Note any atrophy or fasciculations (spontaneous quivering movements caused by firing of muscle motor units) of the tongue while it is resting on the floor of the mouth. Ask the patient to stick their tongue straight out and note whether it curves to one side or the other. Ask the patient to move their tongue from side to side and push it forcefully against the inside of each cheek.
Fasciculations and atrophy are signs of lower motor neuron lesions. Unilateral tongue weakness causes the tongue to deviate toward the weak side. Tongue weakness can result from lesions of the tongue muscles, the neuromuscular junction, the lower motor neurons of the hypoglossal nerve (CN XII), or the upper motor neurons originating in the motor cortex. Lesions of the motor cortex cause contralateral tongue weakness.