Narrowed Disc Spaces
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. Without any neurologic deficits, remember that the pet hasn't been diagnosed with neurologic disease (yet), so spinal pain can be due to bone, joint, nerve, spinal cord, muscle or meningeal in origin. 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)" on the report. What do you do with this?
Does a narrowed disc space indicate a disc herniation?
No, sorry. 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 fibrosis, 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 fibrosis (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 displaces, 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) or fibrocartilaginous embolism (FCE) are two more forms of disc herniation where in 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.
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.
I hope you enjoyed this week's TidBit Tuesday! We're almost into March which, if you're a long-time patron, you know means we're approaching St. Patrick's day. My girls are Irish Dancers and, as such, usually perform in over 30 shows in the month of March. That means this proud mamma has a more limited work schedule due to all of the driving so PLEASE reach out if you cannot find a suitable time on the online scheduler. I may (often can) shift things a bit to accommodate your request! Have a great week and stay safe out there.