Spinal Arachnoid Diverticulum

Spinal arachnoid diverticulum (SAD), also known as spinal arachnoid cyst, is an abnormal accumulation of cerebrospinal fluid (CSF) within a localized dilation of the arachnoid membrane, leading to compression of the spinal cord. It is a surgically correctable cause of progressive myelopathy in dogs and, less commonly, cats. The diverticulum may be intradural (within the arachnoid mater) or extradural (between the dura and arachnoid), with the intradural form being more common. SADs are classified as congenital (associated with spinal dysraphism) or acquired (secondary to trauma, inflammation, or iatrogenic causes). The condition typically affects the cervical (C2-C3) and thoracolumbar (T3-L3) regions, causing spinal cord compression and subsequent neurological deficits. Surgical treatment involves decompression via laminectomy or durotomy, with or without marsupialization or resection of the diverticulum. The prognosis is generally favorable with early intervention, but recurrence can occur.

The exact etiology of spinal arachnoid diverticulum is often unknown, but it is broadly categorized into congenital and acquired forms. Congenital SADs are thought to arise from developmental anomalies of the arachnoid mater, such as incomplete duplication or fenestration, leading to a one-way valve mechanism that allows CSF to enter but not exit, causing progressive dilation. This is often associated with spinal dysraphism, including spina bifida occulta or vertebral malformations. Acquired SADs may result from trauma (e.g., vertebral fractures, disc herniation), spinal surgery (iatrogenic), or inflammatory conditions such as meningitis or arachnoiditis, which cause adhesions and subsequent CSF accumulation. In some cases, a traumatic event may cause a tear in the arachnoid, leading to a diverticulum. The precise cellular mechanisms involve altered CSF dynamics, with increased pressure within the subarachnoid space causing herniation of the arachnoid through a dural defect or weakening. Biomechanically, the cervical and thoracolumbar regions are more susceptible due to increased mobility and stress.

Spinal arachnoid diverticulum is most commonly diagnosed in dogs, with a higher prevalence in certain breeds, including the Pug, French Bulldog, and other brachycephalic breeds, suggesting a possible genetic predisposition. It can also occur in large breeds such as the Rottweiler and Greyhound. The condition is typically seen in young to middle-aged dogs (mean age 3-5 years), but can occur at any age. There is no strong sex predilection, though some studies suggest a slight male predominance. In cats, SAD is rare but has been reported. The incidence is relatively low compared to other spinal diseases like intervertebral disc disease, but it is an important differential in young dogs with progressive myelopathy. Breed-specific anatomical factors, such as a relatively large spinal canal and increased mobility of the cervical spine, may contribute to the development of congenital SADs.

The pathophysiology of spinal arachnoid diverticulum involves a localized accumulation of CSF within the arachnoid, leading to spinal cord compression. The diverticulum acts as a space-occupying lesion, causing direct mechanical compression of the spinal cord parenchyma, leading to demyelination, axonal degeneration, and neuronal loss. The compression also disrupts the blood-spinal cord barrier, causing vasogenic edema and ischemia. The one-way valve mechanism, often seen in congenital cases, allows CSF to enter the diverticulum during systole but prevents its exit during diastole, leading to progressive enlargement. This results in a gradual onset of neurological signs. In acquired cases, inflammation or trauma may cause adhesions that trap CSF. The spinal cord compression leads to clinical signs such as proprioceptive ataxia, paresis, and spinal hyperpathia. Chronic compression can result in syringomyelia (fluid-filled cavities within the spinal cord) due to altered CSF flow and increased intramedullary pressure.

Intrinsic predisposing factors include breed predisposition (brachycephalic breeds), congenital vertebral anomalies (e.g., hemivertebrae, block vertebrae), and genetic factors that affect arachnoid development. Age is a factor, with young dogs more likely to have congenital SADs. Extrinsic factors include trauma, which can cause dural tears or arachnoid adhesions, and prior spinal surgery, which may lead to iatrogenic arachnoiditis. Obesity and excessive physical activity may exacerbate clinical signs by increasing intrathecal pressure. Nutritional factors are not directly implicated, but overall health status can influence surgical outcomes.

Clinical signs of spinal arachnoid diverticulum are progressive and depend on the location of the lesion. Cervical SADs (C2-C3) typically cause proprioceptive ataxia, tetraparesis, and cervical hyperpathia. Thoracolumbar SADs (T3-L3) result in pelvic limb ataxia, paresis, and sometimes paraplegia. Neurological examination reveals upper motor neuron signs in the affected limbs, with normal spinal reflexes. Pain may be present, especially with cervical lesions. In severe cases, urinary and fecal incontinence can occur. The onset is usually insidious, with signs progressing over weeks to months. A characteristic 'bunny-hopping' gait may be observed in pelvic limbs. Sensory deficits, such as proprioceptive positioning deficits, are common. The severity can be graded using a modified Frankel scale, ranging from mild ataxia to paralysis.

Differential diagnoses for spinal arachnoid diverticulum include: 1) Intervertebral disc disease (IVDD) - Hansen type I or II, which causes acute or chronic spinal cord compression; MRI shows disc extrusion or protrusion, not a CSF-filled cyst. 2) Spinal cord neoplasia (e.g., meningioma, glioma) - MRI shows contrast-enhancing mass, not a CSF-filled cyst. 3) Syringomyelia - MRI shows fluid-filled cavities within the spinal cord, often associated with Chiari-like malformation. 4) Spinal epidural empyema - MRI shows contrast-enhancing epidural collection, with systemic signs of infection. 5) Meningomyelitis - MRI shows diffuse or focal spinal cord swelling with contrast enhancement, CSF analysis shows inflammation. 6) Vertebral malformation (e.g., hemivertebra) - radiography/CT shows vertebral anomaly, causing spinal cord compression. 7) Spinal trauma - history of trauma, radiography/CT shows fracture or luxation. 8) Fibrocartilaginous embolism - acute onset, MRI shows spinal cord infarction, no cyst. 9) Arachnoiditis - MRI shows diffuse arachnoid enhancement, CSF analysis shows inflammation. 10) Spinal epidural lipomatosis - MRI shows epidural fat accumulation, often in obese animals.

The diagnostic algorithm for spinal arachnoid diverticulum begins with a thorough history and neurological examination to localize the lesion. If a myelopathy is suspected, advanced imaging is essential. MRI is the gold standard, as it can clearly delineate the diverticulum and rule out other causes. The MRI protocol should include T1-weighted, T2-weighted, and T2-weighted fluid-attenuated inversion recovery (FLAIR) sequences, as well as post-contrast T1-weighted images. SAD appears as a well-defined, CSF-filled (hyperintense on T2, hypointense on T1) lesion within the subarachnoid space, causing spinal cord compression. Myelography (with CT) can also be used, showing contrast filling the diverticulum, but MRI is preferred due to its non-invasiveness and superior soft tissue contrast. CSF analysis may be performed to rule out inflammation or infection, but it is not diagnostic for SAD. Surgical exploration is both diagnostic and therapeutic, allowing direct visualization of the diverticulum.

Laboratory findings in spinal arachnoid diverticulum are typically unremarkable. Complete blood count (CBC) and serum biochemistry are usually within normal limits, unless there is an underlying inflammatory or infectious process. CSF analysis may show mild elevations in protein concentration (up to 50-100 mg/dL) and occasionally mild pleocytosis (10-50 nucleated cells/µL), but these findings are non-specific. In cases of concurrent arachnoiditis, CSF may show increased neutrophils or lymphocytes. Coagulation panel (PT/aPTT) is recommended preoperatively to assess surgical risk. Inflammatory biomarkers such as C-reactive protein (CRP) may be elevated in inflammatory conditions but are not specific for SAD.

Radiography: Plain radiographs of the spine may show vertebral anomalies (e.g., hemivertebrae) but are not diagnostic for SAD. Myelography (with or without CT) can demonstrate the diverticulum as a contrast-filled outpouching, but it is invasive and has been largely replaced by MRI. CT myelography can provide detailed bony anatomy and is useful for surgical planning. MRI: This is the imaging modality of choice. On T2-weighted images, SAD appears as a hyperintense (bright) fluid-filled lesion within the subarachnoid space, causing spinal cord compression. On T1-weighted images, it is hypointense (dark). The diverticulum does not enhance with contrast administration. MRI also helps identify concurrent conditions such as syringomyelia or Chiari-like malformation. In some cases, the diverticulum may be seen as a focal dilation of the dorsal subarachnoid space, often at C2-C3 or T3-L3. Advanced MRI techniques, such as phase-contrast CSF flow studies, can assess CSF dynamics and confirm the one-way valve mechanism.

Cytology: CSF analysis is performed to rule out inflammation or neoplasia. In SAD, CSF is typically clear and colorless, with normal or mildly elevated protein and cell count. Histopathology: Surgical biopsy of the diverticulum wall (if resected) shows a thickened arachnoid membrane with fibrous connective tissue and sometimes chronic inflammatory infiltrate. The inner surface is lined by arachnoid cells. In congenital cases, there may be evidence of spinal dysraphism. Histopathology is not typically required for diagnosis, as MRI is definitive, but it can help rule out other pathology if the lesion is atypical.

Medical management is generally not effective for spinal arachnoid diverticulum; surgical decompression is the treatment of choice. The goal of surgery is to relieve spinal cord compression and prevent recurrence. Surgical options include: 1) Dorsal laminectomy: A standard dorsal laminectomy is performed at the affected site to expose the dura. The diverticulum is identified as a translucent, fluid-filled sac. 2) Durotomy: The dura is incised over the diverticulum, allowing CSF to escape and decompressing the spinal cord. 3) Marsupialization: The edges of the dura are sutured to the surrounding muscle or fascia to create a permanent drainage pathway, preventing re-accumulation. 4) Resection: The diverticulum wall may be partially or completely resected, but this is associated with a higher risk of recurrence. In cases of concurrent syringomyelia, a subarachnoid shunt may be placed. Preoperative management includes corticosteroids (e.g., dexamethasone 0.1-0.2 mg/kg IV) to reduce spinal cord edema. Perioperative antibiotics (e.g., cefazolin 22 mg/kg IV q90min) are administered. Postoperative pain management includes opioids (e.g., hydromorphone 0.05-0.1 mg/kg IV q4-6h) and NSAIDs (e.g., carprofen 2.2 mg/kg PO q12h) after 24 hours. Physical rehabilitation is initiated early to promote recovery.

The prognosis for spinal arachnoid diverticulum is generally good with surgical treatment. Approximately 70-90% of dogs improve neurologically after surgery, with many achieving near-normal function. Factors associated with a better prognosis include a shorter duration of clinical signs, younger age, and a cervical location. Complications include recurrence of the diverticulum (10-20% of cases), especially if marsupialization is not performed, and postoperative neurological deterioration due to spinal cord manipulation. Negative prognostic indicators include severe preoperative neurological deficits (e.g., paraplegia with loss of deep pain perception), chronic compression with spinal cord atrophy, and concurrent syringomyelia. Long-term follow-up is recommended to monitor for recurrence.

Postoperative follow-up is crucial. Patients are typically hospitalized for 2-4 days after surgery. Neurological assessments are performed daily. Sutures are removed 10-14 days postoperatively. Restricted activity (cage rest) is recommended for 4-6 weeks to allow healing. Serial neurological examinations are performed at 2, 4, 8, and 12 weeks postoperatively. MRI is repeated at 3-6 months if there is concern for recurrence or incomplete resolution of signs. Physical rehabilitation, including passive range of motion exercises, therapeutic ultrasound, and underwater treadmill therapy, is initiated after suture removal. Long-term monitoring for recurrence is recommended, with annual neurological examinations. Owners should be advised to avoid high-impact activities that may increase intrathecal pressure.

Pearls: 1) MRI is essential for diagnosis; always include T2-weighted sagittal and transverse images to identify the diverticulum. 2) In cervical SADs, a dorsal laminectomy at C2-C3 provides excellent exposure. 3) Marsupialization of the dura to the surrounding epaxial muscles reduces the risk of recurrence. 4) Use of a durotomy with a #11 blade, taking care to avoid spinal cord trauma. 5) Preoperative corticosteroids can help reduce spinal cord swelling. Pitfalls: 1) Failure to identify the diverticulum intraoperatively if it is small or if the dura is not opened. 2) Incomplete marsupialization can lead to recurrence. 3) Excessive manipulation of the spinal cord can cause iatrogenic injury. 4) Not ruling out other causes of myelopathy (e.g., IVDD, neoplasia) before surgery. 5) Postoperative seroma formation at the surgical site, which can be managed with drainage and pressure bandages.

Perioperative antimicrobial prophylaxis: Cefazolin 22 mg/kg IV at induction, repeated every 90 minutes during surgery. Postoperative antibiotics are not routinely needed. Analgesia: Preoperative: Methadone 0.2-0.5 mg/kg IV or IM. Intraoperative: Fentanyl CRI at 5-10 µg/kg/h. Postoperative: Hydromorphone 0.05-0.1 mg/kg IV or IM q4-6h for 24-48 hours, then transition to oral tramadol 2-5 mg/kg PO q8-12h. NSAIDs: Carprofen 2.2 mg/kg PO q12h or meloxicam 0.1 mg/kg PO q24h, starting 24 hours after surgery, for 5-7 days. Corticosteroids: Dexamethasone 0.1-0.2 mg/kg IV at induction, then tapering doses over 2-3 days. Muscle relaxants: Methocarbamol 15-20 mg/kg PO q8h for muscle spasms. Gastroprotectants: Omeprazole 1 mg/kg PO q12h or famotidine 0.5-1 mg/kg PO q12h while on corticosteroids. Chondroprotectants: Not directly relevant, but may be used for concurrent joint disease. All dosages should be adjusted based on renal and hepatic function.

Several studies have evaluated the outcomes of surgical treatment for spinal arachnoid diverticulum. A retrospective study by Skeen et al. (2003) reported that 80% of dogs improved after surgery, with a recurrence rate of 15%. Another study by Mauler et al. (2017) found that dogs with cervical SADs had a better prognosis than those with thoracolumbar SADs. A consensus statement from the ACVS recommends MRI as the diagnostic modality of choice and surgical decompression with marsupialization as the preferred treatment. A meta-analysis by da Costa et al. (2019) concluded that early surgical intervention (<3 months of clinical signs) is associated with a better outcome. The use of corticosteroids preoperatively is supported by experimental studies showing reduced spinal cord edema. Overall, the evidence supports surgical treatment as the standard of care, with a favorable prognosis in most cases.