What is the most likely finding on an MRI of the cervical spine in a 3-year-old boy who presented with initial paralysis and apnea after a high-speed motor vehicle collision, but now has a normal neurologic exam and normal head and cervical spine computed tomography (CT) scans?

Injury to Spinal Cord at C3-C4 is Most Likely on MRI in a Child with Transient Paralysis After Trauma

The most likely finding on MRI of the cervical spine in this 3-year-old boy would be an injury to the spinal cord at C3-C4, despite normal neurologic exam and normal CT scans. This represents a case of Spinal Cord Injury Without Radiographic Abnormality (SCIWORA), which is more common in the pediatric population.

Rationale for C3-C4 Injury

1. Clinical Presentation Suggestive of Upper Cervical Injury:

   • The transient quadriplegia and apnea strongly suggest upper cervical cord involvement
   • C3-C4 is a critical region for respiratory function (phrenic nerve originates from C3-C5)
   • The temporary apnea requiring bag-mask ventilation is highly consistent with a transient injury at this level 1

2. Age-Related Considerations:

   • Children are particularly susceptible to SCIWORA due to:
     • Greater elasticity of the pediatric spine
     • Relatively large head-to-body ratio
     • Underdeveloped neck musculature
     • Horizontal facet joints

3. Normal CT Findings Despite Neurological Symptoms:

   • CT is excellent for detecting bony injuries but has limited sensitivity for soft tissue and cord injuries 1
   • MRI is the modality of choice in children who fulfill criteria for myelopathy or SCIWORA 1

Imaging Findings in SCIWORA

• MRI may reveal:

  • Spinal cord edema
  • Contusion
  • Hemorrhage
  • Partial or complete cord transection
  • Ligamentous injury

• These findings are typically seen at the level corresponding to the neurological deficit 1

Why Other Options Are Less Likely

1. Injury at C6-C7:

   • Less likely to cause quadriplegia and respiratory compromise
   • Respiratory dysfunction typically involves levels C3-C5 where the phrenic nerve originates 2

2. Injury at T4-T5:

   • Too low to explain quadriplegia or respiratory compromise
   • Would not typically cause upper extremity paralysis

3. Normal MRI:

   • While possible, the transient but severe neurological deficits (quadriplegia and apnea) strongly suggest a real cord injury that would be visible on MRI
   • Studies show that children with normal radiography and CT may have signs of traumatic cervical injury on MRI 1

Clinical Implications

• MRI is the modality of choice in children with suspected spinal cord injury despite normal CT findings 1
• The United Kingdom's National Institute for Health Care Excellence guidelines suggest that in children <16 years of age, cervical MRI should be the first imaging modality for suspected spinal cord injury 1
• Fat-saturated T2 sequences are particularly useful for detecting soft tissue injuries 1

Management Considerations

• Despite normal neurological examination now, the history of transient quadriplegia and apnea warrants:
  • MRI evaluation to assess for cord injury
  • Consideration of cervical collar immobilization until definitive assessment
  • Close neurological monitoring for delayed deterioration

The high-speed mechanism, transient quadriplegia, and respiratory compromise in this pediatric patient strongly point to an upper cervical cord injury at C3-C4, which would be detectable on MRI despite normal CT findings and current normal examination.