It's a Tuesday morning and on your schedule is a 2 year old MC Labrador retriever with a 2 week history of back pain. The neurologic examination is normal. now is a good time to remember that spinal pain, without neurologic deficits, can be due to bone, joint, nerve, spinal cord, muscle or meningeal sources. Given the normal exam, you elect to take sedated spinal radiographs and send them off to the radiologist. A radiologist will report what they see, which often includes "narrowed disc space(s)" . What do you do with this information?
Does a narrowed disc space indicate a disc herniation?
Alas, no. A narrowed disc space could be positional (often), beam angling (often), due to disc degeneration, or disc herniation. Disc degeneration occurs when hydration leaves the annulus fibrosus, thus causing the disc to shrink a little. This is a normal aging process and does not indicate a herniation. Herniation occurs when part of the disc (annulus fibrosus (AF) or nucleus pulposus (NP)) leaves it's normal position. When the NP leaves, we call it a Type I disc herniation. Type I disc herniations are common in chondrodystrophic dogs and about 30% of non-chondrodystrophic dogs. Clinical signs often include calcification of the NP which might be visible on radiographs. Remember, in situ calcification is appropriate and normal for chondrodystrophic and some non-chondrodystrophic dogs and does not indicate herniation. Calcification in the canal suggests that the disc has herniated but does not indicate that the current clinical signs are due to THAT disc herniation. This means we cannot diagnose a type I disc herniation strictly on radiographs. An MRI, CT or myelogram is needed to diagnose a type I disc herniation.
When the AF is displaced, it is called a Type II disc herniation. This is more common in non-chondrodystrophic dogs and is often part of disc associated wobblers and lumbosacral disease. No calcification occurs for this form of herniation therefore it cannot be seen radiographically. A narrowed disc space often (but not always) accompanies a type II disc herniation but is not always radiographically visible. An MRI, CT or myelogram is needed to diagnose a type I disc herniation.
What about FCE or ANNPE?
Acute noncompressive nucleus pulposus extrusion (ANNPE) and fibrocartilaginous embolism (FCE) are two more forms of disc herniatio. In this case, a small amount of disc material leaves its normal location, often under great force. A narrowed disc space can be seen with these types of disc herniation as well but would NOT be diagnostic for either disease. An MRI is needed to diagnose an FCE or ANNPE. Myelogram and CT are not able to diagnose this form of disc herniation as reliably.
What causes of back pain CAN you diagnose on radiographs then??
The most common causes of spinal pain are 2 "D"s, 2 "T"s and an "M" (as I teach it to students). Discospondylitis, disc herniation, trauma, tumor, and meningitis. Of these, only discospondylitis, vertebral neoplasia and fracture/subluxation can be diagnosed on plain radiographs.
The patient above could have discospondylitis on their differential diagnoses list therefore radiographs are absolutely indicated. However, please be cautious reading too much into a "narrowed disc space" and instead look for discospondylitis, vertebral neoplasia or signs of trauma.
Thanks for reading! I hope you have a great week and look forward to working with you soon!