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.
• Difficulty interpreting if suboptimal radiograph.

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

• MRI imaging of spine.
• CT imaging.
• Discography.
• Epidurography.

Preparation

• 15-20 minutes and time for general anesthesia.

Procedure

• 30 minutes 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 foramen magnum as pressure relieved.
• Contraindicated if myelomalacia.
• Contraindicated in meningitis Meningomyelitis: bacterial and fungal.

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.

Minimum consumables

• Radiographic film.
• Water soluble non-ionic contrast agent Radiography: contrast media eg iohexol or iopamidol 180-300 mg I/ml.
• Needle 22 guage/1.5 in spinal needle.
• 5 ml syringe for injecting contrast.
  Use only non-ionic water soluble contrast media, eg iohexol or iopamidol - ionic contrast will kill the patient.

Ideal consumables

• Specialized spinal needle.

• Diazepam Diazepam has been recommended as a pre-medication to reduce seizures associated with myelography.
• Probably not essential with modern contrast agents.
  Avoid acepromazine.

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

• Clip and surgically prepare skin over site of needle entry.
• Cisternal puncture: atlanto-occipital injection.
• Lumbar puncture: usually L5-L6 or L6-L7.
• Cisternal puncture is technically easier but contrast may not extend round lesion - particularly in lumbar area.
• Lumbar puncture is preferred for lumbar or thoracolumbar lesion as injection may be made under pressure.
• Both cisternal and lumbar puncture may be required to identify extent of lesion.

• 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 and flex spine.
  In fat animals it may be easier to position patient in sternal recumbency and pull hindlimbs forward.

Step 1 - Introduce needle

• Using aseptic technique insert needle into subarachnoid space.
• Cisternal puncture:
  • Palpate the wings of the atlas and occipital crest.
  • 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.
  • Angle bevel of needle caudally.
  • 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 occipital plate into atlanto-occipital space.
• Lumbar puncture:
  • Palpate the L5 or L6 dorsal spinous process.
  • Insert needle at caudal aspect of L5 or L6 spinous process and advance at 45° angle cranioventrally towards the midline.
  • Advance needle - a 'pop' may be felt as needle penetrates ligamentum flavum to dura.
  • If CSF visible remove some for analysis.
  • If no CSF visible advance needle through spinal cord until it contacts floor of vertebral canal.
  • A tail twitch or a leg jerk may result.
  • Withdraw the needle slightly until flow of CSF is visible.
  • Angle bevel of needle caudally.

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.
  If CSF appears cloudy do not inject contrast - may indicate meningitis.
• If CSF appears abnormal perform cytological examination before proceeding with myelography.

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-0.5 ml/kg contrast (depending on the anticipated distance of the lesion from the site of puncture):
  • 0.2-0.3 ml/kg for cervical study.
  • 0.3-0.45 ml/kg for thoracolumbar study.
  • 0.5 ml/kg for lumbar study.
  Contrast should flow easily - stop if resistance felt.
• Withdraw needle.

Step 4 - Obtain radiographic views

• Take lateral and ventrodorsal views of all levels of the spine.
  Oblique projection may be useful if asymmetrical lesion involving nerve roots.

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.
  • May require lumbar injection in addition to cisternal puncture in some lumbar or thoracolumbar lesions if extent of lesion is not clear.

Step 2 - Additional radiographic procedures

• May be useful to take extended and flexed views to gain additional information.
• Oblique views may be useful, eg some vertebral disc prolapses .

Potential complications

• Ataxia may be seen for up to 24 hours after myelographic studies (but usually resolves spontaneously).
• Seizures - treat immediately with intravenous diazepam Diazepam.
• Myelography may exacerbate pre-existing neurological signs.

• Epidural contrast placement.

Refereed papers

• Recent references from PubMed and VetMedResource.
• Widmer W R & Blevins W E (1991) Veterinary myelography - a review of contrast media, adverse effects and techniques. JAAHA 27 (2), 163-77 VetMedResource.
• Pardo A P & Morgan J P (1988) Myelography in the cat - a comparison of cisternal versus lumbar puncture using metrizamide. Vet Radiol 29 (2), 89-95 Wiley Online Library.
• Wheeler S, Clayton-Jones D G & Wright J A (1985) Myelography in the cat. JSAP 26 (3), 143-152 Wiley Online Library.