Unilateral localized weakness of leg muscles

Unilateral local weakness of the leg muscles is often the initial stage of a lesion that later becomes generalized. This is especially true for, for example, amyotrophic lateral sclerosis, which often debuts with unilateral distal or proximal weakness; spinal muscular atrophy (rare), polymyositis, and myasthenia.

The causes of a local motor defect most often lie in dysfunction of the roots, plexuses, or peripheral nerves. Motor disturbances are usually accompanied by pain and sensory disturbances. With a typical topography of the distribution of disturbances and, especially, when there is a classic anamnesis, the diagnosis is usually not in doubt. Difficulties arise when the clinical examination does not reveal typical causes for local disturbances, and the distribution of motor disturbances is atypical or is not accompanied by loss of sensitivity.

Unilateral weakness of the thigh muscles and hip flexors caused by lumbar plexus lesions is observed in retroperitoneal hematoma and diabetes mellitus, which are usually accompanied by severe pain. A similar situation may be found in superior subperiosteal traumatic hematoma of the upper ilium. A rare form of lumbosacral plexus lesions after X-ray therapy or some retroperitoneal tumors also causes weakness, rarely without pain, but with corresponding loss of sensitivity. Femoral nerve lesions leading to paresis of the quadriceps femoris and m. ileopsoas (manifested by the inability to raise the thigh and the absence of a knee reflex) are accompanied by loss of sensitivity in the thigh and medial surface of the leg. Such lesions may be observed, for example, after surgical interventions (herniorrhaphy, etc.). Repeated intramuscular injections in newborns may cause contracture of the quadriceps muscle (without paresis). In adults, injections can lead to paresis of the gluteus medius and minimus muscles (Duchenne or Trendelenburg claudication), often without pain.

In the lower leg, acute paresis of the dorsiflexors of the foot and toes is observed as a result of ischemia in lesions of the anterior tibial artery (due to excessive muscle activity or painful, initially ischemic swelling of the anterior tibial region). This syndrome is characterized initially by the absence of pulse in the posterior artery of the foot, often accompanied by transient ischemia of the perforating branch of the peroneal nerve, and later by contracture of the muscles of the anterior region, leading to a clawed posture of the big toe (which prevents the development of foot drop). Rupture of the Achilles tendon (an extremely painful condition) causes incomplete impairment of plantar flexion of the foot (since the posterior tibialis and peroneus muscles act together as flexors of the foot).

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