Overview
The subarachnoid space is a compartment within the spinal that normally contains cerebrospinal fluid (CSF). This fluid flows continuously and plays a vital role in cushioning the spinal cord and removing waste. However, when CSF flow becomes disrupted or irregular, it can lead to the formation of a subarachnoid diverticulum (SAD) β a fluid-filled pocket or cyst that develops within the subarachnoid space. This localized buildup of fluid can lead to compression, inflammation, and damage to the spinal cord.
Causes
The exact cause of SADs is not fully understood, but is believed to arise from either congenital (present at birth) or acquired factors:
- Acquired subarachnoid diverticula typically develop in middle-aged to older dogs and are often linked to vertebral malformations, spinal trauma, or chronic inflammation. Repeated or long-standing meningeal inflammation can lead to adhesions, which disrupt the normal flow of cerebrospinal fluid and contribute to diverticulum formation. You can think of these adhesions and the resulting cerebrospinal fluid buildup like a dam β blocking the flow and causing CSF to build behind it.
- Congenital subarachnoid diverticula are more common in young dogs and may result from developmental abnormalities in the arachnoid membrane, one of the spinal cordβs protective linings.
Certain breeds, such as French Bulldogs, Pugs, and Rottweilers, are more predisposed to developing SADs.
Clinical Signs
Subarachnoid diverticula often cause progressive neurological symptoms as they compress the spinal cord, typically worsening over weeks to months. Common signs include loss of coordination and weakness of the hind limbs. Urinary and/or fecal incontinence is frequently observed. Affected dogs are usually not painful.
These signs usually progress over time if left untreated, as the diverticulum enlarges and continues to compress the spinal cord, further impairing neurological function.
Diagnosis
MRI is considered the gold standard for diagnosing subarachnoid diverticula. It provides detailed images of the fluid-filled pocket and how it may be compressing the spinal cord. While the underlying adhesions or exact cause of the diverticulum may not always be visible, the MRI is invaluable in assessing the severity of the condition and guiding treatment decisions.
Treatment
In mild cases or in dogs who are not ideal surgical candidates, medical (conservative) management may be pursued initially. This typically involves corticosteroids to reduce inflammation, along with restricted activity. Close monitoring is essential during this period to detect any progression of neurological signs.
While some dogs may experience mild or temporary improvement with conservative treatment, surgery is generally considered the treatment of choice β particularly for those with moderate to severe or progressive neurological signs. Surgery is aimed at relieving spinal cord compression and restoring normal CSF flow. After gaining access to the spinal cord, the most common surgical approach involves removing adhesions that are disrupting CSF circulation. If an underlying condition such as spinal instability or vertebral malformation is identified during surgery, it may also be addressed at the same time to improve long-term outcomes.
While surgery often leads to meaningful clinical improvement, results can vary and a full return to normal function is not typically achievable. The primary goal is to improve neurological function enough to ensure a good quality of life. It’s also important to note that recurrence is possible, making long-term monitoring essential.
Prognosis
The prognosis for subarachnoid diverticula (SADs) depends on several factors, including the severity and duration of neurological deficits. Surgery often leads to significant improvement or stabilization, and the overall prognosis with surgery is generally considered fair to good. However, recurrence or ongoing neurological decline is possible, especially if the underlying causes cannot be fully resolved.
Dogs managed with medical therapy alone tend to have a variable but generally guarded prognosis. While some may reach a stable neurological plateau, others may continue to deteriorate over time.
Ongoing monitoring and long-term follow-up are important to detect any signs of recurrence early and to support the best possible long-term outcome.