If nothing is seen, is anything there?

The radiograph was discovered more than 100 years ago. It has been a stalwart of diagnostic imaging in veterinary medicine. We have grown accustomed to its use. We use it to "rule out" numerous diseases. Unfortunately, we have known that it takes a marked change in the electronic density of the material to be seen radiographically. Some estimates indicate that in bone, radiographic density must change by 30-50 percent (either increase or decrease) in order to be visible on a standard radiograph. Therefore, numerous diseases can be present and escape the detection limit of a radiographic change. We must remember that just because we don't see it, doesn't mean it is not there.

Figure 1: Contrast myelogram. (1) Sagittal image through the thoracolumbar region. T9 is labeled. No significant abnormalities are detected. (2) Ventrodorsal views through the same region. Again, T9 is labeled. No abnormalities are seen. (3) Contrast material in the dorsal epidural space of the cervical spine is seen. The animal was on his back during radiography, hence, the pooling in the dorsal subarachnoid space.

Magnetic resonance imaging (MRI) provides a different method of diagnostic imaging. MRI visualizes the chemistry of the various tissues of the body. The chemical change that occurs with numerous disease states can be detected at an earlier state than the secondary radiographic lesion.

"Tucker" is an 8-year-old Boxer with intermittent paraparesis, spinal pain, and at times, some right forelimb lameness. Tucker responded initially to nonsteroidal anti-inflammatory medication. Tucker's condition became worse and he underwent a radiographic study, including a contrast myelogram and CSF analysis. Figures 1A-C are the high quality myelograms that were obtained. No significant abnormalities were seen in the contrast columns within the subarachnoid space in the thoracolumbar spine, with the exception of apparent extravasation of the contrast material into the epidural space in the cervical spine. Since the tap was performed with an L5-6 space, and no significant epidural injection was encountered at that site, this would imply that a lesion must be present within the dura, allowing the material to pass from the subarachnoid space to the epidural space. The CSF analysis indicated mild inflammation. With this information and some mild forelimb signs, a cervical magnetic resonance imaging study was performed. That study did not reveal a significant lesion from C1 through T2.

Little clues

Tucker then proceeded to become unable to rise and a subsequent MRI study was performed at the IAMS Pet Imaging Center in Vienna, Va. Tucker underwent a standard screening magnetic resonance imaging study that studies the spinal column from T3 to S3. Numerous pulse sequences are used to visualize the spine. Some of these are high contrast, lower resolution, while other studies are lower contrast, high resolution. The studies revealed an abnormality at T9-10. There was an increased fluid signal in the intervertebral disc space at this site and increased fluid signal within the paravertebral musculature. Following the administration of gadolinium contrast agent, marked enhancement could be seen of these tissues (Figures 2A-D). Since the intervertebral disc space and the caudal endplate of T9 and the cranial endplate of T10 were involved, discospondylitis was diagnosed.

New evidence

Figure 2: Magnetic resonance study performed two weeks after the myelogram. (1) T2-weighted sagittal image of the thoracic spine. T9 is labeled and the fluid accumulation at the T2 intervertebral disc is readily seen. (2) STIR sequence that is T2-weighted and suppresses the fat signal. The fluid in the normal intervertebral disc can be seen and the destruction of the disc space and end plates at T9-10 is readily seen. Also, notice the subarachnoid fluid columns are obliterated in that region. (3) T1-weighted sagittal view following the administration of contrast agent with fat suppression. The contrast enhancement of the disc region is noted as well as ventrally and dorsally in the paraspinal musculature. Also, contrast enhancement within the spinal canal. (4) Transverse view, post-contrast, with fat suppression. The spinal cord is labeled. There is spinal cord compression and contrast enhancement of the meninges, epidural space, and the surrounding soft tissues.

Discospondylitis occurs in dogs from numerous bacterial and fungal conditions. The most common bacterial condition is Staph sp. Other conditions, including diseases such as Brucella canis., need to be considered. Culture of the cerebral spinal fluid, blood or urine may yield the organism and allow sensitivity testing. Often, these cultures are not successful. Recently, urine cultures have been indicated as being the most likely to yield results. Discospondylitis is often associated with chronic genital urinary tract disease. Discospondylitis can also occur from penetrating wounds or foreign body (grass awn) migration. Long-term antibiotic therapy is generally successful if the condition is not too advanced.

Diagnosis and treatment

Following the MRI diagnosis, Tucker was placed on antibiotics and within two days, left the hospital walking relatively normally. Tucker was rechecked two weeks following discharge from the hospital and was back to normal. A radiographic examination at that time revealed slight narrowing of the intervertebral disc space, but still no radiographic evidence of the lysis of the endplates that allow the diagnosis of discospondylitis.

A radiographic study, contrast myelogram and CSF analysis were unable to make the diagnosis in this case. The superior visualization of abnormal tissues with MRI allowed the diagnosis to be made at an early stage and the proper therapy instituted. The degree of cellulitis and soft tissue involvement appears greater than expected from our radiographic experience, yet with MRI the degree of involvement in this case is typical.

Suggested Reading

It appears reasonable to assume early diagnosis will lead to better therapeutic outcomes.

The diligence of the veterinarian to pursue this case to reach a diagnosis and proper therapy is commendable. Magnetic resonance imaging of patients with undiagnosed conditions that are not getting better or getting worse should be considered. Early diagnosis and proper therapy can decrease the overall cost to the client.