Myelography

Myelography is a radiographic or CT study of the spinal canal, the thecal sac, the spinal cord, and the nerve roots. It is performed after a water soluble, non-ionic iodinated contrast agent (for example 10 to 15 mL of a 180 to 300 mg iodine in each mL agent) is injected into the subarachnoid space.

The injection is made by lumbar puncture (usually L3-L4 or L4-L5, with a 22 to 25 gauge needle), or by cervical puncture (at C1-C2 under fluoroscopy) when the lumbar route is blocked. Fluoroscopic images are taken with the patient tilted to move the contrast along the canal, and CT myelography is done at once and at about 4 to 6 hours or later to show the details.

Intrathecal gadolinium and radionuclide cisternography are used for cerebrospinal fluid leaks. The effective dose is about 3 to 6 mSv for plain myelography, and 6 to 12 mSv for myelography combined with CT of a region.

MR imaging has replaced myelography for most spinal conditions, as it is non-invasive, shows the cord and discs in detail, and does not use ionising radiation.

Myelography with CT remains valuable:

• Where MR cannot be performed (pacemakers, metal implants, claustrophobia).
• Where metal artefact degrades MR.
• Where MR findings do not fit the symptoms.
• For nerve root avulsion after brachial plexus injury, spinal dural fistula and cerebrospinal fluid leaks.
• In the dynamic assessment of canal stenosis in different postures.

Indications. Suspected spinal canal stenosis, nerve root compression or disc herniation when MR is contraindicated or not available. Planning of surgery in complex spinal stenosis (including dynamic studies in flexion and extension). Pre-operative and post-operative assessment where metal causes artefact on MR. Brachial plexus injury (root avulsion shown by pseudomeningoceles).

Suspected arachnoiditis, spinal arteriovenous malformation or dural fistula (with angiography). Cerebrospinal fluid leak (spontaneous intracranial hypotension, post-operative or traumatic). Suspected spinal tumours and syrinx when MR is not available. Cord or root lesions after trauma where MR is contraindicated. Not for first line investigation of back pain, for which MR is used.

Contraindications and precautions. Local skin infection at the puncture site, suspected meningitis, or bacteraemia. Raised intracranial pressure (risk of herniation after lumbar puncture) and intracranial mass lesions: image the brain first.

Coagulopathy (international normalised ratio over about 1.5 for lumbar puncture, platelets under 50 000 in each microlitre). Anticoagulant treatment (stopped for the time set by the local rule, usually 12 to 24 hours for low molecular weight heparin, and 5 days for warfarin with a normal international normalised ratio).

Allergy to iodinated contrast (intrathecal non-ionic agents are not interchangeable with intravascular use, and the dose and the type are checked). Recent lumbar puncture (within about 1 to 2 weeks, with leakage of contrast and headache).

Seizure disorders, because contrast lowers the seizure threshold; drugs that lower seizure threshold (tricyclic antidepressants, phenothiazines, tramadol and others) are withheld for 48 hours before and after. Pregnancy: avoid. Lying flat for a short time after the study, with the head raised by 30 to 45 degrees, reduces headache and seizure risk, and encouraging fluids helps.

Preparation and technique. Informed consent, a check of medication and coagulation, and hydration. The patient lies prone (or on the side) and the lumbar needle is placed under fluoroscopy with local anaesthetic.

Cerebrospinal fluid is sampled for tests if needed, the contrast is injected slowly, and the table is tilted to run the contrast along the canal, with images in the anteroposterior, lateral and oblique projections, including flexion and extension views for the cervical and lumbar spine. After fluoroscopy, CT is done at the area of interest, with sections of 1 to 3 mm and multiplanar reformats.

The patient rests with the head raised for several hours and drinks plenty of fluids.

Normal appearances. The contrast fills the thecal sac as a smooth column with a symmetrical outline, with the spinal cord as a central filling defect (cord diameter at the cervical enlargement about 10 to 13 mm, thoracic 8 mm and conus tip at L1 to L2). The lumbar nerve roots form the cauda equina, which are thin, linear filling defects within the sac.

Nerve root sleeves are filled with contrast for a short distance, are symmetrical, and taper smoothly. The anteroposterior diameter of the lumbar canal is over 12 mm, and the cross-sectional area of the thecal sac is over 100 mm2 in the lumbar spine, with a cervical canal diameter over 13 mm (under 10 mm is stenotic). The dural sac may show mild indentation at the disc levels.

Interpretation.

• Extradural lesions: disc herniation, osteophyte (a defect in the contrast column with indentation of the sac, cutoff of the nerve root sleeve, or an "hourglass" deformity of the sac).
• Spinal stenosis: effacement of the contrast column, a "waist" at the stenosis, and a nerve root redundancy proximal to it.
• Facet joint hypertrophy and ligamentum flavum thickening.
• Metastases or abscess with a block of contrast flow ("paintbrush" or "comb" pattern of the block margins).
• Intradural extramedullary lesions: meningioma and nerve sheath tumour (neurofibroma, schwannoma) show a smooth, rounded mass that widens the subarachnoid space on one side with displacement of the cord to the opposite side ("capping" of contrast like a meniscus or "cup" of the lesion). Drop metastases show multiple nodules.
• Intramedullary lesions: widening of the cord, with thin rims of contrast on both sides (a "fusiform" enlargement) in tumours such as ependymoma and astrocytoma, and in syrinx, oedema or inflammation (post-myelographic CT shows contrast entering a syrinx on a delayed scan).
• Arachnoiditis: clumped, thickened or adherent nerve roots that stick to the sac wall ("empty sac" sign, "candle guttering" appearance), loss of root sleeves, and a block to contrast.
• Nerve root avulsion: a pseudomeningocele at the root sleeve (a contrast filled outpouching) or a missing root, common after brachial plexus injury.
• Spinal arteriovenous malformation and dural fistula: serpiginous filling defects on the dorsal surface of the cord (a tangle of dilated vessels).
• Cerebrospinal fluid leak: extradural contrast collection (extravasation) on delayed CT, and a meningeal diverticulum or a nerve root sleeve cyst as a source of leak.
• Tethered cord and diastematomyelia: low conus (lower than L2 to L3), thick filum, a bony or fibrous septum dividing the cord.
• Chiari malformation and syrinx: crowded foramen magnum with tonsillar descent, and cord cavities.
• Post-operative: fibrosis, retained fragments, pseudomeningocele, and recurrent stenosis in the failed back surgery syndrome.
• Complete block: contrast stops (a total block shows a cutoff with a "paintbrush" appearance), and a second injection on the cranial side of the block (via the cervical route) defines the upper extent; cervical puncture is used when a lumbar block is suspected.

Critical values and findings needing immediate action.

• Spinal cord compression (a complete or near complete block with a mass, abscess, haematoma or epidural metastasis): urgent neurosurgical referral and corticosteroids as directed by the neurosurgeon, as delay causes permanent paralysis.
• Cauda equina syndrome (saddle numbness, urinary retention, bilateral leg weakness): emergency decompression.
• Spinal epidural abscess or discitis with block: antibiotics, urgent drainage and neurosurgical review.
• Intrathecal contrast reaction: seizure (diazepam or lorazepam), encephalopathy, aseptic meningitis, hallucinations and coma if high dose reaches the brain: supportive care, anticonvulsants and neurological review.
• Severe post-dural puncture headache, meningitis or haemorrhage: supine rest, fluids, analgesia, and blood patch if persisting; urgent treatment for meningitis.
• Findings of intradural tumour with progressive deficit: urgent neurosurgical review.

Limitations and artefacts.

• Myelography is invasive and carries risks: headache (up to 30%, mostly mild, lasting 1 to 3 days), nausea and vomiting, back pain, seizure (rare), meningitis (rare), bleeding, nerve root injury and arachnoiditis, and contrast reactions; the risk is reduced with a fine needle and hydration.
• Plain myelography shows the sac and roots only indirectly and cannot show the cord substance or the disc itself; MR and CT are needed for this. A nerve root that is compressed laterally in the foramen may not show on the myelogram, as contrast may not fill the root sleeve.
• A failed puncture, extradural or subdural injection, and air bubbles give a poor study, and contrast outside the sac mimics lesions.
• A complete block prevents the contrast reaching the region on the cranial side, and a second cervical puncture can be dangerous in cord compression (the cord may deteriorate after lumbar puncture caudal to a block).
• Positioning and tilt change the contrast distribution, so a single view can miss lesions; dynamic views and CT improve sensitivity.
• Dilution of contrast on delayed CT, and flow artefact in cerebrospinal fluid, degrade images.
• Metal artefact causes less trouble than in MR, but streak artefacts around screws occur on CT.
• Radiation dose is moderate, and the examination is not repeated without reason.

Special groups.

• Older adults: stenosis of the canal, osteoporotic fractures and degenerative disease are the commonest conditions; renal function is checked less critically for intrathecal use, but hydration is encouraged.
• Children: rare, MR first; if myelography is needed (for example in cord tethering when MR is not possible), sedation or anaesthesia is used with lower contrast doses.
• Patients with pacemakers or metal implants: myelography with CT is the alternative to MR.
• Patients on anticoagulants or antiplatelet drugs: withheld as the local rule requires, and the study may be replaced with another test.
• Pregnancy: avoid.
• Regions: tuberculous spondylitis (Pott disease) with cord compression, spinal epidural abscess and delayed presentation of tumours are common, and myelography with CT may be the only method to define a block when MR is not available.

Follow up tests.

• MR of the spine and cord with contrast.
• CT of the spine, and CT myelography after plain myelography.
• Radionuclide cisternography or intrathecal gadolinium MR for cerebrospinal fluid leak.
• Spinal angiography for vascular lesions.
• Nerve conduction studies and electromyography.
• Cerebrospinal fluid analysis (cells, protein, glucose, culture, cytology).
• Full blood count, erythrocyte sedimentation rate, C-reactive protein and tuberculosis tests.
• Referral to neurosurgery, orthopaedic spine surgery or neurology.