Arachnoid diverticuli (AD) are an increasingly common diagnosis on MRI, specifically for Pugs, but also for many small breed dogs. Arachnoid diverticuli are a focal dilation of the arachnoid space in the meninges. The result of this dilation is an abnormal CSF flow through the region and spinal cord compression. If you don’t remember the layers of the meninges, let’s take a quick moment to remember. The outer most layer is the dura mater, which is a very tough, fibrous protective layer. Above the dura mater is the epidural space (think epidural anesthesia here) and under the dura mater is the subdural space, which doesn’t have much purpose. Deep to the subdural space is the arachnoid layer. The arachnoid layer is connected through spider-y like connections to the underlying pia mater (the 3rd and last layer of the meninges). Between the arachnoid layer and the pia mater is the subarachnoid space. Cerebrospinal fluid courses through the subarachnoid space. Getting back to the pia mater, it is stuck right up against the spinal cord therefore there isn’t a space between it, and the underlying cord. Can you picture this now? I hope so!
So, what happens when the arachnoid layer becomes dilated? Clinical signs often include gait deficits such as proprioceptive ataxia, and paresis, and frequently fecal and/or urinary incontinence. The later signs are notable because these animals often drop or dribble but are still ambulatory animals. For an animal with a disc herniation, this type of incontinence wouldn’t be expected unless they were non-ambulatory approaching a plegic state. Fecal or urinary incontinence in a walking dog is usually a red flag for me to consider an arachnoid diverticulum in a patient! This disease is not typically painful, but occasionally we might detect spinal discomfort on palpation.
How is it diagnosed? MRI. That’s a pretty simple answer but there isn’t much else to add here.
What do we do about AD? There are medical and surgical treatment options. Medical treatment options are geared at decreasing CSF production, reducing inflammation, and managing pain. Prednisone, or other corticosteroids, are the cornerstone for treatment due to the suspected CSF-reducing properties and potent anti-inflammatory properties. Surgical treatment options haven't been very encouraging thus far, however a recent study outlining a retrospective, non-randomized treatment trial was published in Veterinary Surgery (2025: 1-11) comparing two surgical techniques. The first surgical intervention is the more common one and that is a durotomy followed by either durectomy or debridement of the dura. The other treatment option was a durotomy with shunt placement. Twelve dogs were enrolled in the durotomy alone group (which they called the control group, and we will too) and 14 were enrolled in the shunting group. Breeds were similar, as were neurologic grade between both groups. Here were some highlights from the paper:
Immediate post operative (< 24 hour) outcome: not significantly different between groups
Short term outcome (4-8 week recheck): no significant difference between groups
Long-term outcome (> 6 months): the shunting group was statistically more likely to have improved than the control group. They reported 12/14 dogs improving (85.7%), compared to 5/12 (41.7%) in the control group.
Recurrence rates were similar between the two groups (5/12 in the control group and 2/14 in the shunting group).
No difference between the groups was noted in long-term presence of urinary or fecal incontinence.
Previous studies have reported significant difficulty with the shunt placement including a greater likelihood of a non-ambulatory state postoperative. This study did not report this difference, but their technique was slightly different. Did that cause the improvement? It’s hard to say because the number of dogs is so small in each group. The take away for me is that there may be hope on the horizon for a different, more successful, surgical intervention for AD than we’ve had thus far. It was notable that the dogs that experienced signs of recurrence were placed on prednisone and all improved. The ~40% improvement on the standard treatment (not shunt placement), and the possibility of recurrence has historically deterred me from recommending surgery at the start. If patients improve with medical management, I often do not recommend surgical intervention. Perhaps the new shunting techniques will change that recommendation. As with all new things, keep your ear to the ground and I’ll let you know what new data comes down the pipe about corrective surgery for AD.
Thanks for reading! I hope you have a nice week and enjoy some of the weather that March has to offer. Many of you know that March is a busy time for my family, so my hours are a little more restricted than usual. Please be patient with my email responses – I promise I will respond but there may be a slight delay as I play chauffer to two Irish Dancers during St. Patty’s Day season!