At what level should the spinal cord terminate in adults?

Normal Spinal Cord Termination Level in Adults

In adults, the spinal cord should terminate most commonly at the L1-L2 disc space, with the lowest acceptable normal level being the middle third of the L2 vertebral body. 1

Anatomical Standards

Primary Termination Level

• The conus medullaris ends most commonly opposite the disc space between the first and second lumbar vertebrae (L1-L2 disc space) by 2 months after birth and remains at this level throughout adulthood. 1
• The lowest normal level (95% confidence limits) is opposite the middle third of the L2 vertebra. 1
• A conus medullaris that ends below the middle third of L2 is considered radiographically tethered, though clinical correlation is required to determine if this represents pathology. 1

Range of Normal Variation

• Recent high-quality MRI studies in living adults demonstrate that the spinal cord virtually never ends below the mid-body of L2 in normal individuals. 2
• The termination level shows a tightly grouped distribution with the average at the lower third of L1 and the mode at the L1-L2 disc space. 2
• While older cadaveric studies suggested wider variation (ranging from L1 to L3), modern MRI studies in living subjects show much less variability. 3, 4

Clinical Implications

Safety for Spinal Procedures

• Spinal anesthesia at or below the L2-L3 interspace is safe in more than 99% of patients, with only 0.7% incidence of neuraxial risk. 3
• Approximately 18.9% of adults have a conus medullaris at or below the L1-L2 interspace, making the L2-L3 level a safe target for spinal procedures. 3
• Tuffier's line (intercristal line) corresponds to the L4-L5 interspace in 99.8% of patients and serves as a reliable anatomical landmark for identifying safe puncture sites. 3

Pathological Considerations

• Any conus medullaris ending below the middle third of L2 should raise suspicion for tethered cord syndrome, particularly if accompanied by cutaneous markers or neurological symptoms. 1
• Congenital spinal abnormalities are the only significant predictor of having a lower conus medullaris termination. 3
• Demographic variables including age, sex, height, and BMI are limited in predicting conus medullaris level. 3

Important Caveats

Embryological Context

• The spinal cord below S2 and the filum terminale are formed from secondary neurulation, which is distinct from the formation of the cord above this level. 1
• During fetal development, the spinal cord undergoes "ascent" relative to the vertebral column due to differential growth rates, placing the conus progressively higher. 1

Imaging Considerations

• MRI provides the most accurate assessment of conus medullaris level in living subjects compared to cadaveric studies or other imaging modalities. 4, 2
• Whole-spine MRI is recommended when surgical intervention is being considered to identify the exact termination level and rule out associated neural axis abnormalities. 5