Brown-Séquard Syndrome (BSS) is a rare condition that affects the spinal cord, resulting in a distinctive pattern of motor and sensory impairments. Understanding the anatomy behind this syndrome requires a closer look at the structure of the spinal cord, the way it functions, and how damage to one side of the cord can create such a unique array of symptoms.
In this post, we will delve into the anatomy of the spinal cord, explore the specific structures involved in Brown-Séquard Syndrome, and examine how an injury to these areas leads to the characteristic neurological deficits.
The spinal cord is a cylindrical structure that extends from the base of the brain, through the vertebral column, and down the back. It serves as a critical conduit for communication between the brain and the rest of the body, transmitting motor signals from the brain to the muscles and sensory signals from the body back to the brain. This flow of information is what allows us to move, feel, and react to our environment.
The spinal cord is made up of several key components:
White matter: The outer layer of the spinal cord consists of white matter, which contains myelinated nerve fibers (axons) that transmit signals up and down the spinal cord. These axons are organized into distinct tracts or pathways, each responsible for specific types of information (e.g., pain, temperature, touch, motor control).
Gray matter: Inside the white matter is the gray matter, which consists of nerve cell bodies and is organized into horns. The gray matter is responsible for processing and relaying signals between neurons within the spinal cord.
Ascending and descending tracts: The white matter contains both ascending tracts (which carry sensory information to the brain) and descending tracts (which carry motor signals from the brain to the muscles). These tracts are crucial to the functioning of the nervous system.
Spinal nerves: Pairs of spinal nerves emerge from the spinal cord at regular intervals, connecting the spinal cord to the rest of the body. These nerves are responsible for transmitting motor and sensory information between the spinal cord and the muscles, skin, and organs.
Each region of the spinal cord (cervical, thoracic, lumbar, and sacral) is responsible for innervating different parts of the body, which is why the location of a spinal cord injury can dramatically impact the symptoms experienced.
The key to understanding Brown-Séquard Syndrome lies in the concept of lateralization within the spinal cord. This syndrome occurs when damage is confined to one side of the spinal cord, a condition known as hemicord injury. The spinal cord is arranged in a way that certain functions are split between the two sides of the body, which means that a lesion on one side can cause asymmetrical deficits.
In BSS, the damage to one side of the spinal cord disrupts both the motor pathways and certain sensory pathways, but in different ways on each side of the body. This creates the syndrome's hallmark presentation:
Ipsilateral motor loss: The side of the body where the injury occurred experiences motor weakness or paralysis below the level of the injury. This happens because the corticospinal tract, which controls voluntary motor movement, crosses over from one side of the brain to the opposite side of the spinal cord in the medulla. However, below the level of the injury, the tract is interrupted, preventing motor signals from reaching the muscles.
Contralateral sensory loss: On the opposite side of the body, there is a loss of pain and temperature sensation, but not motor function. This is due to the interruption of the spinothalamic tract, which carries pain and temperature signals from the body to the brain. These signals cross over to the opposite side of the spinal cord shortly after entering it, so a lesion on one side affects the sensory information on the opposite side.
This split between motor and sensory function is a key feature of Brown-Séquard Syndrome and is a direct result of how the spinal cord is organized.
The corticospinal tract is one of the most important motor pathways in the body. It begins in the motor cortex of the brain, descends through the brainstem, and crosses over at the level of the medulla before continuing down the opposite side of the spinal cord.
Function: The corticospinal tract is responsible for transmitting motor signals from the brain to the muscles, allowing for voluntary movement. Damage to this tract, as seen in Brown-Séquard Syndrome, results in weakness or paralysis on the side of the body where the injury occurred.
Effect of Injury: In BSS, when the corticospinal tract is damaged on one side of the spinal cord, the individual experiences ipsilateral motor loss—meaning that the side of the body corresponding to the side of the lesion is affected. This motor loss can range from mild weakness to complete paralysis, depending on the severity of the injury.
The spinothalamic tract carries sensory information related to pain, temperature, and crude touch from the body to the brain. Unlike the corticospinal tract, the spinothalamic tract crosses over to the opposite side of the spinal cord almost immediately after entering it.
Function: The spinothalamic tract is responsible for transmitting sensory signals that allow us to perceive pain and temperature. These signals travel from the skin, muscles, and organs to the spinal cord and then up to the brain.
Effect of Injury: In Brown-Séquard Syndrome, damage to the spinothalamic tract on one side of the spinal cord results in contralateral sensory loss—meaning that the side of the body opposite the lesion loses the ability to perceive pain and temperature. This loss occurs below the level of the injury.
Another critical structure affected in Brown-Séquard Syndrome is the dorsal columns, which are part of the white matter in the spinal cord. The dorsal columns carry sensory information related to fine touch, vibration, and proprioception (the sense of body position).
Function: The dorsal columns are responsible for transmitting sensory signals that allow us to perceive fine touch (e.g., feeling the texture of an object) and proprioception (e.g., knowing where our limbs are in space without looking at them). This information is vital for coordinated movement and spatial awareness.
Effect of Injury: In BSS, the dorsal columns are affected on the same side as the injury, resulting in a loss of fine touch and proprioception on the ipsilateral side below the level of the lesion. This means that the individual may have difficulty sensing fine touch or knowing the position of their limbs on the side of the injury, which can affect balance and coordination.
Brown-Séquard Syndrome can be caused by a variety of injuries or conditions that damage one side of the spinal cord. The underlying mechanisms can include:
Trauma: Penetrating injuries, such as stab wounds or gunshot wounds, are the most common causes of BSS. In these cases, the injury is typically confined to one side of the spinal cord, disrupting motor and sensory pathways.
Tumors: Tumors that grow on one side of the spinal cord can compress the hemicord and lead to the symptoms of Brown-Séquard Syndrome. These tumors may be benign or malignant, and the severity of the symptoms depends on the size and location of the tumor.
Infections and Inflammatory Conditions: Certain infections, such as spinal tuberculosis, and inflammatory conditions, like multiple sclerosis, can cause lesions or abscesses on one side of the spinal cord, resulting in hemicord damage.
Ischemia: Reduced blood flow to one side of the spinal cord, such as in a spinal stroke, can also cause the asymmetrical deficits characteristic of Brown-Séquard Syndrome.
Degenerative Conditions: Conditions like herniated discs or severe osteoarthritis can sometimes result in spinal cord compression on one side, leading to the development of BSS symptoms.
The location of the injury along the spinal cord plays a significant role in determining the specific symptoms and their severity. The spinal cord is divided into different regions—cervical, thoracic, lumbar, and sacral—each of which innervates a different part of the body.
Cervical Region: Injuries to the cervical region (the neck) affect motor and sensory function in the arms, hands, chest, and legs. A cervical injury in Brown-Séquard Syndrome can result in paralysis or weakness in the arms and legs, as well as sensory loss in large portions of the body.
Thoracic Region: Injuries to the thoracic region (the upper back) affect the trunk and legs. In BSS, a thoracic injury may cause weakness or paralysis in the legs, along with sensory loss in the torso and lower body.
Lumbar and Sacral Regions: Injuries to the lumbar and sacral regions (the lower back) primarily affect the legs and pelvic organs. A lumbar injury in BSS may result in weakness or paralysis in the legs, while a sacral injury can affect bowel, bladder, and sexual function.
What sets Brown-Séquard Syndrome apart from other spinal cord injuries is the distinct pattern of neurological deficits that occur when only one side of the spinal cord is damaged. This
unique pattern includes:
This combination of deficits allows healthcare providers to identify Brown-Séquard Syndrome based on clinical examination alone, although imaging studies (such as MRI) are often used to confirm the diagnosis.