Chiari-Like Malformation and Syringomyelia

Chiari-like malformation (CM) is a congenital developmental disorder of the craniocervical junction in dogs, characterized by a small, shallow caudal fossa (occipital bone) and a subsequent herniation of the cerebellum and brainstem through the foramen magnum. This malformation obstructs the normal flow of cerebrospinal fluid (CSF) at the craniocervical junction, leading to altered CSF dynamics and the formation of fluid-filled cavities within the spinal cord parenchyma, known as syringomyelia (SM). SM is a progressive condition that can cause neuropathic pain, sensory deficits, and motor dysfunction. The disease is most commonly seen in brachycephalic toy and small breeds, particularly the Cavalier King Charles Spaniel (CKCS), but can affect other breeds. Surgical management aims to decompress the craniocervical junction, restore CSF flow, and alleviate clinical signs.

The primary etiology of CM/SM is congenital and developmental, with a strong genetic predisposition. The condition arises from an underdevelopment of the occipital bone, resulting in a reduced volume of the caudal fossa. This bony malformation leads to caudal displacement of the cerebellum and brainstem, obstructing the foramen magnum. The obstruction impedes the normal pulsatile flow of CSF between the cranial and spinal subarachnoid spaces, causing increased CSF pressure in the cranial compartment and forcing CSF into the central canal of the spinal cord. This hydrodynamic pressure leads to the formation and progression of syrinxes. Secondary factors may include trauma, but the primary cause is genetic. In CKCS, a hereditary basis has been identified, with multiple genes implicated. The condition is not acquired but may be exacerbated by factors such as obesity, chronic coughing, or strenuous activity that increase intracranial pressure.

CM/SM is most prevalent in small and toy breed dogs, with the Cavalier King Charles Spaniel being the most commonly affected breed, with an estimated prevalence of up to 95% in some populations. Other predisposed breeds include the Griffon Bruxellois, Chihuahua, Maltese, Pomeranian, Yorkshire Terrier, and Miniature Poodle. The condition is less common in large breeds. There is no sex predilection, but age of onset of clinical signs typically ranges from 6 months to 3 years, although many dogs remain asymptomatic. The high prevalence in CKCS suggests a strong genetic component, with a complex mode of inheritance. In a study of CKCS, over 90% of dogs had CM on MRI, and about 50% had SM. The condition is increasingly recognized due to advanced imaging availability, but many affected dogs are asymptomatic, making true incidence difficult to determine.

The pathophysiology of CM/SM involves a cascade of anatomical and hydrodynamic abnormalities. The primary defect is a small caudal fossa due to occipital bone hypoplasia, leading to overcrowding of the caudal fossa contents. The cerebellum is herniated through the foramen magnum, and the brainstem is compressed. This obstruction at the craniocervical junction impedes the normal systolic pulsatile flow of CSF from the cranial to the spinal subarachnoid space. The altered CSF dynamics result in increased intracranial pressure and a pressure gradient across the foramen magnum. This pressure gradient forces CSF into the central canal of the spinal cord, particularly at the level of the cervical and thoracic segments, leading to the formation of syrinxes. The syrinx expands over time, causing damage to the spinal cord parenchyma, including the spinothalamic tracts, which mediate pain sensation. This leads to neuropathic pain, allodynia, and hyperesthesia. Additionally, the syrinx can cause disruption of the autonomic pathways, leading to autonomic dysfunction. The progression of SM is variable, and some dogs may remain stable or even have spontaneous resolution of syrinxes, but in many, it is progressive.

Intrinsic predisposing factors include breed-specific conformational traits, such as brachycephaly and a small caudal fossa, which are genetically determined. Age is a factor, with clinical signs often appearing in young adulthood, but asymptomatic dogs can develop signs later. Obesity may exacerbate clinical signs by increasing intracranial pressure. Extrinsic factors include trauma, which can worsen the condition, and chronic coughing or straining, which can increase CSF pressure. Prior surgery, such as ventriculoperitoneal shunting, may alter CSF dynamics. Additionally, concurrent conditions like hydrocephalus or atlantoaxial instability may be present and complicate the clinical picture.

Clinical signs of CM/SM vary depending on the severity of the malformation and the presence and extent of syringomyelia. Common signs include cervical hyperesthesia, which is often manifested as phantom scratching (air scratching) without touching the skin, especially when excited or on a leash. Dogs may exhibit signs of pain such as yelping, crying, or reluctance to move the head and neck. They may also show signs of weakness, ataxia, and proprioceptive deficits in the limbs. In severe cases, there may be scoliosis, thoracic limb muscle atrophy, and even facial nerve paralysis. Neurological examination may reveal decreased postural reactions, spinal hyperpathia, and sometimes cranial nerve deficits. The pain is often neuropathic and may be refractory to standard analgesics. The onset can be acute or gradual, and signs may be intermittent or progressive.

Differential diagnoses for CM/SM include: 1) Atlantoaxial instability (AAI) - a congenital or traumatic instability of the atlantoaxial joint, causing cervical pain and paresis; MRI shows dorsal displacement of the axis and spinal cord compression. 2) Cervical intervertebral disc disease (IVDD) - herniation of the intervertebral disc causing spinal cord compression; MRI shows disc extrusion or protrusion. 3) Meningoencephalitis of unknown origin (MUO) - inflammatory brain disease; MRI shows multifocal lesions, CSF analysis shows pleocytosis. 4) Intracranial neoplasia - brain tumors such as meningioma or glioma; MRI shows a contrast-enhancing mass. 5) Syringomyelia secondary to spinal cord trauma or infection - history of trauma or infection, MRI shows syrinx but no CM. 6) Hydrocephalus - ventricular enlargement; MRI shows dilated ventricles, often with periventricular edema. 7) Chiari malformation type II (myelomeningocele) - rare in dogs, associated with spinal dysraphism. 8) Cervical spinal cord tumor - such as meningioma or nerve sheath tumor; MRI shows an intramedullary or extramedullary mass. 9) Otitis interna/media - can cause head tilt and pain, but MRI shows middle ear disease. 10) Temporomandibular joint disease - causes pain on opening the mouth, but MRI shows joint changes.

The diagnostic algorithm for CM/SM begins with a thorough history and neurological examination. If cervical hyperesthesia or signs of myelopathy are present, the next step is advanced imaging, preferably MRI of the brain and cervical spinal cord. MRI is the gold standard for diagnosing CM and SM, as it allows visualization of the caudal fossa, cerebellar herniation, and syrinx formation. T1-weighted and T2-weighted sagittal and transverse images are essential. CSF analysis may be performed to rule out inflammatory disease, but it is not diagnostic for CM/SM. CT can be used to evaluate bony abnormalities but is inferior to MRI for soft tissue and CSF evaluation. In some cases, myelography may be used if MRI is unavailable, but it is less sensitive. Genetic testing may be available for CKCS to assess risk, but it is not diagnostic. The diagnostic algorithm should also include screening for concurrent conditions such as hydrocephalus or atlantoaxial instability.

Routine laboratory findings in CM/SM are typically unremarkable. Complete blood count (CBC), serum biochemistry, and urinalysis are usually within normal limits. CSF analysis may show normal or mildly elevated protein levels, but cell counts are typically normal. In some cases, there may be mild pleocytosis if there is concurrent inflammation. Coagulation panel (PT/aPTT) is recommended before surgery to assess bleeding risk. Inflammatory biomarkers such as C-reactive protein (CRP) are not specific for CM/SM. Synovial fluid analysis is not relevant. Blood gas analysis may be performed if there are respiratory concerns, but it is not routinely indicated.

MRI is the imaging modality of choice for CM/SM. Findings include: 1) Caudal fossa crowding with herniation of the cerebellum through the foramen magnum (often >2 mm in dogs). 2) Indentation of the brainstem by the occipital bone. 3) Syringomyelia, which appears as a hyperintense (on T2-weighted) or hypointense (on T1-weighted) fluid-filled cavity within the spinal cord, most commonly in the cervical region. 4) Altered CSF flow, which can be assessed with phase-contrast MRI. 5) Hydrocephalus may be present. CT can show the bony malformation, such as a small occipital bone and a short basicranium, but it is less sensitive for syrinx detection. Radiography is not useful for diagnosing CM/SM but may show signs of cervical spondylosis or other concurrent conditions. Ultrasonography is not used. In some cases, fluoroscopy may be used to assess CSF flow dynamics, but it is not standard.

Cytology and histopathology are not typically performed for CM/SM, as the diagnosis is based on imaging. However, if surgery is performed, histopathological examination of the dura mater or bone may be done, but it is not diagnostic. In research settings, histopathology of the spinal cord may show syrinx cavities, gliosis, and axonal degeneration. CSF cytology may be performed to rule out inflammation or neoplasia, but it is usually normal in CM/SM.

Medical management is the first line of treatment for CM/SM. It includes: 1) Corticosteroids (e.g., prednisone at 0.5-1 mg/kg/day PO, tapering) to reduce inflammation and CSF pressure. 2) Non-steroidal anti-inflammatory drugs (NSAIDs) such as carprofen (2.2 mg/kg PO q12h) or meloxicam (0.1 mg/kg PO q24h) for pain relief. 3) Analgesics such as gabapentin (10-20 mg/kg PO q8-12h) or pregabalin (2-4 mg/kg PO q8-12h) for neuropathic pain. 4) Diuretics such as furosemide (1-2 mg/kg PO q12h) or acetazolamide (5-10 mg/kg PO q8h) to reduce CSF production. 5) Omeprazole (0.5-1 mg/kg PO q12h) may be used to reduce CSF production. 6) Weight management and activity restriction. Surgical treatment is indicated for dogs with progressive neurological deficits or severe pain refractory to medical management. Surgical options include: 1) Foramen magnum decompression (FMD) - a suboccipital craniectomy and dorsal laminectomy of the atlas (C1) to enlarge the foramen magnum and decompress the brainstem and cerebellum. The dura mater may be opened (durotomy) and a dural patch graft (e.g., porcine small intestinal submucosa) applied to expand the dura. 2) Ventriculoperitoneal (VP) shunting - used for concurrent hydrocephalus. 3) Syrinx shunting - placement of a shunt from the syrinx to the subarachnoid space or peritoneum, but this is less commonly performed. The surgical approach for FMD involves a dorsal midline incision from the occipital protuberance to the C2-C3 region. The occipital bone is removed using a high-speed burr, and the dorsal lamina of C1 is removed. The dura is opened, and a dural graft is sutured in place with 5-0 or 6-0 polydioxanone (PDS) or polypropylene. Postoperative care includes pain management, antibiotics, and strict rest. Complications include CSF leakage, infection, and worsening of neurological signs.

The prognosis for CM/SM is variable. Medical management can control clinical signs in many dogs, but the disease is often progressive. Surgical decompression can improve clinical signs in about 70-80% of dogs, but recurrence is possible. Factors associated with a poorer prognosis include severe syringomyelia, long duration of clinical signs, and presence of neurological deficits. The overall long-term prognosis is guarded, with many dogs requiring lifelong medication. In a study of CKCS with SM, about 50% of dogs remained stable or improved with medical management, while others progressed. Surgical treatment may provide better outcomes in selected cases, but complications are not uncommon.

Postoperative follow-up is essential. After surgery, patients should be monitored for 24-48 hours in the hospital. Suture removal is typically at 10-14 days. Neurological examinations should be performed at 2, 4, 8, and 12 weeks postoperatively. MRI may be repeated at 3-6 months to assess syrinx size and CSF flow. Activity should be restricted for 4-6 weeks, with gradual return to normal. Physical therapy may be beneficial. Long-term monitoring includes regular neurological examinations and imaging if clinical signs recur. Medical management may need to be adjusted based on response.

Pearls: 1) MRI is essential for diagnosis and surgical planning. 2) Foramen magnum decompression should be performed with a high-speed burr to avoid iatrogenic injury. 3) A dural patch graft can reduce the risk of adhesions and CSF leakage. 4) Postoperative pain management should include multimodal analgesia. 5) Early surgical intervention may improve outcomes. Pitfalls: 1) Failure to identify concurrent hydrocephalus may lead to poor surgical outcomes. 2) Incomplete decompression can lead to recurrence. 3) CSF leakage can cause subcutaneous fluid accumulation and infection. 4) Overly aggressive durotomy can cause cerebellar herniation. 5) Not addressing obesity can worsen clinical signs.

Perioperative drug protocols based on Plumb's Veterinary Drug Handbook: 1) Prophylactic antibiotics: cefazolin (22 mg/kg IV) administered 30 minutes before incision and repeated every 90 minutes during surgery. 2) Postoperative antibiotics: amoxicillin-clavulanate (13.75 mg/kg PO q12h) for 7-10 days. 3) Analgesics: fentanyl CRI (2-5 mcg/kg/hr IV) during surgery and for 24 hours postoperatively; then transition to oral opioids such as tramadol (2-5 mg/kg PO q8-12h) or codeine (1-2 mg/kg PO q6-8h). 4) NSAIDs: carprofen (2.2 mg/kg PO q12h) or meloxicam (0.1 mg/kg PO q24h) for 3-7 days, starting after surgery. 5) Neuropathic pain: gabapentin (10-20 mg/kg PO q8-12h) or pregabalin (2-4 mg/kg PO q8-12h) for chronic pain. 6) Corticosteroids: prednisone (0.5 mg/kg PO q12h) tapering over 2-4 weeks to reduce inflammation. 7) Gastroprotectants: omeprazole (0.5-1 mg/kg PO q12h) or famotidine (0.5 mg/kg PO q12h) to prevent gastric ulcers. 8) Antiemetics: maropitant (1 mg/kg PO q24h) if needed. 9) Diuretics: furosemide (1-2 mg/kg PO q12h) or acetazolamide (5-10 mg/kg PO q8h) to reduce CSF production, but use with caution. 10) For refractory pain, consider amantadine (3-5 mg/kg PO q24h) or ketamine CRI (0.5 mg/kg/hr IV) for 24 hours.

Key studies: 1) Rusbridge et al. (2000) described the clinical and MRI findings in CKCS with CM/SM, establishing the diagnostic criteria. 2) Dewey et al. (2005) reported on foramen magnum decompression in dogs with CM/SM, showing improvement in 80% of cases. 3) Driver et al. (2010) evaluated the long-term outcome of medical vs surgical treatment, finding that surgery may be beneficial in severe cases. 4) Cerda-Gonzalez et al. (2009) investigated the genetic basis of CM in CKCS, identifying multiple loci. 5) A consensus statement from the ACVS (2015) recommends MRI for diagnosis and surgical decompression for dogs with progressive signs. 6) A meta-analysis by Knowler et al. (2017) found that FMD improves clinical signs but has a complication rate of up to 30%. 7) Recent studies have explored the use of dural patch grafts to reduce adhesions and improve CSF flow. Overall, the evidence supports surgical decompression for refractory cases, but careful patient selection is crucial.