Contributors: Fraser McConnell

 Species: Canine   |   Classification: Miscellaneous

Indication

  • To diagnose and localize spinal cord compression.

Contraindication

  • Turbid or discolored CSF or inflammation or infectious CSF disease.
  • Coagulopathy.
  • Severe skin infection over site of injection.

Technical considerations

  • Aseptic spinal tap technique required.
  • Only use non-ionic iodine based contrast medium, general anesthesia mandatory.
  • Contrast rapidly dissipates from the subarachnoid space so examination should be performed quickly.
  • Myelography should follow plain survey radiography.
  • The CSF should be collected for analysis Cerebrospinal fluid: sampling. If CSF discolored or turbid, myelography should not be performed.

Cisternal injection

  • The head should be elevated during myelography to minimize cranial extension of contrast media.
    • Preferred for cervical cord and lumbosacral evaluation (better delineation/ lack of epidural contamination in LS area).
    • Cisternal puncture myelography has higher incidence of post myelography seizures, presumably due to cranial flow of contrast medium into ventricular system.
    • Compressive spinal lesions force the contrast media towards the ventricles which increases the risk of seizures Seizures and prevents outline of spinal lesion.

Lumbar injection

  • Preferred for visualization of thoracolumbar region.
  • Generally lower seizure incidence.
  • Can be forced to pass a compressive cord lesion under pressure which facilitates outline a lesion surrounded by an extensive spinal cord edema.
  • Doses: 0.30ml/kg body weight for cisternal and 0.30-0.45 ml/kg body weight for lumbar injection.
  • Contrast media is heavier then CSF so can be moved within the subarachnoid space by altering the positioning of the animal, eg elevation of the head and hindquarters will cause the contrast to pool in the caudal cervical spine.

Myelographic contrast media

  • Requirements:
    • High radiopacity with a relatively small dose.
    • Water soluble.
    • Low viscosity.
    • Reasonably slow resorption from the subarachnoid space.
    • Minimal neurotoxic and other harmful side-effects.
  • Only low osmolar non-ionic water soluble iodine contrast media fulfill those criteria as:
    • They easily mix with CSF.
    • They contain Iodine which results in high radiographic density.
    • They are non-ionic. By maintaining their original number of particles they do not increase the osmotic pressure, hence they are low osmolar. They do not contain neurotoxic sodium ions and due to the low tonicity the blood-brain-barrier remains unharmed which reduces the neuropenetratration of other neurotoxic substances.
  • Side-effects:
    • Post-myelographic seizures. This can be minimized by preventing contrast reaching the cranial subarchnoid space (lifting the head, lumbar puncture), prolonged anesthesia.
    • Hypersensitivity reactions to non-ionic contrast media are very rare and mild in dogs.
  • Iohexol (Omnipaque®), Iopamidol (Niopam®), Ioversol (Optiray®) are now commonly used for myelography in a concentration of 200-350 mg Iodine/ml.

Principles of interpretation

  • Contrast medium mixes with subarachnoid CSF surrounding spinal cord.
  • Lateral view: dense dorsal and ventral contrast column.
  • Ventrodorsal view: left and right lateral contrast column.
  • Interpretation is based on filling defects in or around subarachnoid space.
  • Subarachnoid filling defects can be divided into three types:
    • Extradural
    • Intradural-extramedullary
    • Intramedullary.

Extradural compression

Major findings

  • Thinning of a contrast column, possibly completely lost.
  • Central deviation of a contrast column.
  • Split of a contrast column (not necessarily the case).
  • Reduction in spinal cord diameter on 1 view and possibly widening (divergence of contrast column) on orthogonal view.

Disk prolapse

  • Lateral view:
    • Dorsal deviation of the ventral contrast column at the level of the affected disk space. Dorsal contrast may be thinned or lost. Deviation may be longer than the actual disk space.
      Lateral extrusion of disk material may only cause thinning of both contrast columns in lateral view.
    • Split of the ventral contrast column on the lateral view suggests ventrolateral extradural compression.
  • Ventrodorsal view:
    • Lateral contrast columns may appear thinned and/or deviated laterally. This reflects the ovoid cross-sectional shape of the dural sac caused by the ventral compression.
      Lateral extrusion of disk material causes central deviation & thinning of ipsilateral contrast column.
      In small dogs the ventral contrast column often conform to the ventral surface of the verterbral canal resulting in slight undulation and thinning over the disk space. This is normal and should not be mistaken for pathology.

Canine cervical spondylomyelopathy

  • (Wobbler syndrome Cervical spondylopathy.)
  • Dorsal and ventral extradural compression at the level of several cervical disk spaces due to ligamentous hypertrophy.

Extradural neoplasia

  • (Vertebral tumor Spine: neoplasia, lymphoma Lymphoma.)
  • Larger area of extradural compression.
  • Less likely centered over disk space.
  • May see vertebral changes (lysis in new bone production) in some vertebral tumors.

Dorsal extradural compression alone

  • Rare finding.
  • Differential diagnosis includes:
    • Extradural neoplasia.
    • Dermoid cyst.
    • Dorsal spinal instability due to articular facet malformation, hypoplasia or aplasia.
    • Extradural empyema (over the lenghth of several vertebrae).
    • Synovial cyst of the dorsal articular facet joint.
    • Hemorrhage/hematoma.
    • Subdural contrast injection (artifactual).
    • Extradural neoplasia such as lymphosarcoma.

Lateral extradural compression alone

  • Uncommon finding.
  • Differential diagnosis includes:
    • Lateralized disk protrusion or extrusion.
    • Nerve root / nerve root sheet tumor.
    • Extradural hemorrhage/hematoma.
    • Extradural compression such as lymphosarcoma.

Extensive extradural compression

  • Differential diagnosis includes:
    • Disk extrusion with disk material propulsed into the spinal canal.
    • Disk associated extradural hemorrhage.
    • Non-disk associated extradural or subdural hemorrhage or hematoma.
    • Extradural empyema.
    • Extradural neoplasia such as lymphosarcoma.
    • Vertebral neoplasia with soft tissue swelling in the spinal canal.

Pitfalls

  • The dorsal contrast column decreases it size at the caudal end of C2. The ventral column shows often a kink at C2-C3 with a mild protrusion.
    Don't confuse subarachnoid contrast with epidural and subdural contrast (see artifacts).

Intradural-extramedullary lesion

  • Relatively rare finding.
  • Filling defect within or abnormal shape of the subarachnoid contrast column.
  • Golf tee sign:
    • Subarachnoid contrast passes the filling defect bilaterally within one contrast column reminding of a golf tee in cross-section. May be seen in meningeal neoplasia and nerve root tumors and also (rarely) disk disease or inflammatory disease. Contralateral contrast column is often thinned and deviated abaxially due to spinal cord compression.
      Do not confuse with splitting of the contrast column seen with asymmetrical extradural lesions.

Subarachnoid & dermoid cysts

  • Rare developmental abnormality in the cervical or thoracic canal of dogs.
  • Can cause spinal cord compression.
  • Dorsal contrast column ends prematurely with a drop like sac.
  • May also occur ventrally or circumferentially surround the spinal cord (most common) in caudal cervical spine of Rottweilers.