Discovery of a Novel Yadav-Kunal Reflex in Context of Upper Motor Neuron Lesions: A Report of Three Cases.

Introduction: Numerous reflexive responses have been documented as alterations to the Babinski sign within upper motor neuron lesions. However, scant attention has been given to reflexes beyond these, which exhibit independence from the extensor plantar response. These reflexes predominantly form polysynaptic arcs, with nociceptive stimuli acting as afferents. Case Report: The reflex was serendipitously discovered in an 18-year-old female patient who presented with spastic paraplegia with bowel and bladder involvement, as a consequence of an aneurysmal bone cyst of the D3 (dorsal) vertebrae, and the same was named after the authors as "Yadav-Kunal reflex" which can be defined as: "In individuals with spastic paraparesis, forcibly plantarflexing the toes will result in sudden jerky flexion of the knee and hip on the same side." This novel reflex was further investigated and validated in two additional patients with spastic paraplegia: one, a 45-year-old female with D9-D10 Pott's spine and bowel and bladder involvement, and the other, a 65-year-old male with D10-D11 compressive myelopathy and bowel and bladder involvement. This reflex was meticulously tracked until the abatement of spasticity following surgical intervention. Notably, its manifestation was evident in individuals experiencing spastic paraparesis, dissipating concomitantly with the resolution of spasticity - a direct clinical correlation. Conversely, the reflex was conspicuously absent in cases of flaccid paraplegia. Conclusion: Spasticity, characterized by an increase in muscle tone on swift stretching movements, is a manifestation of a stretch reflex disorder. This condition is primarily induced by lesions affecting upper motor neurons. The activation of muscle spindles in toe dorsiflexors (primarily governed by the L5 nerve) occurs during forceful elongation caused by plantarflexion.