More detailed anatomy, physiology of Brown-Séquard

Studying the pathways in the spinal cord damaged in Brown-Séquard can help one more thoroughly understand the associated symptoms.

The lateral corticospinal tract is injured in Brown-Séquard syndrome.   This tract carries motor information initiated from the precentral gyrus and other nearby areas of the brain.  It crosses over from the left side of the brain to the right side of the brain (or vice versa) in the medulla, eventually synapsing with neurons on that new side—in our first example, to the right side.  If the right side of the spinal cord is transected, motor neurons at synapsing at that level are unable to function.  Information from the tract is also unable to travel further down the spinal down the spinal cord.  This eventually results in a spastic paralysis with increased muscle tone right below the level of the injury.  Note that this usually follows an initial period of spinal shock, with muscle flaccidity instead of spasticity and without hyperactive reflexes.  Also note that at the actual injury level, primary motor neuron damage results in flaccid paralysis with low muscle tension, not spastic paralysis as occurs further down the cord.

The posterior column is also interrupted in classical Brown-Séquard.  This region is the major pathway for most forms of sensory information, including tactile, proprioceptive, vibratory, and positional information.  This pathway begins out in the body, where the receptors take in sensory information.  The axons of these neurons enter the spinal cord and travel up the spinal cord in the posterior columns.  In the brainstem, these neurons synapse with other neurons that cross over before ascending to the thalamus.  Similar to the lateral corticospinal tract, if the right side of the spinal cord is transected, the sensory neurons at that level and at any level below are unable to communicate with the brain.  If the injury is on the right side of the spinal cord, as in our example, an individual will have loss of this sensory information on this side.

Injury to the anterolateral pathway in Brown-Séquard results in a different pattern.  This pathway contains the spinothalamic tract, which conveys pain and temperature information from the body to the brain.  In this pathway, axons of neurons from sensory receptors in the body synapse with other neurons in the spinal cord.  These next neurons travel on the same side of the spinal cord for a level or two before crossing over to the other side of the spinal cord.  Here they ascend up the spinal cord and eventually synapse again in the thalamus.  Thus pain and temperature sensation is lost on the opposite side from the injury—in our example, on the left side.  Because of the way these fibers cross over, the diminished sensations begin a segment or two below the level of damage.

Patients with Brown-Séquard usually show a fair amount of motor recovery.  This is thought to be due to the spared motor axons in the opposite and undamaged side of the cord, which may, in time, take over some of the function of the damaged axons.

Patients with Brown-Séquard tend to do better than those with other partial cord syndromes.  For example, in anterior cord syndrome, the anterior two-thirds of the spinal cord is damaged, usually due to compression.  These patients can feel some crude sensations, since their posterior columns are intact, but more detailed sensation is lost.  Pain and motor pathways are lost below the level of the lesion, bilaterally, as the spinothalamic and anterolateral pathways on both sides are damaged.  These patients have only a twenty percent chance of motor recovery.  Brown-Séquard also carries a better prognosis than other partial cord syndromes like central core syndrome or posterior cord syndrome.

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