Donald E. Thrall, DVM, PhD, DACVR (Radiology, Radiation Oncology)
Spinal pain is a common clinical problem and spinal radiographs are often made in such patients to look for a cause. There are: 1) positioning problems that lead to a false diagnosis of spinal disease, 2) causes of spinal pain for which radiographs have a low yield, 3) causes of spinal pain for which radiographs are highly specific, and 4) various spinal radiographic changes that are not clinically significant.
Sensible radiography in a patient with spinal pain depends on accurate neuroanatomic localization of the lesion. Thus, it is critical that basic neurologic examination principles be understood.
It is important to realize that it in most patients will not be possible to determine the cause of a spinal neuropathy from survey radiographs. This is because of the relatively poor inherent soft tissue contrast of radiographs, and the inability to visualize the spinal cord radiographically without intrathecal contrast medium (Myelogram). CT and/or MRI are usually necessary; to determine the cause of spinal pain. However, spinal radiographs are very important for screening, and it is in that context that spinal radiography is discussed herein.
Sedation or anesthesia is critical for obtaining adequately-positioned radiographs. The spine is complex and good positioning is needed to reduce confusion. Proper positioning is just not possible in patients that are not sedated or anesthetized. Positioning errors can either mask a diagnosis or create a lesion. At best, poorly positioned radiographs are totally useless. At worst, they are misleading.
A basic principle of radiography is that two views made at right angles to each other, termed orthogonal views, are needed to visually recreate an image of a three-dimensional patient. This applies to the spine. However, if a spinal fracture or instability is a clinical concern, then only lateral views should be made initially, followed by VD views once instability has been ruled out.
The main indications for spinal radiography are spinal pain, or a spinal neuropathy. Spinal neuropathies should be categorized into the segment of spinal cord most likely to be injured, based on the clinical signs. These well recognized syndromes are C1–C5 neuropathy, C6–T2 neuropathy, T2–L3 neuropathy and L3–S1 neuropathy.
Common Abnormalities in Survey Radiographs
Abnormalities likely to be encountered in survey spinal radiographs are disc space abnormalities, spondylosis and articular process degenerative joint disease.
Disc herniation is the most common cause of spinal neuropathy or spinal pain. Herniation of an intervertebral disc cannot be diagnosed from survey radiographs, but survey radiographs should always be made in patients with spinal neuropathy for screening purposes. Finding a narrowed disc space is common. A radiographically narrowed disc space is good evidence that the disc has herniated, but a narrowed disc space is not evidence that disc herniation at this site is the cause of the neuropathy. Many chronic herniations do not have associated clinical signs. A negative radiographic screening in a patient suspected of having a herniated disc should be followed by referral to a specialist.
Myelography is useful for diagnosing herniated discs but it is tedious and time consuming and most practitioners choose not to become proficient in this technique. Setting out to do myelography in your practice should only be done if one is also prepared to do spinal decompressive surgery. Most disc herniations are now diagnosed using CT or MRI and myelography is performed less commonly than in the past, even in specialty practices.
Mineralization of discs is common. Often these discs remain in their normal location, i.e., in situ mineralization. These mineralizations are not significant.
In lateral radiographs, mineralized material is often seen superimposed on the intervertebral foramen. Because of the complex anatomy, the vertebral canal cannot be evaluated easily in ventrodorsal radiographs. Thus, it is usually not possible to discern where this mineralized material is located. Often it is lateral to the vertebral canal, created by lateral spondylosis becoming superimposed on the intervertebral foramen.
Vertebrae articulate at the disc, but also dorsally at the articular process joint. These small joints are diarthrodial joints, characterized by articular cartilage, synovial fluid and a joint capsule. These joints are commonly affected by degenerative joint disease. The radiographic signs of degenerative joint disease of the articular process joint are the same as for other diarthrodial joints, namely periarticular osteophyte formation, joint space narrowing and subchondral bone sclerosis. Articular process degenerative joint disease can cause pain but this is difficult to diagnose and specific treatment directed at articular process degenerative joint disease is unusual. The exception is in cervical spondylomyelopathy (Wobbler Syndrome), where hyperostosis of articular processes can cause impingement on the spinal cord.
Spondylosis is a very common radiographic finding. Spondylosis is characterized by new bone forming on the ventral aspect of the vertebral body, near the endplates. This is due to annular degeneration and tearing of ventral annular attachments. Usually spondylosis is not clinically important, except at L7–S1 where it is a component of lumbosacral instability.
Lumbosacral instability (cauda equina syndrome) is a complex problem characterized by disc protrusion/herniation, lumbosacral instability, vertebral canal stenosis and spondylosis. The spondylosis can contribute significantly to foraminal nerve root compression at this site. Affected dogs have pelvic region pain, pelvic limb lameness and difficulty rising. The degenerative changes at the lumbosacral junction are visible radiographically but the extent of nerve root compression cannot be assessed; this is best accomplished using MRI. Myelography is not effective for examining the lumbosacral junction in dogs as the dural sac ends cranial to this location. CT can be used but is not as sensitive as MRI.
Less Common Abnormalities in Survey Radiographs
Less frequently encountered radiographic abnormalities are endplate lysis, vertebral malformations and aggressive bone lesions.
Discospondylitis is an infection of the intervertebral disc with adjacent endplate osteomyelitis; the endplate infection leads to endplate lysis. This endplate lysis is the hallmark radiographic sign of discospondylitis. Dogs with discospondylitis usually have back pain but associated neuropathy is uncommon unless there is a coexisting vertebral canal abscess, or the disc infection leads to instability and subluxation. Discospondylitis is usually due to infection with a Staphylococcus species, or Brucella canis; Brucella canis should be considered a zoonotic organism but human infections are rare. There is often concurrent urinary tract infection.
Cervical spondylomyelopathy (Wobbler disease) is a common vertebral anomaly in some breeds, notably the Doberman pinscher and Great Dane. Clinically, cervical spondylomyelopathy is a progressive cervical myelopathy. The underlying problems are cervical instability or malformation, or both. Radiographs can be suggestive of the disease but it is best assessed with MRI. Survey radiographic features are vertebral canal stenosis, assessed by looking at the relationship of the dorsal vs. ventral limit of the vertebral canal (usually over C5–C6–C7) and articular process degenerative joint disease. There may also be premature spondylosis and cervical malalignment. Again, these radiographic signs are only suggestive and MRI is needed for complete characterization of the disease
Atlantoaxial malalignment is also a vertebral malalignment. Atlantoaxial malalignment can be caused by trauma in any breed, but is most commonly seen in toy breeds where there is a congenital predisposition due to an abnormality affecting one of the ligaments that stabilizes the atlantoaxial joint. Radiographically there is increased distance between the dorsal lamina of C1 and C2; the laminae are not parallel. The dens may also be small or absent. Atlantoaxial malalignment is best visualized on a lateral view. Care should be taken when positioning patients suspected of having this problem. It is advisable to obtain just a lateral view first. If signs of atlantoaxial instability are present, the VD view should not be made as there is a risk of damaging the spinal cord because of the instability.
Primary and secondary vertebral tumors are not common but these will occasionally be seen. A vertebral tumor will usually produce radiographic signs of an aggressive bone lesion. Mesenchymal tumors are common, but reticuloendothelial tumors also occur. It is worth obtaining a histologic diagnosis of aggressive vertebral tumors as isolated reticuloendothelial tumors can be cured in some patients using radiation therapy.