Syringomyelia

01Pathophysiology

Syringomyelia refers to the development of a syrinx, a longitudinal fluid-filled cavity that forms within the substance of the spinal cord, most frequently in the cervical and upper thoracic segments. The most common underlying cause is Chiari I malformation, in which the cerebellar tonsils herniate more than 5 mm below the foramen magnum. This herniation obstructs the normal flow of cerebrospinal fluid (CSF) at the craniocervical junction. Over time, the altered CSF dynamics create a pressure gradient that forces fluid into the central canal of the spinal cord, leading to progressive cavitation.

The reason this disease produces such a distinctive clinical pattern lies in the anatomy of the spinal cord cross-section. The syrinx begins in the central region of the cord, where it first disrupts the anterior white commissure. This is the structure where second-order spinothalamic neurons cross from one side to the other before ascending. Because these fibers carry pain and temperature information, bilateral destruction of the crossing fibers produces the classic finding: loss of pain and temperature sensation in a dermatomal band across the shoulders, arms, and upper trunk, while light touch and proprioception (carried by the dorsal columns, which are located posteriorly) remain intact. This selective pattern is called dissociated sensory loss, and it is the hallmark feature tested on exams.

As the syrinx enlarges, it expands outward and begins to involve adjacent structures. Anterior horn cell destruction at the level of the lesion produces lower motor neuron (LMN) signs in the upper extremities, including muscle wasting, weakness, fasciculations, and decreased reflexes, particularly in the hand intrinsics (thenar and hypothenar muscles). If the cavity extends laterally into the lateral corticospinal tracts, upper motor neuron (UMN) signs appear in the lower extremities, including spasticity, hyperreflexia, and extensor plantar responses (Babinski sign). In cervicothoracic syrinxes, disruption of the ciliospinal center of Budge (C8-T2) can interrupt the sympathetic chain, producing an ipsilateral Horner syndrome (ptosis, miosis, anhidrosis).

The key concept for exam purposes is that syringomyelia produces a "central cord" pattern that evolves from the inside out: crossing spinothalamic fibers first, then anterior horn cells, then corticospinal tracts, and finally dorsal columns (a late and uncommon finding).

02Classification and Clinical Manifestation

By Etiology

By Stage of Expansion

03Diagnostic Workup

The best initial test is an MRI of the entire spine. This is also the most accurate (confirmatory) test. On T2-weighted images, the syrinx appears as a well-defined, hyperintense, longitudinal cavity within the spinal cord that follows CSF signal intensity. The T1-weighted sequence will show the cavity as hypointense. Gadolinium contrast is added to rule out an enhancing intramedullary tumor (such as ependymoma or hemangioblastoma) as the cause of the syrinx, since tumors enhance while a simple syrinx does not.

Once a syrinx is identified, the next step is to determine the underlying etiology. An MRI of the brain including the craniocervical junction is ordered to evaluate for Chiari I malformation. The hallmark finding is cerebellar tonsils descending more than 5 mm below the foramen magnum on sagittal imaging.

A cine-phase contrast MRI (CSF flow study) can be performed preoperatively. This dynamic study evaluates CSF flow at the foramen magnum and is helpful in surgical planning, demonstrating whether there is a flow obstruction that would benefit from decompression.

EMG and nerve conduction studies are not required for diagnosis but can be useful when the clinical picture is unclear, as they can confirm anterior horn cell involvement and help distinguish syringomyelia from conditions such as cervical radiculopathy, carpal tunnel syndrome, or amyotrophic lateral sclerosis.

A critical exam point: a syrinx that is identified incidentally on MRI does not always require treatment. Small, stable, asymptomatic syrinxes can be monitored with serial imaging.

04Management and Treatment

Step 1: Confirm diagnosis and identify the cause. Once MRI confirms the syrinx, the underlying etiology dictates the treatment plan. The most common testable scenario is a syrinx secondary to Chiari I malformation.

Step 2: Observation vs. intervention. If the patient is asymptomatic and the syrinx is discovered incidentally, the appropriate next step is clinical and radiographic surveillance. Serial MRI is performed every 6 to 12 months initially. Surgery is reserved for patients with progressive neurological deficits, worsening syrinx size on imaging, or debilitating symptoms.

Step 3: Surgical treatment for Chiari-associated syringomyelia. The first-line surgical intervention is posterior fossa decompression, which involves a suboccipital craniectomy, C1 (and sometimes C2) laminectomy, and duraplasty. This procedure restores normal CSF circulation at the craniocervical junction, which in the majority of patients leads to stabilization or reduction of the syrinx. The goal is to address the obstruction causing the syrinx, not to drain the syrinx directly.

Step 4: Refractory cases. If neurological deterioration continues or the syrinx does not decrease in size after posterior fossa decompression, a syringosubarachnoid or syringoperitoneal shunt may be placed. However, shunts carry risks of obstruction, infection, and recurrence, so they are considered second-line.

Step 5: Address associated conditions. In tumor-related syringomyelia, the treatment is resection of the intramedullary tumor. In post-traumatic cases, surgical lysis of adhesions or untethering may be necessary. Arachnoiditis-related syrinxes are challenging and may require shunting.

Step 6: Symptomatic management. Neuropathic pain is treated with gabapentin (starting at 300 mg daily, titrated up to 3600 mg/day in divided doses) or pregabalin (starting at 75 mg twice daily, up to 300 mg twice daily). Physical therapy and occupational therapy are important adjuncts for patients with motor deficits. Patients should be counseled to avoid activities that increase intrathoracic pressure (heavy lifting, straining, Valsalva maneuver), as these can worsen the syrinx.

Contraindications and cautions: Lumbar puncture should be performed with caution in patients with Chiari malformation due to the risk of tonsillar herniation. In patients with known Chiari I, a brain MRI should always be reviewed before any LP.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most common way students get questions about syringomyelia wrong is by failing to recognize the dissociated sensory loss pattern. When a vignette describes a young patient with bilateral loss of pain and temperature in the upper extremities but preserved light touch, many students default to diagnoses like MS, cervical radiculopathy, or peripheral neuropathy. The key is recognizing that selective loss of pain and temperature with intact light touch and proprioception points to disruption of crossing spinothalamic fibers, and the bilateral, suspended, cape-like distribution is the signature of a central cord process.

A second common trap is confusing syringomyelia with an intramedullary tumor. Both can look alike clinically and even on initial MRI sequences. The vignette will often include the detail that the lesion "follows CSF signal on all sequences" and "does not enhance with gadolinium." If the lesion enhances, think tumor. If it does not, think syrinx.

Students also frequently miss the Chiari I connection. When a question describes a patient with occipital headaches worsened by coughing or straining, combined with the cape-like sensory loss, the expected answer links the syrinx to Chiari I malformation, and the appropriate management is posterior fossa decompression rather than direct drainage of the syrinx.

Another testable pearl: the "painless burn" clue. Vignettes may describe a patient who repeatedly injures or burns their hands without feeling pain. This is a classic setup for syringomyelia, and it tests whether you understand that pain fibers are selectively lost while tactile sensation remains.

Finally, remember the motor pattern: LMN signs in the upper extremities (atrophy, weakness, decreased reflexes) combined with UMN signs in the lower extremities (spasticity, hyperreflexia, Babinski) is the hallmark of a cervical spinal cord lesion expanding from the center outward. This combination of LMN-at-the-level and UMN-below-the-level is heavily tested and should immediately prompt consideration of syringomyelia or a central cord process.

The core competency being tested is the ability to localize the lesion to the central spinal cord based on the pattern of neurological deficits, connect that localization to the correct diagnosis, identify the underlying cause, and select the appropriate management sequence.