinal cord frequently allows for precise neuroanatomic localization of signs and symptoms.

Examined in cross section, a central, distinctive butterfly-shaped gray matter core is seen surrounded by white matter tracts. A small, ependyma-lined central canal runs the length of the cord and nearly all neurons that cross from one side of the spinal cord to the other do so in the anterior commissure that lies anterior to this canal.

There is a general tendency in the spinal cord for motor structures to be located anteriorly and sensory structures posteriorly. Thus, the posterior gray matter of the spinal cord receives the dorsal (sensory) roots, and the anterior gray of the spinal cord contains the anterior horn cells (motor neuron cell bodies), which give rise to the anterior (motor) roots. The descending motor (corticospinal) tracts are located posterolaterally and the sensory white matter tracts are located both anteriorly and posteriorly (dorsal and ventral spinothalamic tracts and dorsal columns).

Each spinal nerve is named for its adjacent vertebral body. This leads to two problems in nomenclature. Because there is an additional pair of spinal nerve roots as compared to the number of vertebral bodies, the first seven spinal nerves are named for the first seven cervical vertebrae, each nerve exiting through the intervertebral foramen above its correspondingly named vertebral body. The spinal nerve exiting below the level of C7, however, is referred to as the C8 spinal nerve (the "extra" spinal root), though no eighth cervical vertebra exists. Because of this "extra" nerve root, all subsequent roots exit below the vertebral body for which they are named, beginning with T1. The 8 cervical roots, 12 thoracic roots, 5 lumbar roots, 5 sacral roots, and 1 coccygeal root total 31 spinal nerve root pairs. All of these contain both motor and sensory roots with the exception of C1, which lacks a sensory component (explaining the absence of a "C1 dermatome").

After leaving the spine, the C5-T1 nerve roots from the brachial plexus behind the clavicle. The 12 paired motor-sensory thoracic roots continue as intercostal nerves. In the lower extremity, the L1-S3 roots come together to form the lumbosacral plexus.

The other issue in spinal root nomenclature arises from the relative positions of the spinal nerves with respect to their vertebral bodies. During embryonic development, the spinal segments are closely aligned to their corresponding vertebral segments. But later, the bony spinal column's downward growth outpaces that of the spinal cord. This differential growth gives rise to the appearance that the lower portion of the spinal cord has "ascended" in the spinal canal relative to the vertebral column. Indeed, because the adult spinal cord ends as the conus medullaris at approximately the L1 level, the lumbar and sacral roots must plunge downward below the termination of the spinal cord to find their respective intervertebral foramina, forming the distinctive cauda equina (horse tail). As a consequence, a pathologic process at the level of the L4 vertebral body would be potentially in close proximity to both the L4 nerve root and the other lower spinal roots of the cauda equina; however a lesion at the L4 level cannot affect the spinal cord.

The vascular supply to the spinal cord consists of a single, larger anterior spinal artery and two smaller posterior spinal arteries. The anterior spinal artery supplies the anterior two-thirds of the cord. The two smaller posterior spinal arteries supply a wedge-shaped area constituting the posterior third of the cord.

Functional Neuroanatomy of the Spinal Cord

While a large number of ascending and descending tracts have been identified and mapped in the spinal cord, the three most important of these in terms of neuroanatomic localization of spinal cord lesions are the corticospinal tracts, spinothalamic tracts, and the dorsal columns (see figure below).

The corticospinal tract arises from neurons whose cell bodies are located in the precental gyrus, the primary motor cortex). These neurons are known as the upper motor neurons. Once having crossed in the lower medulla, the axons of the corticospinal tract move posteriorly to the posterolateral funiculi of the spinal cord. Most of these neurons are destined to synapse on the anterior horn cells located in the anterior gray of the spinal cord, the cell bodies of the lower motor neurons. These descending corticospinal tract fibers are laminated in the spinal cord in a clinically important arrangement, with fibers destined for the lower limbs traveling more superficially in the cord and fibers destined for upper limbs traveling more deeply in the cord. Because they have already crossed, damage to these corticospinal tract neurons in the spinal cord results in ipsilateral clinical findings such as spastic weakness, increased deep tendon reflexes, and a Babinski sign. When there is damage to the anterior horn cells (lower motor neurons), ipsilateral clinical findings occur at the level of the affected segments, including flaccid weakness, muscle wasting, decreased deep tendon reflexes, and fasciculations.

There are two major ascending systems that transmit conscious sensory information in the spinal cord: the spinothalamic tracts and the dorsal columns (a.k.a. posterior columns). The first order neurons of both of these afferent systems begin as sensory structures situated in end organs (e.g., sensory receptors in skin and stretch receptors in muscle). The cell bodies of the first order neurons of these sensory pathways are located in the dorsal root ganglia of the spinal nerves. These ganglia are seen as distinctive prominences on the dorsal nerve roots just proximal to the point where the anterior and dorsal branches join in the intervertebral foramen to form the peripheral spinal nerve.

The spinothalamic tracts transmit pain and temperature sensation, commonly tested at the bedside in the form of pinprick and cold sensation. As the axons of these neurons enter the spinal cord, most rise one or two levels before entering the dorsal gray of the spinal cord where they synapse (explaining why a cord lesion at T3 may result in a level at T5). They then cross immediately in the anterior commissure of the spinal cord and ascends in the anterolateral funiculus as the lateral spinothalamic tract. As a result, when the anterolaterally located spinothalamic tract is damaged in the spinal cord, the patient experiences sensory symptoms in the contralateral half of the body. This is contrary to the case of injuries to the motor system described above, where the symptoms are ipsilateral.

Clinical Spinal Cord Syndromes

•  Complete Transection

•  Hemisection (Brown-Sequard)

•  Combined Multisystem Degeneration (Vitamin B12 deficiency)

•  Anterior Horn Cell Disease

•  Central Cord

•  External Compression

•  Anterior Spinal Artery Occlusion