The spinal column is comprised of numerous bones called vertebrae. The lower back section, known as the lumbar spine, contains five of these bones, labeled L1 through L5. Damage to the spinal nerve roots that exit between the fourth (L4) and fifth (L5) lumbar vertebrae causes a condition called radiculopathy. This compression or irritation is a frequent source of discomfort and altered function that extends from the lower back into the leg. Understanding the specific patterns of these symptoms helps determine which nerve root is affected.
The Function of the L4 and L5 Nerve Roots
The nerve roots at the L4 and L5 levels are communication lines between the spinal cord and the lower body, controlling movement and sensation. These roots contribute significantly to the formation of the sciatic nerve, which descends through the buttocks and down the leg. The L4 root primarily carries signals for the muscles that move the knee and sensation for the inner part of the lower leg. The L5 root controls the muscles that lift the foot and sensation across the top of the foot. When the space these nerves exit through becomes narrowed, their function is compromised, leading to a predictable set of neurological symptoms.
Sensory Manifestations of Damage
When the L4 or L5 nerve roots are compressed, the first noticeable effects involve sensory changes like pain, numbness, or tingling, which follow distinct pathways called dermatomes. The pain is frequently described as sharp, shooting, or burning and may be worsened by actions such as coughing or sneezing. If the L4 nerve root is irritated, pain and altered sensation typically radiate down the front of the thigh, crossing the knee and extending into the inner, or medial, aspect of the lower leg. This pathway helps clinicians differentiate L4 compression from other nerve issues affecting the leg.
Compression of the L5 nerve root results in a sensory pattern that travels along a different route. Symptoms usually track down the outer, or lateral, side of the thigh and calf. The numbness or tingling commonly wraps across the top surface of the foot and extends into the great toe. This distribution reflects the area of skin supplied by the L5 nerve root, making the location of the numbness an indicator of the affected level.
Motor Deficits and Reflex Changes
Damage to the L4 and L5 roots can cause specific motor deficits and changes in reflexes due to the affected myotomes, or groups of muscles controlled by a single nerve root. L4 radiculopathy causes weakness in the quadriceps muscle group, resulting in difficulty straightening the knee. This weakness can affect stability when standing or climbing stairs. A related finding for L4 damage is a diminished or absent patellar reflex, commonly known as the knee-jerk reflex.
The primary motor deficit associated with L5 nerve root compression is weakness in the muscles that lift the front part of the foot (dorsiflexion). This weakness often leads to “foot drop,” where the foot drags or slaps the ground during walking. To compensate, an individual may adopt a high-stepping or “steppage” gait, raising the thigh excessively high with each step. The L5 root also supplies the muscles responsible for extending the great toe, so the inability to lift the big toe is an early sign of L5 nerve involvement.
Common Causes and Seeking Medical Care
L4-L5 nerve root damage is usually caused by mechanical compression within the spinal canal. The most frequent cause is a lumbar disc herniation, where the interior material of the disc pushes out and presses directly on the nerve root. Other common causes include spinal stenosis, which is the narrowing of the bony canal that houses the nerves, often due to age-related degeneration. A condition called spondylolisthesis, where one vertebra slips forward onto the one below it, can also pinch the nerve roots as they exit the spine.
If symptoms consistent with L4 or L5 radiculopathy are present, especially motor weakness or foot drop, seeking professional diagnosis is important. While many cases of nerve root irritation resolve with conservative measures, progressive motor deficits indicate a higher degree of compression. Timely evaluation helps prevent potential permanent nerve damage resulting from prolonged pressure on the spinal nerve root.