Basic Principles and Techniques

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The beginning of myelography has its "roots" in the early 1900s when subarachnoid injections of air were performed to localize spinal cord tumors. This technique of pneumomyelography was eventually supplanted by the subarachnoid injection of an iodized poppy seed oil, Lipiodol, the myelographic medium of choice during the 1930s. Lipiodol had its disadvantages, including a very high viscosity, its immiscibility with CSF (thereby making it difficult to perform satisfactory diagnostic studies as well as remove on completion of the study), and its irritating nature to the leptomeninges. In 1940, Pantopaque was introduced. This oil-based contrast agent was less viscid and less irritating to the leptomeninges than Lipiodol. Its lower specific gravity allowed it to flow more freely in the subarachnoid space with less tendency to form large irregular globules; and, accordingly, removal on completion of the study was easier to perform than its predecessor. Pantopaque soon became the contrast medium of choice in the United States until the late 1970s, although it was still far from being an ideal subarachnoid contrast agent. Its relative immiscibility in CSF resulted in its inability to enter nerve root sleeves, thereby making it difficult to evaluate far posterolateral disc abnormalities. A 20-gauge spinal needle or larger was required during Pantopaque insertion and removal, and the use of this relatively large size needle commonly resulted in spinal headaches from CSF leakage at the puncture site. Furthermore, Pantopaque was still difficult to remove in entirety after completion of the study. When left behind, Pantopaque was absorbed slowly at the rate of approximately 1 ml/yr, which could produce local changes of arachnoiditis.

In the early 1930s experimental studies were carried out

Figure 23-17 A, MRV shows loss of flow related signal in the proximal and distal and straight sinuses (SS) and superior sagittal sinus (SSS) in a patient with dehydration and dural sinus thromß, Follow-up MRV showed reconstitution of the SSS and SS.

with Abrodil, a water-soluble organic iodine compound widely used in the Scandinavian countries for lumbar myelography. Abrodil never achieved popularity in the United States as a result of its marked hypertonicity, in comparison to CSF, which produced significant irritation to the leptomeninges and neural elements. Water-soluble myelography was revitalized in the United States during the early 1970s with the introduction of Conray and Dimer-X, salts of iodinated acids. These compounds, like Abrodil, caused significant neurotoxic effects, including meningeal and radicular irritation, as a result of their hypertonicity.

As a result of ongoing investigations, metrizamide was marketed as a synthetic nonionic water-soluble contrast medium that had a much lower osmolality than the salts of iodinated acids. This contrast agent provided safe examination of the entire spinal column without significant neurotoxicity.

Within recent years, in an attempt to further reduce the incidence of serious side effects, other nonionic water-soluble contrast agents, such as Isovue and Omnipaque, have been synthesized and are now in wide use. In many institutions, iopamidol (Isovue) is used for subarachnoid injections. In the concentration used for lumbar myelography (200 mg iodine/ml), Iopamidol is only mildly hyperosmolar when compared with CSF (413 vs. 301 mOsm/kg H2 O) and has dramatically fewer neurotoxic side effects than the salts of iodinated acids. Iopamidol is rapidly excreted in the kidneys with undetectable plasma levels at 48 hours in the absence of renal dysfunction. The most common adverse reactions are headache, nausea, vomiting, and transient worsening of neck, back, or leg symptomatology. Hypotensive collapse and shock are rare, with only isolated reports being found in the literature. The usual recommended adult dose range for iopamidol when performing myelographic procedures is 2000 to 3000 mg iodine. This is equivalent to 10 to 15 ml Isovue-M200, which is used for lumbar myelography, and to 10 ml maximum Isovue-M 300, which is used for cervical myelography via lumbar injection.

Before a myelogram is performed, certain information should be obtained from all patients. In those individuals with a prior history of dye allergy, bronchial asthma, hay fever, and food allergies, premedication with corticosteroids and antihistamines is indicated to prevent or minimize possible allergic reactions. Patients should also be well hydrated before and after the myelographic procedure. Dehydration may contribute to acute renal failure in patients with a history of diabetes, advanced vascular disease, or pre-existing renal disease. Knowledge as to current prescription and nonprescription medications as well as underlying disease states is extremely important. Specifically, elective studies should not be performed on patients who are receiving oral or intravenous anticoagulants, and the use of aspirin should be discontinued for approximately 48 hours before the myelographic procedure. Drugs that lower the seizure threshold such as monoamine oxidase inhibitors, tricyclic antidepressants, and central nervous system stimulants should be withheld for 48 hours before and after the myelographic procedure. Routinely, serum creatinine, blood urea nitrogen, and coagulation profile including prothrombin time, partial thromboplastin time, and platelet counts are obtained within the week before the myelographic procedure.

During lumbar myelography, a 22-gauge spinal needle is inserted either in the midline between the spinous processes or slightly off midline through the rhomboid fossa at the level of L2-L3. This puncture site is preferred over the L3-L4 through L5-S1 levels because of the higher incidence of herniated discs and spinal stenosis at these lower levels. Alternatively, if premyelography imaging studies or certain clinical information is available, the puncture site can be performed at these lower lumbar levels when significant disease is known to exist at the level of L2-L3. The lumbar injection should be performed slowly over a period of 1 to 2 minutes to avoid excessive mixing with CSF and subsequent loss of contrast medium as well as premature cephalad dispersion.

In most instances, cervical myelography can be adequately performed by means of a lumbar puncture with subsequent fluoroscopic guidance of a more concentrated dose of contrast medium into the cervical region by gravity. Alternatively, a lateral C1-C2 puncture can be performed with the needle placement in the posterior third of the spinal canal between the C1-C2 neural arches. At all costs, intracranial entry of a bolus of contrast medium should be avoided because this could possibly lead to acute neurotoxic effects.

Meticulous radiographic technique is essential to obtain quality diagnostic examinations. Low voltage (Kvp) with optimal milliampere-seconds (mas) maximizes contrast detail. Air gaps should also be eliminated to reduce scatter and increase edge sharpness. In both the lumbar and the cervical regions, AP, lateral, shallow, and steep oblique views should be obtained with the addition of AP and lateral views of the conus during lumbar myelography. For thoracic myelography, AP and lateral views are routinely filmed.

Postmyelography CT of abnormal disc levels should be performed in the prone position with imaging deferred for at least 4 hours to reduce the degree of contrast within the desired area to be studied. Before obtaining the postmyelogram CT scan, the patient should be helped to roll over several times to thoroughly mix the contrast medium and avoid pooling in dependent portions of the spinal canal.

Routine postprocedure orders include keeping the patient's head elevated 30 to 45 degrees for 12 to 24 hours with close neurological monitoring. Oral fluids should be encouraged and diet prescribed as tolerated. All movements should be monitored by hospital personnel and performed slowly with the patient's head maintained in an upright position.

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