Myelography with Water-Soluble Contrast Medium

Transcription

1 Acta Radiologica: Diagnosis ISSN: (Print) (Online) Journal homepage: Myelography with Water-Soluble Contrast Medium Joe P. Morgan, Peter F. Suter & Terrell A. Holliday To cite this article: Joe P. Morgan, Peter F. Suter & Terrell A. Holliday (1972) Myelography with Water-Soluble Contrast Medium, Acta Radiologica: Diagnosis, 12:sup319, To link to this article: Published online: 04 Jan Submit your article to this journal Article views: 39 View related articles Full Terms & Conditions of access and use can be found at

2 FROM THE DIVISIONS OF RADIOLOGY AND MEDICINE, DEPARTMENT OF CLINICAL SCIENCES, SCHOOL OF VETERINARY MEDICINE, UNIVERSITY OF CALIFORNIA, DAVIS, CALIFORNIA. MYELOGRAPHY WITH WATER-SOLUBLE CONTRAST MEDIUM Radiographic interpretation of disc herniation in dogs JOE P. MORGAN, PETER F. SUTER and TERRELL A. HOLLIDAY The advent of myelography with water-soluble contrast media has overcome many of the limitations of conventional radiography in diagnosing spinal diseases in the. dog. The trend to perform decompressive surgery in the treatment of canine spinal conditions caused by epidural and spinal masses is increasing the need for a precise way to determine the location and extent of the compression. To date, the potential benefits of myelography have been challenged by technical difficulties and by undersirable effects related to the water-soluble contrast media utilized. Descriptions of the specific information to be gained from myelography are limited, and the meaning of many radiographic findings is unclear. Under these circumstances, it is difficult to evaluate myelography relative to non-contrast radiography in terms af advantages vs. risks. The purposes of this report are to describe the findings associated with herniation of the intervertebral disc in 127 dogs on which myelograms were

3 218 J. P. MORGAN ET COLL. performed and to discuss the value and interpretation of these studies. Appreciation of its diagnostic limitations and benefits will allow informed decisions on when myelography may be an effective supplement to non-contrast radiographic procedures. Literature The technique of myelography in dogs, utilizing water-soluble contrast media injected into the subarachnoid space at a lumbar puncture site, the technique and its side effects (Funkquist 1960, 1961, 1962 a, Bullock 8c Zook 1967), the advantages and disadvantages of different contrast media (Hoerlein 1965, Olsson 1966), the relation of observed signs to underlying pathoanatomic changes, and the value of the information gained by the contrast study (Funkquist 1962 b) have been reported. The study of a large series of cases has shown the superiority of myelographic studies in consistently exposing both the cause and location of the spinal disorder, especially in lesions due to herniated discs (Bullock & Zook 1967). The information gained from myelograms was considered to be of critical importance to the surgeon when decompressive laminectomy was planned, and the inherent risks of the procedure were considered acceptable. One argument against the use of myelography in dogs has been the belief that, if both radiologist and neurologist were familiar with the condition, the occurrence of spinal cord compressive lesions not outlined on regular radiographs was rare (Hoerlein 1965). Because of the lack of an ideal contrast medium, myelography has been advocated only under certain conditions. Such conditions were: negative findings on non-contrast radiographs of a dog with a positive clinical diagnosis; strong evidence of spinal cord compression but with a clinical picture atypical for disc protrusion; and in cases pending surgery in which there was an inability to clearly identify the protruded disc (Olsson 1966). Material and methods All myelography was performed between July 1967 and August 1970 at the Veterinary Medical Teaching Hospital, University of California, Davis. Examinations that were not considered technically, adequate were excluded. Radiographic diagnoses of herniated discs were confirmed by surgery andlor necropsy in 82 of 127 dogs. The other 45 dogs either underwent clinical improvement or were unavailable for post-mortem examination. Since the Veterinary Medical Teaching Hospital serves primarily as a

4 MYELOGRAPHY 219 W I U % < J < '- CERVICAL SPINE THORACIC SPINE LUMBAR SPINE Fig. 1. Location of herniated intervertebral discs as determined in 127 myelograms performed by subarachnoid injection of water-soluble contrast medium. referral hospital, cases tend to be selective. Many cases were referred for disc surgery because of acute and severe nature of the disease or lack of response to conservative therapy; others were referred because of uncertain diagnoses. After clinical examination and non-contrast radiography at the teaching hospital, a further selection was done. If the site and extent of the disc herniation could be determined from a plain radiograph and if radiographic findings agreed with clinical signs, myelography was not considered necessary. Cases in which severe clinical signs were suggestive of hematomyelia were not examined by myelography. Additionally, no myelograms were performed until it was determined that laminectomy or fenestration could be performed if indicated by the myelographic findings. Methiodol sodium* (40 O/o) was diluted with physiologic saline or bacteriostatic water to 2Oo/o (in some cases to 30 O /o) and mixed with 0.5 ml of Xylocaine't:t (2O/o) for every 9.5 ml of diluted contrast! medium. The dosage of the methiodol sodium mixture was 0.3 ml/kg of body weight. In dogs with suspect cervical lesions, the amount of contrast was increased to 0.4 ml/kg. * Skiodan, Winthrop Laboratories, New York, N.Y ** Astra Pharmaceutical, Worcester, Mass.

5 220 J. P. MORGAN ET COLL. Fig. 2. Lateral projection of a myelogram of the thoraco-lumbar region with 3 Type I1 disc herniations. Narrowing of the subarachnoid space, dorsal shifting of the spinal cord, and narrowing of the spinal cord are present at Ti-n-Li and Ts-10. Similar but less severe changes are noted at Lz-La. Tu--lo and Ln--n were considered normal on the plain radiographs. The contrast medium was rapidly injected through an gauge spinal needle, the tip of which had been placed into the subarachnoid space by percutaneous puncture. With only a few exceptions, the punctures were done between vertebrae L5 and LO or L4 and L5. At least five radiographs were made in all cases, according to the following pattern: (1) lateral recumbent view of the thoraco-lumbar region immediately after the injection, prior to removal of the spinal needle; (2) ventro-dorsal view of the thoraco-lumbar region following removal of the needle; (3) opposite lateral recumbent view of the thoraco-lumbar region; (4) coned-down lateral view of the thoraco-lumbar junction; and (5) lateral view of the cervico-thoracic region. Additional views of the cervical region were made if indicated by clinical signs. Among the 127 dogs, only 13 dogs were of large breeds such as the German Shepherd; the rest were both chondrodystrophoid and non-chondrodystrophoid small breeds weighing less than 14 kg. Dachshunds significantly outnumbered all other breeds combined. Results A pattern of seven specific radiographic changes associated with disc herniations was noted: (1) changes in width of the contrast column in the subarachnoid space; (2) obliteration of the subarachnoid space; (3) change in size of the spinal cord; (4) shift in position of the spinal cord with or without concurrent change in the subarachnoid space; (5) persistence of contrast medium in the spinal cord for an extended time; (6) filling of the central canal;

6 MYELOGRAPHY 22 1 Fig. 3. Lateral projection of a myelogram of the thoraco-lumbar region with marked generalized narrowing of the subarachnoid space due to cord edema extending from Ti3 to L3. The spinal cord is lifted off the floor of the spinal canal at Li--2 because of a partially herniated disc. The secondary cord edema is frequently noted in association with Type I disc herniations. On plain radiographs, L3-4 had been considered as the principal lesion. and (7) pooling of contrast medium in the cauda equina. Other, less specific signs that were difficult to relate to patho-anatomic changes were also noticed. The location of the disc herniations observed is presented (Fig. 1). The meaning of the radiographic changes is discussed according to the structures involved. Subarachnoid Space. A marked difference in the width of the subarachnoid and epidural spaces was observed in unaffected portions of the cord among dogs of different breeds. In Dachshunds, the subarachnoid space was narrow and the width of the cord was almost equal to the diameter of the spinal canal at the thoraco-lumbar junction. In large dogs the subarachnoid space was wider, and a well-defined space was usually seen between the cord and the boundaries of the spinal canal throughout the length of the spinal cord. Narrowing of the subarachnoid space by pathological processes was the result of either external pressure on the cord and the meninges from epidural masses, or internal pressure from intramedullary masses. Both of these findings were noted in association with disc disease. Slowly herniating Type I disc lesions or old, fairly stable herniations were seen as a distinct mass that exerted pressure on the spinal cord and narrowed 'the subarachnoid space over a short distance, immediately dorsal to the diseased disc (Fig. 2). The same type of findings were seen in Type I1 disc lesions. The dorsal and lateral outlines of the subarachnoid space appeared normal in some cases and narrowed in others.

7 222 J. P. MORGAN ET COLL Fig. 4. Dorso-ventral projection of a myelogram of the same dog as in Fig. 3. Narrowing of the subarachnoid space from Ti3 to L3 due to cord edema is present. The spinal cord is displaced to the left as a result of herniated disc material at LI--2. The lateral displacement of the cord and the severe degree of compression could only be determined from the dorso-ventral projection, stressing the importance of the second view in determining the appropriate site for surgical intervention. Fig. 5. Dorso-ventral projection of the thoraco-lumbar region with narrowing of the spinal cord at LI--e and a persistence of contrast medium into the spinal cord. This radiographic change is due to a localized hematomyelia and constitutes an unfavorable prognostic sign.

8 MYELOGRAPHY 223 Fig. 6. Lateral projection of a myelogram (same dog as in Figs. 3 and 4) that illustrates the tendency of the contrast medium to accumulate in the cauda equina because of a restriction to cranial flow. More rapidly herniating Type I disc lesions in the chondrodystrophoid breeds caused more extended radiographic lesions because of the accompanying hemorrhage and inflammation (Fig. 3). No longer confined by the structures of the dorsal longitudinal ligament, the disc material was free to accumulate anywhere in the epidural space. Therefore, compressions of the subarachnoid space were found cranial or caudal to the intervertebral spaces from which they originated, and occasionally extended over an entire spinal segment. In other cases, compressions were seen lateral or dorsal to the cord. Quite often, narrowing of the subarachnoid space was recognized only on a lateral or dorso-ventral view. Spreading of the disc material caused a more generalized pressure on the cord and narrowing of a longer subarachnoid segment. Acute herniation also caused cord edema and narrowing of the subarachnoid space from within by a swelling extending over several segments. Because the dura is a rather rigid and non-elastic structure, narrowing of the subarachnoid space is inevitable in cord edema. In some cases, the subarachnoid space was completely obliterated in response to the dynamic changes described, perhaps because of severe cord edema. Severe edema was noted in 58 of the 127 cases, causing a complete collapse of the subarachnoid space. This change extended over many vertebral segments and was usually far more severe than could have been anticipated from the clinical signs. Since an intramedullary mass such as cord edema tended to extend quite evenly in all directions, the narrowing of the subarachnoid space was seen on both the lateral and ventro-dorsal views (Figs. 3,4). A distinct increased resistance to injection of the contrast medium, similar

9 224 J. P. MORGAN ET COLL. Fig. 7. Dorso-ventral projection of a myelogram of the thoracolumbar region (same dog as in Fig. 2) with filling of the central canal by contrast medium. Note that the disc herniation at Tis-L1 is barely discernible on the dorso-ventral projection. A slight shifting of the cord to the left side can be recognized by the wider epidural space on the right. to that encountered when the tip of the needle is misplaced in the dorsal longitudinal ligament, was noted in the cases of extended cord edema. In addition, there was a definite difference in the width of the subarachnoid space between radiographs made immediately after injection and those made 1 to 2 minutes later. The subarachnoid space was forced open by the pressure exerted by injection of the contrast medium and then collapsed again due to the increased local epidural pressure. It was always possible to force the contrast medium beyond the area of compression. In a few cases, it was difficult to decide between a spread-out Type I disc herniation and a localized cord edema. It was, therefore, arbitrarily established that the ques-

10 MYELOGRAPHY 225 Fig. 8. Lateral projection of the thoraco-lumbar region of a German Shepherd dog with a Type I1 herniated disc or long-standing Type I herniation at Tis-Li. The spinal cord is displaced dorsally and slightly narrowed. Contrast medium has accumulated in the epidural space caudal to and, to a lesser degree, cranial to the herniated disc. Note that the affected disc space is not narrowed. tionable lesion was due to generalized edema only if it extended over three or more vertebrae. The width of the subarachnoid space was found to depend upon the positioning of the cord within the spinal canal, which in turn depended upon the positioning of the animal, as will be shown later. Gravitation also appeared to affect the outlining of the subarachnoid space, necessitating a change in positioning between consecutive radiographs. Narrowing of the Spinal Cord. Narrowing of the spinal cord usually was noted in one view only, with or without narrowing of the subarachnoid space. This change was most often due to the presence of a distinct epidural mass of disc material, large enough to cause the cord to drape over the mass and be narrowed (Fig. 2). The several cases of uniform, extended narrowing of the cord were associated with retention of contrast medium and hematomyelia (Fig. 5): Widening of the Spinal Cord. Compensatory widening of the spinal cord on one view was often associated with narrowing of the spinal cord in the opposite perpendicular view, due to draping of the cord over an organized extradural mass. This finding was most typically seen in the lumbar area, where the width of the epidural space allowed a change in conformation of the cord. Widening of the spinal cord on both views did not occur with extradural masses due to disc herniation. Change in Position of the Spinal Cord. Shifting of the affected segment of the spinal cord without corollary narrowing of the subarachnoid space was uncommon. If the extradural mass was large enough, a certain degree of shift

11 226 J. P. MORGAN ET COLL. of the cord was noted in most cases, generally in the dorsal direction (Fig. 2), less often laterally (Fig. 5), and rarely ventrally. Displacement of the cord was most easily seen in areas where the spinal canal was wide enough to allow displacement, such as the lumbar area. A rather common cause of the shifting of large portions of the cord was positioning of the spinal column. Because the spinal cord is attached at the foramen magnum and at the cauda equina, it is free to move dorsally or ventrally or laterally in response to the dog s extension, flexion or lateral bending. This change could be followed on the radiographs and was most easily seen in the cervical area and cervico-thoracic junction; in some cases it was also seen in the thoraco-lumbar area. Persistent Contrast in the Spinal Cord. The persistent presence of contrast medium within the spinal cord was a rare but significant finding, indicating severe damage to the cord substance such as hematomyelia (Fig. 5). In the cases of severe cord damage, the contrast did not stay in the subarachnoid space, but infiltrated the cord tissue and could be seen on radiographs immediately following injection and remaining up to 4 or 5 hours after injection. Filling of the Cauda Equina and the Central Canal with Contrast Material. There was no indication that the presence of a cord edema favored a tendency of the contrast medium to pool in the cauda equina (Fig. 6) or to fill the central canal (Fig. 7). Outlining of the central canal occurred under two distinctly different circumstances. Most often filling of the central canal was due to the placement of the tip of the needle within the cord near the central canal while the contrast medium was injected. Filling of the canal was not seen when the needle was introduced between the 5th and 6th lumbar vertebrae, but occurred in some cases in which the puncture was made between the 4th and 5th lumbar vertebrae. A second type of filling of the central canal, unrelated to the injection site, occurred as a sudden change in location of some of the contrast material from the subarachnoid space into the central canal. In such cases, the central canal was outlined from the disc lesion cranially, whereas caudal to the lesion the cord appeared unremarkable. In some cases, the central canal, when filled with contrast, appeared small and well-defined; in others, it was 1-2 mm wide and rather ill-defined. Death occurred in a high number of cases in which the central canal was outlined, especially when it was ill-defined and presented as a wide shadow. Miscellaneous Findings. In almost all cases a small amount of contrast material was seen in the epidural space. The outlining of the epidural space, especially over the disc area, can cause a rather confusing radiograph. In a few cases, some extradural contrast material extended cranially up to the area of the extradural mass (Fig. 8). The cranial end of the extradural

12 MYELOGRAPHY 22 7 DISPLACEMENT OF (57) OR WIDENING ( I02 1 OF CCKD NARROWING OR OCCLUSION OF THE SUBARACHNOID SPACE (127) Fig. 9. A Venn diagram illustrates the interrelationships between the three major myelographic changes noted to occur with herniated discs. The intersections between circles portray simultaneous occurrence of more than one myelographic change per study. contrast column was, however, a rather unreliable sign for the location of the lesion because stoppage could occur one to two vertebral bodies caudally to the actual lesion. Discussion Technique. Technical factors influence the outcome of a myelogram to a great extent, and the method of rnyelography can be altered in several aspects. In a few cases, the concentration of the contrast medium was increased to 30 O /o, to improve delineation of the subarchnoid space. However, concentrations of over 200/0 involve risks related to the adverse effects of osmotic factors (Funkquist & Obel 1961). In miniature breeds and in dogs with a wide subarachnoid space, dosages based on body weight were too low to give a clear outline up to the cervical area. In stocky or obese dogs, dosages were too high and, in some cases, contrast medium passed into the subarachnoid space of the brain resulting in tonic seizures lasting several hours. Volume of the contrast medium could probably be more accurately calculated on the basis of crown-rump measurement than on body weight.

13 228 J. P. MORGAN ET COLL. A cisternal tap may serve to improve the cervical myelogram (Funkquist, 1961); but it also facilitates the progression of the contrast medium into the subarachnoid space of the brain and diminishes the dilution of the contrast material resulting in a potentially harmful increase in the osmotic pressure. Elevation of the forequarters can be used to minimize those adverse effects (Olsson 1966). Flexion and extension of the vertebral column move the cord within the spinal canal and have been recommended specifically as a technique to assist in the determination of cervico-spinal compressions (Funkquist 196 1). The cord is shifted away from small ventral spinal compressions when the neck is bent dorsally. To visualize lesions in the caudal cervical and cranial thoracic spine, the neck should be bent ventrally. Head and neck should be extended if cranial and middle parts of the cervical lesion are of primary interest (Funkquist 196 1). Occasionally, uneven distribution of contrast medium caused insufficient outlining of the subarachnoid space in non-diseased areas. To overcome this unevenness, radiographs were made in varying positions. Radiographs should be made immediately after the injection; otherwise the extradural mass or the spinal edema may force the contrast medium out of the area of specific interest. A substantial variation between the first and second radiograph has been described (Funkquist 1962 b). Radiographic Findings. Myelograms exactly determine the level of the disc herniation, including cranial and caudal boundaries. Non-contrast radiographs were non-diagnostic in cases. In 21/37 cases, the suspect level of compression was incorrect; and in 16/37 cases, the findings were inconclusive or negative. Ambiguous or incorrect interpretations were often due to more than one suspect area in the same dog. Two discs were suspect in 20 dogs and three discs were suspect in 6 dogs. A myelogram is not necessary if two suspected compression sites can be easily reached by the same incision. However, if suspicious areas are several vertebral bodies apart, a myelogram is required. In previously decompressed dogs, only the myelogram determines whether compression is due to a new herniation or secondary scar formation. In a few cases, malacia of the cord was determined by myelography, and surgical intervention was not considered because of the poor prognosis. Type I protrusions were found in increasing numbers in old dogs of large breeds, usually with signs of degenerative spinal disease. In the small breeds, the myelogram revealed the location and spread of the prolapsed material. The surgeon who is aware of the exact location of the compressing mass has a choice between dorsal laminectomy and which side to approach the compress-

14 MYELOGRAPHY 229 ing mass in hemilaminectomy. Thus, myelography facilitates surgery by diminishing the risks and surgical trauma; and the prognosis is improved if the type of the disc lesion and the extent of cord damage are known in advance. Since disc herniation is common in aged dogs of the chondrodystrophoid breeds, a diagnosis of an extradural mass due to disc herniation must be considered first. Even in non-chondrodystrophoid breeds, the incidence of disc herniation exceeds that of other extra- and intradural masses. Extradural masses originate from bone or soft tissues surrounding the cord and may or may not be neoplastic. Osteolysis in the vertebral body, the vertebral arch, or an increase of the size of the intervertebral foramen are indicative for neoplasm. If bony changes are absent, as occurs in most epidural and medullary tumors, myelography becomes essential. Intramedullary tumors were characterized by a spindle-shaped widening of the cord, by a compromised subarachnoid space, and by absence of cord displacement. Occasionally, the cord was diffusely enlarged. In disc protrusions, displacement of the cord was seen in about half the cases, with about even frequency in lateral and dorso-ventral views. Cord width was altered in about 90 /o of all cases and a change in width of the subarachnoid space was seen in all cases. Change in cord position was less frequent than the other two alterations due to the rather small diameter of the spinal canal in the thoraco-lumbar region in chondrodystrophoid breeds, providing no space for the cord to move. Alteration in cord width was seen in about half of the cases in the lateral view only, in about 25O/o of the cases in the ventro-dorsal view only. In the remaining 25O/o, a narrowing in one view, usually the lateral one, was accompanied by a widening in the perpendicular view, usually the ventro-dorsal one. The changes are subtle and must not be confused with the normally-occurring change in diameter in the lower cervical and lumbar region. In 8Oo/o of the cases, narrowing of the subarachnoid space was visible on two views. In the rest of the cases, it was seen with equal frequency on either view. Cord edema was responsible for an even narrowing of the subarachnoid space in the two projections seen in a high number of cases. Since some of the changes are visible on one view only, it is essential to have good radiographs made in both projections of the area involved. The various combinations of the major radiographic signs of the myelogram are illustrated in a Venn diagram (Fig. 9). Filling of the central canal occurred in about 10 /o of the cases where the injection had been done between L4 and L5. Especially when the central canal appeared dilated and ill-defined, it was considered a grave sign.

15 230 J. P. MORGAN ET COLL. The 127 myelograms constituted a select material but, with this restriction, we suggest that a myelographic study and its inherent risks are warranted more often than is generally accepted. A myelogram should be done in all cases where decompressive surgery is contemplated and whenever clinical and/or radiographic signs leave doubt about the level and extent of the lesion. Myelography is rarely indicated in cases of disc compression that will be treated conservatively unless the diagnosis remains doubtful or clinical response is not as expected. We believe that clinical signs of myelomalacia should exclude myelography. Myelography of the cranial thoracic and cervical area is less successful in large dogs and carries a greater risk than in small breeds; the reason is not known. Additional disadvantages of the procedure in large dogs are difficulties in introducing the needle into the subarachnoid space of the lumbar area and in reading the myelograms, due to heavy rib shadows overlying the thoracic spinal canal and the massive density of the cardiac silhouette. SUMMARY The literature on myelography in the dog is reviewed. The technique is described and the result in 127 cases of disc protrusion is reported. Myelographic findings are correlated with subsequent surgical or post mortem observations. Indications and contraindications are discussed. REFERENCES BULLOCK L. P. and ZOOK B. C.: Myelography in dogs, using water-soluble contrast mediums. J. Amer. vet. med. Ass., 151 (1967), 321. FUNKQUIST B.: Lumbar subarachnoid puncture and injection in the dog. Nord. Vet. med., 12 (1960)* Cervical myelography with a water-soluble contrast medium. An experimental study in dog. Acta Radiol., 56 (1961), Thoraco-lumbar myelography with water-soluble contrast medium in dogs. I. Technique of myelography; side-effects and complications. J. small Anim. Pract., 3 (1962), 53 (a). -Thoraco-lumbar myelography with water-soluble contrast medium in dogs. 11. Appearance of the myelogram in disc protrusion and its relation to functional disturbances and patho-anatomic changes in the epidural space. J. small Anim. Pract., 3 (1962), 67 (b). -and OBEL N. : Effect on the spinal cord of subarachnoid injection of water-soluble contrast medium. An experimental study in dogs. Acta Radiol., 56 (1961), 56. HOERLEIN B. F.: Canine Neurology. Diagnosis and Treatment. W. B. Saunders Go., Philadelphia, OLSON S.-E.: In: Intervertebral Disc Protrusion in the Dog. Edited by G. D. Pettit, p. 82. Appleton- Century-Crofts, New York, 1966.