Flaccid Arm Monoplegia Due to an Upper Motor Neuron Lesion
Flaccid arm weakness due to an upper motor neuron lesion occurs immediately after the insult (Figure 213.1).

A	B

Figure 213.1.—Flaccid right arm weakness. [A] Patient had a seizure and then developed right arm weakness. The patient had a neurological evaluation 10 hours after the seizure. Muscle stretch reflexes and Moro reflex were decreased. [B] MRI revealed a left posterior limb internal capsule stroke.

The site of anatomical involvement may be the brain, brainstem, cerebellum, or upper cervical spine (Figure 213.2 A-D). Neonates with flaccid arm weakness have an asymmetrical Moro reflex due to lack of movement of the affected arm during the maneuver and decreased muscle stretch reflexes.

Figure 213.2.—Sites of possible nervous system injury that can produce arm monoparesis: A: brain to midbrain; B: upper pons; C: lower pons and medulla; D: rostral spinal cord; E: brachial center; F: brachial plexus; G: peripheral nerves; V: ventricles; T: thalamus; UQ: upper quadrant; FN: facial nerve; LQ: lower quadrant; BP: brachial plexus; LSP: lumbosacral plexus. The colored rectangles indicate the location of weakness produced by damage to the different components of the somatic motor system.

Localization of an upper motor neuron lesion to the brain, brainstem, or spinal cord may be difficult. The distribution of the arm weakness is not helpful. Acute upper motor neuron lesions produce weakness that involves the shoulder, arm, forearm, and hand equally, regardless of the location of the lesion within the upper motor neuron. Several other neurological findings may help localize the lesion within the upper motor neuron. Flaccid facial weakness may be present with lesions of the pons, midbrain, and brain. Facial weakness due to a lesion in the brain and midbrain is ipsilateral to the arm weakness. Facial weakness due to a lesion at the pons is contralateral to the arm weakness. Facial weakness will not be present with lesions localized to the cerebellum, medulla, or the cervical spine unless a second anatomical site is involved by the same pathological process. Flaccid arm weakness due to an upper motor neuron lesion may be associated with gaze preference and convulsions. Gaze preference occurs with brain involvement at the cortex because of the close proximity of the cortical area that controls rapid eye movements and arm movements. Gaze preference with eye deviation away from the weak arm usually occurs, but irritative lesions causing seizures may produce eye deviation toward the weak arm associated with nystagmoid movements. Convulsions involving the paralyzed arm with secondary generalization may occur.
The distinction between upper and lower motor neuron flaccid arm monoplegia may be difficult because the neurological manifestations are similar. The distinction relies on finding signs of central nervous system involvement suggesting an upper motor neuron lesion, or the typical arm posture characteristic of a lower motor neuron lesion. Our approach in cases when the anatomical diagnosis can not be made by clinical evaluation is to obtain a brain MRI. We look for the possibility of an upper motor neuron lesion first because the causes of upper motor neuron lesions may require immediate intervention. If brain MRI is normal, we do an EMG of the affected arm and an MRI of the brachial plexus.