Radiography: myelography

• Introduction of contrast into the subarachnoid space to allow visualization of the spinal cord.

• Investigation of:
  • Back or neck pain.
  • Neurological deficits thought to arise from spinal cord compression.
• Localization of site of compression if spinal surgery contemplated.

Advantages

• Relatively accurate for diagnosis of spinal cord compression and localization of lesion.
• May give some indication of likely etiology.

Disadvantages

• Risk of exacerbating lesion.
• Requires general anesthesia.
• Invasive.
• Contraindicated if increased CSF pressure or meningitis.
• Risk of seizures after procedure.

• Requires some experience on part of operator and ideally cadaver practice.

• MRI imaging of spine Magnetic resonance imaging: spine.
• CT imaging Computed tomography.
• Discography.
• Epidurography.

Preparation

• 15-20 min and time for general anesthesia.

Procedure

• 30 min upwards (depending on number of films required).

Criteria for choosing test

• Is the examination appropriate?
• Can you make the diagnosis without it?
• Will your management of the case be affected by the outcome of the examination?

Risk assessment

• Contraindicated if increased intracranial pressure due to risk of brain herniation through canal as pressure relieved.
• Contraindicated if myelomalacia suspected.

Minimum equipment

• X-ray machine.
• Cassette.
• Processing facilities.
• Protective clothing (lead apron) for radiographer.
• Positioning aids (sandbags, cradle and ties).
• Method of labeling film.

Ideal equipment

• Ability to process films during procedure so that repeat radiographs can be taken during course of study if required.
• High output X-ray machine.
• High definition screen.
• Grid for examination of large dog.

Minimum consumables

• Radiographic film.
• Water soluble contrast agent.
• Needle 22 G/1 cm with stylet.
• 5 ml syringe for injecting contrast.

Ideal consumables

• Specialized spinal needle.

• Fast animal for 12 h prior to anesthetic to prevent reflux esophagitis.

• Clip and surgically prepare skin over site of needle entry.
• Cisternal puncture: C1-C2.
• Lumbar puncture: L3 - S1.

• General anesthesia required.

Step 1 - Control Films

• Lateral and ventrodorsal spinal radiographs Radiography: spine :
  • Confirm positioning adequate.
  • Confirm diagnosis not apparent without contrast study.

Step 2 - Prepare contrast

• Warm contrast media to body temperature (holding loaded syringe in palm of hand while animal prepared for myelography is usually sufficient).
  Must ensure that contrast is sterile.

Step 3 - Position patient

• Cisternal puncture:
  • Position patient in straight sternal recumbency and flex neck so that nose is perpendicular to table.
  • Or in lateral recumbency get assistant to flex neck so that nose remains parallel to table and is flexed at right angles to spine.
  
  Ensure that endotracheal tube does not become totally obstructed by kinking during this procedure.
• Lumbar puncture:
  • Position patient in lateral recumbency.

Step 1 - Introduce needle

• Using aseptic technique insert needle into subarachnoid space.
• Cisternal puncture:
  • Palpate the wings of the atlas.
  • Insert needle perpendicular to spine at intersection of a line drawn between wings of atlas and a line from occipital crest through midline of spine.
  • Advance needle through muscle tissue.
  • A distinct 'pop' is usually felt as needle enters dura matter.
  
  If needle contacts bone gently 'walk' needle caudally off edge of vertebra into intervertebral space.
• Lumbar puncture:
  • Palpate the L5-L6 disk space.
  • Insert needle through disk space keeping perpendicular to spine and parallel to table.
  • Advance needle until it contacts floor of vertebral canal.

Step 2 - Ensure correct needle position

• CSF should flow through needle as stylet is withdrawn.
• Collect sample into containers (EDTA for cytological analysis and into plain sterile tube for culture).
• Allow a volume of CSF (50-100% of anticipated injection volume) to flow from the needle before connecting syringe of contrast media.

Step 3 - Inject contrast

• Connect syringe of warm contrast to needle.
  Ensure that needle is not dislodged during this procedure.
• Inject 0.2 ml/kg -0.45 ml/kg contrast (depending on the anticipated distance of the lesion from the site of puncture).
• Withdraw needle.

Step 4 - Obtain radiographic views

• Take lateral and ventrodorsal views of all levels of the spine.

Step 1 - Assess radiographs

• Ensure that contrast agent is with subarachnoid space, ie not in epidural space.
• Has the contrast media reached the area of interest?
  • Tipping the patient to encourage the flow of contrast under gravity may speed passage of contrast.
  • Add more contrast if necessary.

Step 2 - Additional radiographic procedures

• May be useful to take extended and flexed views to gain additional information.

Potential complications

• Ataxia may be seen for up to 24 h after myelographic studies (but usually resolves spontaneously).

• Epidural contrast placement.

Refereed papers

• Recent references from PubMed and VetMedResource.
• Hay C W, Muir P (2000) Tearing of the dura mater in three dogs. Vet Rec 146 (10), 279-282 PubMed.
• McKee W M, Penderis J & Dennis R (2000) Radiology corner - obstruction of contrast medium flow during cervical myelography. Vet Rad Ultra 41 (4), 342-343 PubMed.
• Widmer W R & Blevins W E (1991) Veterinary myelography - a review of contrast media, adverse effects and technique. JAAHA 27 (2), 163-77 VetMedResource.