Conus Medullaris Terminating at L3 in an Adolescent
A conus medullaris terminating at L3 in an adolescent is abnormally low and represents radiographic evidence of a tethered spinal cord that requires clinical correlation and likely neurosurgical evaluation. 1
Normal Anatomy and Definition of Abnormal
By 2 months after birth, the conus medullaris normally ends at the L1-L2 disc space, with the lowest normal level (95% confidence limits) being the middle third of L2. 1
A conus medullaris ending below the middle third of L2 is considered radiographically tethered. 1
In adolescents specifically, the normal range extends from T12 to L1-L2, with the most common location being the middle third of L1 or L1-L2 interspace. 2
A conus at L3 is definitively abnormal and falls well outside the normal range for any age group beyond infancy. 1, 3, 2
Clinical Significance and Required Evaluation
Immediate Assessment Priorities
The finding of a low-lying conus mandates evaluation for tethered cord syndrome, which can cause progressive neurological deterioration affecting morbidity and quality of life if left untreated. 1, 4
Key clinical features to assess include:
Bladder and bowel dysfunction (urinary urgency, incontinence, retention, chronic constipation, or late toilet training history) 1, 4
Lower extremity symptoms including pain (dull, sharp, or dysesthetic), sensory deficits, muscle weakness, or gait abnormalities 1, 4
Orthopedic manifestations such as progressive scoliosis, foot deformities, leg length discrepancies, or exaggerated lumbar lordosis 1, 4
Cutaneous markers overlying the spine (dermal sinus tracts, lipomas, hairy patches, or vascular malformations) that suggest underlying spinal dysraphism 1
Pathophysiology of Clinical Deterioration
Tethered cord syndrome results from physical stretching of the spinal cord leading to impaired blood flow, diminished oxidative metabolism, and metabolic failure at the mitochondrial level. 1
The severity and reversibility of metabolic disturbances correlate with the chronicity of tethering, making early detection critical. 1
Long-standing untreated tethering causes progressive and potentially irreversible musculoskeletal deformities, muscle atrophy, and autonomic changes. 1, 4
Diagnostic Workup
MRI of the complete spine without contrast is the imaging modality of choice to evaluate for underlying spinal dysraphism and associated abnormalities. 1
The MRI should specifically assess for:
Closed spinal dysraphism (lipomyelomeningocele, split cord malformation, dermal sinus tract) 1
Intraspinal masses (lipomas, dermoid/epidermoid cysts) 1
Associated anomalies including Chiari I malformation, syringomyelia, or intrinsic cord tumors 1
Important Clinical Pitfalls
Asymptomatic Does Not Mean Benign
Many adolescents with tethered cord have subtle abnormalities dating to early childhood that were dismissed or attributed to other causes (being "slow" athletically, chronic constipation, minor orthopedic issues). 1
Infants are commonly asymptomatic, with symptoms becoming more apparent as the child grows and the cord experiences increasing traction. 1
Not Related to Idiopathic Scoliosis
Multiple studies confirm that conus position in adolescent idiopathic scoliosis patients is no different from healthy controls, with mean position at lower third of L1. 5, 6
A conus at L3 in a patient with scoliosis suggests the scoliosis may be secondary to the underlying spinal dysraphism rather than idiopathic. 1
Management Implications
Neurosurgical consultation is warranted for any adolescent with a conus terminating at L3, regardless of current symptom status. 1, 4
Surgical detethering may be indicated to prevent progressive neurological deterioration, particularly if any clinical symptoms are present. 1
Even asymptomatic patients require close monitoring, as symptoms can develop or progress during growth spurts or with activities that increase spinal traction. 1
The goal of intervention is to prevent irreversible neurological damage affecting bladder/bowel function, mobility, and quality of life. 1, 4