Westphal's sign is the clinical correlate of the absence or decrease of patellar reflex or knee jerk. Patellar reflex or knee jerk is a kind of deep or stretch reflex where an application of a stimulus to the patellar tendon such as strike by a solid object or hammer caused the leg to extend due to such stimulus causes the quadriceps femoris muscle to contract.
It is named for Carl Friedrich Otto Westphal (1833-1890).
Westphal's sign Wikipedia
Westphal's sign has a clinical significance used in determining neurodisorders or diseases such as:receptor damage, peripheral nerve disease, involving the dorsal(sensory) columns of the spinal cord and cerebellar lesions
lesions present within the motor cortex of the brain or the pyramidal tracts which it combined with muscular spasms
complete interruption of sensory and/or motor impulse transmission in the femoral nerve