Subarachnoid diverticula
This condition occurs due to abnormal development of the arachnoid layer of the meninges, which is thought to result in either splitting of or duplication of the arachnoid membrane.9 This results in disturbance in the flow of cerebrospinal fluid (CSF), potentially due to creation of a one-way valve at the cranial aspect of the defect. This results in fluid dilation in the subarachnoid space, generally dorsal to the spinal cord. The majority of French bulldogs (61 percent), have a concurrent condition at the same site of the diverticulum, such as a vertebral malformation or intervertebral disc extrusion.10 French bulldogs with this condition generally start to show signs of a T3-L3 myelopathy at a young age (less than three years), which often worsens over time, in addition to urinary and fecal incontinence.11
Patients can be managed conservatively with steroids and drugs to reduce CSF production (such as omeprazole, a proton-pump inhibitor), with limited benefit. A variety of surgical techniques have been described and are associated with a relatively good outcome in young, mildly affected dogs, although recurrence is possible.12,13
Intervertebral disc disease
Intervertebral disc extrusion is the most commonly reported neurologic disease in French bulldogs. French bulldogs are chondrodystrophic, meaning that along with dachshunds, beagles, basset hounds, cocker spaniels, and corgis, they have abnormal endochondral ossification, which results in shortening of long bones.14 This dysfunctional endochondral ossification is suspected to be related to an FGF4 retrogene insertion on chromosome 12.15 Chondrodystrophy is linked to intervertebral disc degeneration in these affected breeds. The intervertebral disc is comprised of a nucleus pulposus, which in health is roughly 80 percent water, encased in the annulus fibrosus. With Hansen type I intervertebral disc degeneration, called chondroid metaplasia, the resident population of cells in the nucleus pulposus, notochord cells, recede, and are no longer able to maintain the proteoglycan matrix that retains hydration of the disc. Chondrocytes enter, and begin to produce cartilage. The combined issues lead to disc dehydration and calcification. When the nucleus pulposus has expanded enough to exceed the retaining capacity of the annulus fibrosus, the annulus fibrosus ruptures, and calcified disc material extrudes through the broken disc and enters the vertebral canal.15 There, it compresses the spinal cord, venous plexus, and nerve roots, leading to progressive myelopathy and pain.
When compared with other dog breeds, the average age for French bulldogs experiencing their first intervertebral disc extrusion is substantially younger than other breeds (median age of four years for French bulldogs and median age of six years for dachshunds14). French bulldogs are significantly more likely to develop extensive epidural hemorrhage as a result of a disc extrusion, when compared to dachshunds. When epidural hemorrhage occurs, this added volume in the vertebral canal puts additional pressure on the spinal cord and can cause substantial spinal cord compression.
Often, removal of the compressive hematoma is necessary, and as the blood travels beyond the region of the disc extrusion, multiple areas along the vertebral canal need to be surgically accessed to remove the compression. This can result in multi-site hemilaminectomies that in some French bulldogs may span up to five intervertebral disc spaces.16 This surgical treatment is concerning for two reasons: first, it is a more extensive surgery that could predispose patients to increased risk of infection, inflammation, and instability. Second, the rate of disc extrusion recurrence is higher in French bulldogs than in other breeds.17 Recurrence occurs in approximately 50 percent of French bulldogs, as compared to dachshunds, in which the recurrence rate is roughly 25 percent.17
Given French bulldogs are substantially younger at the time of their first disc extrusion, they have more livable years remaining for additional extrusions, coupled with a severe tendency to develop intervertebral disc disease. When a new intervertebral disc extrudes in these patients, requiring another surgery to treat, the vertebral column is already compromised by the previous multi-site hemilaminectomy. The approach is more challenging and more dangerous when a region of a previous hemilaminectomy must be accessed to retrieve disc material.
In general, dachshunds are more likely to experience a thoracolumbar disc extrusion than a cervical disc extrusion. Contrastingly, in French bulldogs, the rate of cervical disc extrusions nearly matches the rate of thoracolumbar disc extrusions. Cervical disc extrusions are associated with higher medical management failure rates and increased risk for life-threatening surgical complications.
French bulldogs are also more likely to develop extrusions in the mid- to caudal lumbar vertebral column.18 This results in them more commonly experiencing an L4-S3 myelopathy than dachshunds. A lesion in the caudal lumbar spinal cord results in flaccid paraparesis or paraplegia, along with a flaccid, constantly leaking urinary bladder, along with a dilated anus and constantly leaking rectum. This is clinically more difficult to manage than a patient with a T3-L3 myelopathy, and more challenging for pet owners to manage as well.
In patients that do not recover their fecal and urinary continence following treatment for their disc extrusion, a substantially higher risk of severe urinary tract infections remains for life. The tendency for French bulldogs to develop disc extrusions at the L3-L4 disc is concerning for an additional reason, discussed below following a discussion of progressive myelomalacia.
French bulldogs are also more likely to have progression of their neurologic dysfunction over a short period of time compared to other breeds. I have observed French bulldogs evaluated for back pain can progress to being paraplegic without pain sensation in a matter of hours. In general, French bulldogs in this severe category are less likely to gain functional recovery following surgery compared to other dog breeds (46 percent for French bulldogs compared to about 60 percent for other breeds).19
For patients in this most severe category of neurologic impairment from an intervertebral disc extrusion (the paraplegic patient that lacks deep pain sensation), several additional important differences between French bulldogs are other breeds are critical to disclose. Patients with this grade of dysfunction are at risk for developing a condition called progressive myelomalacia, which is characterized by ischemic necrosis of the spinal cord. This process, which cannot be reliably predicted, cannot be prevented, and is not caused by surgery, extends both cranially and caudally from the site of the disc extrusion.
Progressive myelomalacia that proceeds both cranially and caudally is a fatal condition. Approximately 10-15 per cent of paraplegic dogs that lack deep pain sensation due to disc extrusion develop progressive myelomalacia. However, French bulldogs are more likely to become paraplegic and lack deep pain sensation than other dog breeds, and have a higher rate of progressive myelomalacia as well (potentially up to 33 per cent).18 Finally, dogs with L3-L4 disc extrusions have an increased risk of developing poliomyelomalacia, even if they have retained deep pain sensation prior to surgery.19 This is thought to occur due to compression of critical vascular supply to the spinal cord in the region of the L3-L4 intervertebral disc. Given that French bulldogs have a higher risk of L3-L4 disc extrusions compared to other breeds, their risk of poliomyelomalacia is higher, as well.
