A 58-year-old man presents with progressive lower back pain and bilateral leg weakness. Sagittal MRI of the lumbar spine is performed. The structure marked **C** in the diagram (Conus Medullaris) appears to terminate at the L1-L2 level, which is normal for an adult. However, on careful review, the conus is noted to terminate at L3-L4. Which of the following clinical conditions is most strongly suggested by this finding?

Explanation

## Why Tethered spinal cord is right The conus medullaris normally terminates at the L1-L2 level in adults. When the conus terminates below L2 (in this case at L3-L4), it indicates a **low conus**, which is the classic imaging finding for **tethered spinal cord**. This is a key clinical correlation on sagittal MRI of the lumbar spine. Tethered cord occurs when the spinal cord is abnormally fixed or stretched, preventing normal rostral migration during development. Patients may present with progressive neurological deficits, lower back pain, and lower limb weakness. The diagnosis is made directly from the position of the conus on sagittal MRI, as cited in Sutton Radiology. ## Why each distractor is wrong - **Cauda equina syndrome from disc herniation**: While cauda equina syndrome can present with bilateral leg weakness and back pain, it is caused by acute compression of the nerve roots distal to the conus, not by abnormal conus position. The imaging finding of a low conus is not diagnostic of acute cauda equina syndrome. - **Lumbar spinal stenosis with neurogenic claudication**: Spinal stenosis results from narrowing of the central canal or lateral recesses, typically due to degenerative changes, disc herniation, or facet hypertrophy. A low conus position is not the primary imaging finding for stenosis; rather, canal diameter and nerve root compression are assessed. - **Degenerative disc disease with Modic changes**: Modic changes represent vertebral endplate edema, fatty replacement, or sclerosis and are associated with chronic low back pain. These changes do not affect the position of the conus medullaris and are unrelated to a low conus finding. **High-Yield:** Conus medullaris terminates at L1-L2 in adults—a low conus (below L2) = tethered cord until proven otherwise. [cite: Sutton Radiology—Normal Sagittal MRI Lumbar Spine; Key MRI findings: (6) CONUS MEDULLARIS — terminates at L1-L2 in adults — LOW CONUS suggests TETHERED CORD]