Femoral Nerve Mononeuropathy
History. You examine an 8-year-old male golden retriever. The owner complains that the dog cannot bear weight on the right rear leg.
Clinical Examination. Physical examination deficits are limited to the right rear leg, where you find that the quadriceps femoris muscles are much smaller than those of the left rear leg.
The dog cannot bear weight on the right rear leg because the right quadriceps femoris muscles are paralyzed. When you tap on the left patellar tendon with a reflex hammer, the knee briskly extends (the knee jerk or muscle stretch reflex). However, when you lap on the right patellar tendon, no movement occurs.Comment. The quadriceps femoris muscle group is one of the major antigravity muscle groups of the leg causing the stifle joint (knee joint) to extend. The paralysis in this animal’s quadriceps muscle is the reason why he cannot bear weight on the leg. The small size of the right quadriceps muscle is caused by atrophy, or muscle wasting, which in turn is caused by the loss of α motor neurons to the extrafusal muscle fibers in the quadriceps muscle belly (see Chapter 9). This would also cause a loss of the muscle stretch reflex, because even though the spindle detected the stretch of the muscle belly caused by either gravity or the reflex hammer, the α motor neurons returning to the quadriceps muscle are unable to signal the muscle to contract, thus completing the reflex arc. This syndrome could occur if the femoral nerve is damaged by a tumor or trauma. If the pathological lesion were in the peripheral nerve rather than only in the ventral roots, there would likely be some sensory loss in addition to the motor deficits.
Treatment. This is a femoral nerve mononeuropathy. Its treatment depends on the cause of the nerve damage (e.g., trauma, neoplasia, inflammation).