What is a Syrinx?
A syrinx (also called syringomyelia when referring to the disease state) is a fluid-filled cavity within the spinal cord that forms when cerebrospinal fluid accumulates in the central canal, creating a cystic dilation that can cause progressive neurological damage.
Pathophysiology and Formation
The syrinx represents an abnormal collection of cerebrospinal fluid (CSF) within the spinal cord parenchyma. Based on recent evidence, the fluid originates from the subarachnoid space through transparenchymal bulk flow when CSF pathway obstruction occurs 1. This mechanism has been demonstrated through contrast studies showing that more subarachnoid contrast enters syringes when associated with CSF pathway obstruction (such as in Chiari malformation) compared to tumor-related syringes 1.
The formation involves:
- Blockage of CSF flow at the craniocervical junction or along the spinal canal
- Pressure-wave-induced fluid flux across the pial membrane surrounding the spinal cord 2
- Accumulation in the central canal creating progressive expansion
Clinical Context and Associations
Syringes most commonly occur in association with:
Chiari Malformation Type I (Most Common)
- CIM may cause syringomyelia through obstruction of CSF flow at the foramen magnum 3
- Present in a significant proportion of Chiari patients
- The cervico-dorsal region is most frequently affected (31.4% of cases), followed by cervical (30.4%) and dorsolumbar segments (21.7%) 4
Spinal Subarachnoid Space Obstruction
- Any obstruction along the spinal canal can lead to syrinx formation
- Demonstrates similar transparenchymal CSF flow patterns as Chiari-related syringes 1
Intramedullary Tumors
- Tumor-related syringes have different fluid dynamics - the fluid does NOT originate from the subarachnoid space in these cases 1
- Requires different management approach
Post-Traumatic (Spinal Cord Injury)
- Can develop after spinal cord contusion or injury
Imaging Characteristics
On MRI (the gold standard for diagnosis):
T2-weighted imaging characteristics:
- Appears as centrally located, continuous fluid-filled cavity within the spinal cord 4
- Typically extends beyond 3 cm in length (55.6% of cases) 4
- All syringes are centrally located with no eccentric or skip-pattern cavities observed 4
Key imaging features to assess:
- Syrinx nodularity - suggests possible associated mass
- Syrinx septations - internal divisions within the cavity
- Spinal cord signal abnormality separate from the syrinx
- Size correlation - larger syringes show positive correlation with presence of mass lesions 5
Important caveat: T2-weighted imaging alone has 100% sensitivity and 100% negative predictive value for detecting syrinx-associated masses 5. Contrast-enhanced imaging may not be required if T2 sequences show no suspicious features (no nodularity, no septations, no separate cord signal abnormality) 5.
Clinical Manifestations
Neurological deficits result from:
- Compression of spinal cord tracts as the cavity expands
- Lower motor neuron features (55% of cases) - more common than upper motor neuron signs 4
- Upper motor neuron features (45% of cases) 4
Specific symptoms depend on syrinx location and extent but may include:
- Progressive weakness
- Sensory loss (particularly dissociated sensory loss - loss of pain/temperature with preserved light touch)
- Bladder/bowel dysfunction
- Pain
- Gait disturbance
Management Implications
Critical distinction: Symptom resolution and syrinx resolution do not correlate directly 3. This means:
- A patient's symptoms may improve even if the syrinx persists radiographically
- Conversely, syrinx reduction doesn't guarantee symptom improvement
- Treatment decisions should prioritize clinical symptoms over radiographic appearance alone
Surgical timing for persistent syrinx: According to 2023 Congress of Neurological Surgeons guidelines, surgeons may perform additional neurosurgical intervention 6-12 months following initial surgical treatment in patients who have not demonstrated radiographic improvement 3 (Grade B recommendation, Class II evidence).
Predictors of persistent syrinx requiring reoperation:
- Syrinx diameter >10 mm at thickest point
- Extension over more than 10 vertebrae
- Starting from upper cervical region extending to upper thoracic region These features may predict need for second surgery before initial posterior fossa decompression 6