The muscles of mastication can be observed as an infant sucks and swallows. The corneal reflex can test cranial nerves V ophthalmic division and VII at any age. Facial sensation of light touch and pain can be determined with cotton gauze and pinprick. Facial sensation can be functionally assessed in an infant by gently brushing the cheek, which will produce the rooting reflex (turns head and neck with mouthing movement, as if seeking to nurse).
Facial muscles are assessed by observing the face during rest, crying, and blinking. At older, cooperative ages, children can be asked to smile, blow out their cheeks, blink forcibly, and furrow their foreheads. Weakness of all unilateral muscles of the face, including the forehead, eye, and mouth, indicates a lesion of the ipsilateral peripheral facial nerve (Bell palsy).Because the upper third of the face receives bilateral cortical innervation, if the weakness affects only the lower face and mouth, a contralateral lesion of upper motor neuron in the brain (tumor, stroke, abscess) must be considered.
Lesions of cranial nerve VIII cause deafness, tinnitus, and vertigo. Normally, alert neonates blink in response to a bell or other abrupt, loud sound. Four-month-old infants turn their head and eyes to localize a sound. Hearing can be tested in a verbal child by whispering a word in one ear while covering the opposite ear. If there are any concerns about hearing, formal audiologic assessment is indicated.
Lesions of the vestibular component of cranial nerve VIII produce symptoms of vertigo, nausea, vomiting, diaphoresis, and nystagmus. Nystagmus is an involuntary beating eye movement with a rapid phase in one direction and a slow phase in the opposite direction. By convention, the direction of the nystagmus is defined by the fast phase and may be horizontal, vertical, or rotatory.
The gag reflex is brisk at all ages except the very immature neonate. An absent gag reflex suggests a lower motor neuron lesion of the brainstem, cranial nerves IX or X, neuromuscular junction, or pharyngeal muscles. Weak, breathy, or nasal speech; weak sucking; drooling and inability to handle secretions; gagging and nasal regurgitation of food are additional symptoms of cranial nerve X dysfunction.
Observing the infant’s posture and spontaneous activity assesses the functions of the trapezius and sternocleidomastoid muscles. Head tilt and drooping of the shoulder suggest lesions involving cranial nerve XI. In later childhood, strength in these muscles can be tested directly and individually.
Atrophy and fasciculation of the tongue, usually indicating a lesion of the anterior horn cells (spinal muscular atrophy [SMA]), can be observed at any age and are assessed most reliably when the infant is asleep. The tongue deviates toward the weak side in unilateral lesions.
In infants, power is assessed by observation of spontaneous movements and movements against gravity. Arm and leg movements should be symmetrical, seen best when the infant is held supine with one hand supporting the buttocks and one supporting the shoulders. The limbs should easily lift up. Power is graded as follows: 5 Normal 4 Weak but able to provide resistance 3 Able to move against gravity but not against resistance 2 Unable to move against gravity