Why is the lateral spinothalamic tract (LST) often spared in syringomyelia?

Why the Lateral Spinothalamic Tract is Not Involved in Syringomyelia

The lateral spinothalamic tract is typically spared in syringomyelia because the central cavity formation primarily affects the central gray matter and crossing fibers in the anterior commissure, while the lateral spinothalamic tract is located more peripherally in the anterolateral white matter of the spinal cord.

Anatomical Basis for Selective Involvement

Syringomyelia is characterized by the development of fluid-filled cystic cavities within the spinal cord. These cavities have a distinct pattern of expansion that explains the classic dissociated sensory loss seen in this condition:

• Central Cavity Formation: The syrinx typically begins as a central canal dilation (hydromyelia) that expands outward 1
• Anatomical Spread Pattern: The cavity preferentially expands into the central gray matter and crosses the anterior commissure 2
• Affected Pathways: This expansion pattern primarily affects:
  • Decussating pain and temperature fibers in the anterior commissure
  • Central gray matter structures
  • Anterior white commissure fibers

Spinothalamic Tract Anatomy and Preservation

The lateral spinothalamic tract has several anatomical characteristics that explain its relative preservation:

• Peripheral Location: The tract is located in the anterolateral quadrant of the white matter 3
• Distance from Initial Expansion: Being more laterally positioned, it is farther from the initial central expansion of the syrinx
• Fiber Organization: The already-crossed fibers in the lateral spinothalamic tract are more compact and resistant to compression than the crossing fibers in the anterior commissure

Clinical Manifestations Supporting This Pattern

The clinical presentation of syringomyelia provides evidence for this selective involvement:

• Classic Dissociated Sensory Loss: Loss of pain and temperature sensation with preservation of touch, vibration, and proprioception 2
• Cape-Like Distribution: Bilateral sensory loss in a cape-like distribution over the shoulders and upper extremities
• Segmental Pattern: The sensory deficits typically follow a segmental pattern corresponding to the levels affected by the syrinx

Imaging Confirmation

MRI findings in syringomyelia support this understanding:

• Cavity Configuration: On axial MRI, the syrinx typically appears as a central or slightly eccentric cavity 4
• Expansion Pattern: The cavity expands centrally first, then dorsally and ventrally, before expanding laterally 5
• Red Flag Finding: As noted in Brain (2019), spinal cord cavities are a "red flag" finding that distinguishes syringomyelia from other conditions like multiple sclerosis 4

Pathophysiological Mechanisms

The underlying mechanisms for this selective involvement include:

• CSF Flow Disturbances: The prevailing theory is that disturbances in CSF flow dynamics lead to syrinx formation 1
• Pressure Gradients: Abnormal pressure gradients cause fluid to accumulate in the central canal first
• Tissue Resistance: The white matter tracts offer more resistance to expansion than the central gray matter
• Decussating vs. Decussated Fibers: Fibers in the process of crossing (in the anterior commissure) are more vulnerable than those already crossed and organized in the lateral spinothalamic tract

Clinical Implications

Understanding this selective involvement has important clinical implications:

• Diagnostic Value: The dissociated sensory loss pattern is a key diagnostic feature of syringomyelia
• Monitoring Disease Progression: Extension of sensory deficits laterally may indicate expansion of the syrinx
• Surgical Planning: The goal of surgery is to re-establish physiological CSF pathways, typically by enlarging the craniocervical junction with duraplasty in Chiari-associated syringomyelia 4

Associated Conditions

Syringomyelia is often associated with other conditions:

• Chiari I Malformation: The most common association, requiring posterior fossa decompression 4
• Spinal Dysraphism: Found in 2-4% of cases initially thought to be idiopathic 6
• Trauma: Post-traumatic syringomyelia can develop after spinal cord injury 7

Understanding the anatomical basis for the sparing of the lateral spinothalamic tract in syringomyelia helps explain the characteristic dissociated sensory loss and guides appropriate diagnostic and therapeutic approaches.