A spinal cord injury (SCI) involves damage to the intricate network of nerves that transmit signals between the brain and the rest of the body. Such an injury can profoundly disrupt various bodily functions, including motor control, sensation, and the regulation of internal organs. Among these widespread impacts, the ability to control bladder function is frequently affected, leading to different forms of urinary incontinence. This article explores how the specific level of spinal cord injury influences bladder dysfunction and the resulting manifestations of incontinence.
Spinal Cord’s Role in Bladder Control
Normal bladder function relies on a complex interplay between the brain, spinal cord, and peripheral nerves that innervate the bladder and its sphincters. During the storage phase, the bladder muscle (detrusor) relaxes to allow urine to fill, while the urethral sphincters remain contracted to prevent leakage. Sensory nerves in the bladder wall send signals to the spinal cord and then to the brain, indicating bladder fullness.
The brain coordinates the decision to void. When urination is desired, the brain sends signals down the spinal cord to inhibit sympathetic activity and activate parasympathetic nerves, primarily from the sacral spinal cord (S2-S4). This prompts the detrusor muscle to contract and the urethral sphincters to relax, allowing urine to be expelled.
Anatomy of Spinal Cord Injury Levels
The spinal cord is a cylindrical bundle of nervous tissue extending from the brainstem down to the upper lumbar region of the spine. It is organized into distinct regions: cervical (C1-C8), thoracic (T1-T12), lumbar (L1-L5), and sacral (S1-S5), with an additional coccygeal segment. Each segment gives rise to pairs of spinal nerves that branch out to different parts of the body, transmitting both sensory and motor information.
Spinal cord injuries are typically classified by the lowest segment of the spinal cord where sensory and motor function remain intact. For instance, a cervical injury occurs in the neck region, affecting signals to the arms, trunk, and legs, while a lumbar injury impacts the lower back, primarily affecting the legs and pelvis. The spinal cord itself is shorter than the vertebral column, terminating around the L1-L2 vertebral level, with nerve roots continuing downwards as the cauda equina.
SCI Levels and Bladder Dysfunction
Injuries to the spinal cord disrupt the communication pathways between the brain and bladder, leading to a condition known as neurogenic bladder. The specific level of injury dictates the type of bladder dysfunction experienced. This is because different parts of the spinal cord control distinct aspects of bladder function.
Injuries affecting the sacral spinal cord (S2-S4) or the conus medullaris (the lowest part of the spinal cord) often result in a flaccid, or underactive, bladder. In these cases, the lower motor neurons that directly control the bladder muscle and external sphincter are damaged, preventing the bladder from contracting effectively to empty. The bladder may overfill, becoming distended, and lose its ability to sense fullness. The external urethral sphincter might also become weak or flaccid, further compromising control.
Conversely, spinal cord injuries above the sacral level, such as those in the thoracic (T1-T12), lumbar (L1-L5), or cervical (C1-C8) regions, typically lead to a spastic, or overactive, bladder. While the brain’s ability to control urination is lost due to interrupted descending signals, the reflex arc within the sacral spinal cord remains intact. This means the bladder muscle can still contract involuntarily once a certain volume of urine is reached, without conscious control.
A complication in spastic bladders is detrusor-sphincter dyssynergia (DSD). DSD occurs when the detrusor muscle contracts to expel urine, but the external urethral sphincter simultaneously contracts rather than relaxing. This uncoordinated action creates an obstruction, causing high pressures within the bladder and preventing complete emptying, which can lead to urine backing up towards the kidneys.
Manifestations of Post-SCI Incontinence
The type of bladder dysfunction caused by a spinal cord injury directly influences how urinary incontinence presents. Individuals with a flaccid bladder, typically from sacral or lower lumbar injuries, commonly experience overflow incontinence. This occurs because the bladder cannot empty effectively, leading to overfilling and then passive leakage or dribbling of urine when bladder capacity is exceeded. The bladder essentially “spills over” due to its inability to contract.
In contrast, those with spastic bladders, resulting from higher-level injuries in the thoracic or cervical spine, often experience urge incontinence or reflex incontinence. Here, the bladder muscle contracts involuntarily and without warning, leading to a sudden, strong urge to ur urinate followed by involuntary leakage. Detrusor-sphincter dyssynergia can worsen this, as the uncoordinated contraction of the bladder against a closed sphincter can lead to incomplete emptying and subsequent leakage, along with increased bladder pressure.