Arm monoparesis is characterized by decreased frequency and strength of upper extremity movements due to weakness. Arm monoparesis, by definition, excludes arm weakness in the context of hemiparesis, upper extremity diplegia, and decreased arm movements due to restricted range of motion or pain.

Arm monoparesis is due to arm motor system injury (Figure 201.1). The arm motor system has a central component and a peripheral component. The central component of the arm motor system consists of the cerebral cortical motor neurons, brainstem neurons whose axons make contact with the motor neurons of the peripheral component of the arm motor system, and cerebellar neurons that influence the arm motor system cortical and brainstem neurons (upper motor neuron). The peripheral component of the arm motor system consists of motor neurons in the cervical enlargement of the spinal cord whose axons innervate the upper extremity muscles (lower motor neuron).

Figure 201.1.— Schematic representation of the motor systems of the face, arms, and legs, and central and peripheral nervous systems structures involved in limb movements. The colored rectangles indicate the location of weakness produced by damage to the different components of the somatic motor system. U: upper motor neurons; V: ventricles; T: thalamus; UQ: upper quadrant; FN: facial nerve; LQ: lower quadrant; L: lower motor neurons; BP: brachial plexus; LSP: lumbosacral plexus.