Main page/Posterior fossa/Foramen magnum/FM Clinical

FM Clinical

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Foramen magnum

Surgical approaches to the foramen magnum

  • Posterior operative approach is commonly selected for intradural lesions.
  • Anterior approach is frequently selected for extradural lesions situated anterior to the foramen magnum.
  • Lateral approach may be selected for intradural lesions located lateral to and/or in front of the brainstem, especially if they involve or are contiguous with the temporal bone.
  • Lateral approaches directed through the temporal bone are considered in a later section of this issue.

Foramen magnum lesions

Differential diagnosis

Neoplastic: Most foramen magnum (FM) region tumors are extra-axial.

  • Extra-axial tumors
    • Meningioma (38–46% of FM tumours)
      • The anterior lip of the foramen magnum is the second most common site of origin of posterior fossa meningiomas
    • Chordoma
      • A mass behind the dens compressing the spinal cord is a chordoma until proven otherwise
    • Neurilemmoma
    • Epidermoid
    • Chondroma
    • Chondrosarcoma
    • Metastases
  • Exophytic component of a brainstem tumor

Non-neoplastic lesions

  • Aneurysms or ectasia of the vertebral artery
  • Odontoid process in cases of basilar invagination
  • Pannus from involvement of the odontoid with rheumatoid arthritis or old nonunion of fracture
  • Synovial cyst of the quadrate ligament of the odontoid

Presentation

  • In the pre-imaging era (i.e., before CT & MRI) these lesions were often diagnosed relatively late due to the unusual associated clinical syndromes and the rarity of visualizing this region on myelography.

Clinical findings

Symptoms

  • Sensory
    • Craniocervical pain:
      • An early symptom
      • Commonly in neck and occiput.
      • Aching in nature.
      • Worsen with head movement
    • Sensory findings:
      • Usually occur later.
      • Numbness and tingling of the fingers
  • Motor
    • Spastic weakness of the extremities:
        • Syndrome
        Deficit Pattern
        Anatomical compression site
        Classic Cause
        Hemiplegia cruciata
        Paralysis of one arm & contralateral leg
        Pyramidal decussation (lower medulla) at the sides of the cord
        Trauma/lesion at lower medulla
        Cruciate paralysis
        Bilateral upper limb paralysis, lower limbs spared
        Cervicomedullary junction (above decussation) at midline
        Trauma at craniovertebral junction
        Rotating paralysis
        "Alternating" or shifting limb deficit (non-standard)
        After decussation at the sides of the cord
        Rare, not defined neuroanatomically
      • Rotating paralysis
        • Weakness usually starts in the ipsilateral UE → ipsilateral LE → contralateral LE → contralateral UE
        • Postulation
          • Lesion occurs at the level of the cord/medulla after decussation of the corticospinal tract
          • Lesion causing compression on the right side of the cord → lesion pushes the cord towards the opposite side (left). The venous return of the cord on the opposite side gets compressed (here on the left side) causing venous oedema of the affected portion of the cord
            • Arrangement of the nerve fibres in cord (CTLS) - UL I/L is affected followed by the LL.
          • Once the spinal cord has no more space to move under the compressive force to the opposite side, then to direct compressive effect C/L LL followed by the UL
      • Different hemiplegia cruciata
        • Compression on one side may produce the similarly named but clinically (spastic palsy of one UE and the contralateral LE)
      • Cruciate paralysis
        • Aka Bells paralysis
        • Due to
        • Need to differentiate from central cord syndrome
        • Mech
            • Compression in midline above the pyramidal decussation (located in the inferior medulla oblongata) → the medial part of the pyramid decussation is arm → bilateral UE weakness + atrophy of the hands with sparing of the LEs
            notion image

Signs

  • Sensory
    • Dissociated sensory loss: loss of pain and temperature contralateral to lesion with preservation of tactile sensation
    • Loss of position and vibratory sense, greater in the upper than the lower extremities
  • Motor
    • Spastic weakness of the extremities
    • Atrophy of the intrinsic hand muscles:
      • A lower motor nerve finding
      • It had been postulated that long tract findings were due to direct compression at the cervicomedullary junction, and that lower motor nerve findings in the upper extremities were due to central necrosis of the gray matter as a result of compression of arterial blood supply. Anatomic study suggests that it is actually venous infarction at lower cervical levels (C8–1) that is responsible for the lower motor neuron findings.
    • Cerebellar findings may rarely be present with extensive intracranial extension
  • Long tract findings
    • Brisk muscle stretch reflexes (hyperreflexia, spasticity)
    • Loss of abdominal cutaneous reflexes
    • Neurogenic bladder: usually a very late finding
  • Ipsilateral Horner syndrome:
    • Due to compression of cervical sympathetics

Images

notion image