Which nerve conduction parameter is key to diagnosing Guillain-Barré syndrome?

The key nerve conduction parameter for diagnosing Guillain-Barré syndrome is a significant decrease in conduction velocity while maintaining normal sensory amplitudes. This presentation is particularly characteristic of Guillain-Barré syndrome, where patients often experience demyelination of peripheral nerves.

In this condition, the nerve conduction studies typically reveal reduced conduction velocity due to the damage to the myelin sheath surrounding the nerves. However, sensory nerve action potentials may remain relatively preserved, which helps differentiate Guillain-Barré syndrome from other neuropathies that may cause similar symptoms but with significant changes in sensory amplitudes as well.

This pattern helps clinicians confirm the diagnosis by indicating that despite the slowing of conduction speed, there is still an intact capacity for sensory transmission, which aligns with the clinical profile of Guillain-Barré syndrome, typically presenting with ascending weakness and sensory alterations.