The spinal cord plays a central role in nearly every function of the body, serving as the primary pathway for communication between the brain and the rest of the body. Its complex structure allows for the transmission of both motor signals (which control voluntary movement) and sensory information (such as pain, temperature, and touch). When an injury like Brown-Séquard Syndrome (BSS) occurs, the effects on the body can be profound, as the syndrome disrupts this vital flow of information.
In this blog post, we'll explore the intricate role of the spinal cord in Brown-Séquard Syndrome, how the spinal cord is structured, and why a lesion on just one side of the spinal cord can cause such a distinctive pattern of symptoms.
The spinal cord is a cylindrical bundle of nerve fibers housed within the vertebral column, extending from the brainstem to the lower back. Its primary function is to relay signals between the brain and various parts of the body. These signals control everything from voluntary movements (like lifting an arm) to autonomic functions (such as regulating heart rate and breathing). The spinal cord also processes reflexes independently of the brain, enabling quick reactions to stimuli without the need for conscious thought.
To understand Brown-Séquard Syndrome, it’s essential to grasp the two key functions of the spinal cord:
Motor Functions: The spinal cord transmits signals from the brain to the muscles via descending motor tracts. These signals are responsible for voluntary movement, posture, and coordination.
Sensory Functions: It also sends sensory information from the body back to the brain through ascending sensory tracts. This includes sensations such as pain, temperature, touch, and proprioception (the sense of body position).
Brown-Séquard Syndrome is the result of a hemicord injury—damage to one side of the spinal cord. This leads to a very particular combination of symptoms, primarily because the spinal cord is arranged in such a way that different types of signals cross over at different levels.
For example, motor signals and fine touch sensations do not cross the midline until they reach the brain, so an injury to one side of the spinal cord will affect those functions on the same side of the body. Meanwhile, pain and temperature signals cross over to the opposite side of the spinal cord shortly after entering it, meaning that an injury to one side will impair those sensations on the opposite side of the body.
Several critical components of the spinal cord are involved in the motor and sensory deficits seen in Brown-Séquard Syndrome:
The Corticospinal Tract: This tract controls voluntary movement, running from the brain down through the spinal cord. In BSS, the lesion interrupts the corticospinal tract on one side of the spinal cord, resulting in ipsilateral motor weakness or paralysis. This means that the muscles on the same side as the injury lose their ability to function properly.
The Dorsal Column-Medial Lemniscus Pathway: This pathway carries signals related to fine touch, vibration, and proprioception. Because these signals remain on the same side of the spinal cord until they reach the brain, damage to the dorsal column results in the loss of these sensations on the ipsilateral side.
The Spinothalamic Tract: Responsible for transmitting pain and temperature sensations, the spinothalamic tract crosses over to the opposite side of the spinal cord shortly after entering it. In BSS, damage to this tract leads to contralateral loss of pain and temperature sensation, meaning that the side opposite the injury loses the ability to feel these sensations.
Sympathetic Pathways: The spinal cord also contains pathways that control autonomic functions, such as blood pressure regulation and sweating. Depending on the location of the injury, BSS can sometimes affect autonomic function, leading to complications such as changes in blood pressure or abnormal sweating patterns.
Voluntary movement is governed by the brain, but the spinal cord acts as the crucial highway for motor signals. When the brain sends a command to move, the signal travels down the spinal cord through the corticospinal tract. This tract crosses over from one side of the brain to the opposite side of the body at the level of the medulla (in the brainstem), but once in the spinal cord, it stays on the same side.
In Brown-Séquard Syndrome, an injury to the hemicord interrupts this pathway, meaning that the brain can no longer communicate properly with the muscles on the same side of the body below the level of the injury. As a result, individuals experience motor deficits such as weakness, loss of coordination, or even paralysis on the side of the body where the injury occurred.
The spinal cord is responsible for transmitting various sensory signals from the body to the brain. These signals include:
Pain and Temperature: These sensations are carried by the spinothalamic tract, which crosses over to the opposite side of the spinal cord shortly after the sensory nerves enter the cord. Therefore, in BSS, damage to one side of the spinal cord results in the loss of pain and temperature sensation on the opposite side of the body (known as contralateral sensory loss).
Touch, Vibration, and Proprioception: These sensations travel through the dorsal columns. Unlike pain and temperature, these signals do not cross over immediately; they ascend the spinal cord on the same side before crossing at the level of the brainstem. As a result, in Brown-Séquard Syndrome, individuals lose the ability to perceive fine touch, vibration, and proprioception on the same side as the injury (known as ipsilateral sensory loss).
This dual loss of sensation—pain and temperature on one side, fine touch and proprioception on the other—is one of the hallmark characteristics of Brown-Séquard Syndrome.
Because of the way the spinal cord is structured, a hemicord injury creates a distinct set of motor and sensory deficits that are unique to Brown-Séquard Syndrome. These include:
Ipsilateral Motor Loss: Weakness or paralysis on the same side as the injury, caused by damage to the corticospinal tract.
Ipsilateral Loss of Fine Touch and Proprioception: Loss of fine touch, vibration, and body position sense on the same side as the injury, due to damage to the dorsal columns.
Contralateral Loss of Pain and Temperature Sensation: Loss of pain and temperature sensation on the opposite side of the injury, caused by damage to the spinothalamic tract.
The asymmetry in the deficits seen in Brown-Séquard Syndrome may seem counterintuitive at first glance. How can damage to one side of the spinal cord affect different sensations and functions on opposite sides of the body? The answer lies in the decussation (crossing over) of certain nerve pathways.
For motor function and fine touch, the pathways cross at the brainstem. For pain and temperature, the pathways cross at the level of the spinal cord. Therefore, damage to one side of the spinal cord interrupts motor and fine touch signals on the same side, but pain and temperature signals on the opposite side.
In addition to its role in motor control and sensation, the spinal cord is also essential for reflexes. Reflexes are automatic, involuntary movements in response to stimuli, and they often bypass the brain altogether, traveling only through the spinal cord. For example, when you touch something hot, your hand pulls away before you even consciously register the pain.
The spinal cord processes these reflexes, coordinating the rapid contraction of muscles to prevent injury. In Brown-Séquard Syndrome, reflexes below the level of the injury may be impaired or exaggerated, depending on which side of the spinal cord is damaged.
The spinal cord's role in coordinating movement, sensation, and reflexes means that damage can lead to significant long-term effects, particularly in Brown-Séquard Syndrome. Individuals may experience:
Motor Impairment: Weakness or paralysis on one side of the body can make it difficult to walk, balance, or perform everyday tasks.
Sensory Loss: The loss of sensation—particularly pain and temperature perception—can increase the risk of injury, as individuals may not be able to feel heat, cold, or pain on one side of the body.
Spasticity and Muscle Stiffness: Some individuals with spinal cord injuries experience spasticity, which causes muscles to become stiff and resist movement.
Bladder and Bowel Dysfunction: Depending on the level of the injury, some individuals may experience difficulties with bladder and bowel control, leading to incontinence or the need for catheterization.
Rehabilitation for individuals with Brown-Séquard Syndrome focuses on physical therapy to improve strength, coordination, and mobility. Occupational therapy helps individuals adapt to sensory and motor impairments in daily life, using adaptive equipment or learning new ways to perform tasks.
In cases where reflexes are exaggerated or spasticity occurs, treatment may involve stretching exercises, medications to relax muscles, and strategies to manage pain or discomfort.
The spinal cord’s complex organization explains the unique constellation of symptoms seen in Brown-Séquard Syndrome. While BSS may present with a distinct pattern of deficits, the severity of the condition varies depending on the level and extent of the spinal cord injury.
Ultimately, the spinal cord’s central role in the body underscores the importance of early diagnosis and appropriate treatment in Brown-Séquard Syndrome. For individuals living with BSS, understanding the spinal cord’s role in their condition can be empowering, helping them better manage their symptoms and work towards improved quality of life.