Cranial Nerves XI and XII: Accessory and Hypoglossal

Mastering the accessory (CN XI) and hypoglossal (CN XII) nerves is essential for any clinician, as they are the final purely motor nerves in the cranial series and are critical for fundamental actions like head turning, shoulder shrugging, and speech articulation. Damage to these nerves provides clear, localizing neurological signs that can pinpoint the site of a lesion, from the brainstem to the surgical neck.

Anatomical Pathways and Nuclei

The spinal accessory nerve has a unique origin, arising from both cranial and spinal roots. The cranial root originates from the nucleus ambiguus in the medulla, but its fibers quickly join the vagus nerve (CN X) to supply the muscles of the larynx. The clinically significant spinal root arises from motor neurons in the anterior horn of the upper cervical spinal cord (C1-C5). These nerve fibers ascend through the foramen magnum into the cranial cavity, briefly join the cranial root, and then exit the skull through the jugular foramen. The nerve then descends through the posterior cervical triangle—a notably superficial and vulnerable course—to innervate the sternocleidomastoid (SCM) and trapezius muscles.

In contrast, the hypoglossal nerve is a purely somatic motor nerve. Its nucleus is located in the medulla oblongata. Axons exit the medulla as rootlets between the olive and the pyramid, which then converge to form CN XII. The nerve exits the skull via the hypoglossal canal and descends into the neck, where it loops around the occipital artery before passing deep to the posterior belly of the digastric muscle to reach the tongue. This entire pathway innervates all intrinsic and most extrinsic muscles of the tongue.

Muscle Functions and Clinical Examination

Each nerve commands specific, testable movements. The spinal accessory nerve exclusively innervates two major muscles: the sternocleidomastoid and the trapezius.

• Sternocleidomastoid (SCM): This muscle has two actions depending on whether one or both sides contract. To test it, ask the patient to turn their head to one side against resistance. The contracting SCM you are palpating is on the opposite side; the right SCM turns the head to the left. A weak right SCM will cause difficulty turning the head to the left.
• Trapezius: This large back muscle elevates, retracts, and rotates the scapula. The classic test is shoulder shrugging against downward resistance. Weakness leads to a drooping shoulder and difficulty raising the arm above the horizontal.

The hypoglossal nerve is the master controller of the tongue. It innervates all intrinsic muscles (which alter the tongue's shape) and all extrinsic muscles except the palatoglossus (which is innervated by CN X). The key extrinsic muscles include the genioglossus, hyoglossus, and styloglossus. To examine CN XII, have the patient protrude their tongue. Observe for asymmetry, atrophy (which causes a wrinkled or "bag of worms" appearance), and fasciculations. Then ask them to move their tongue from side to side against a tongue depressor to test lateral pushing strength.

Interpreting Lesions and Tongue Deviation

The physical exam findings are direct windows into the underlying neuroanatomy. A lesion of the spinal accessory nerve causes ipsilateral weakness. The patient will have a drooped shoulder (weak trapezius) and difficulty turning their head away from the side of the lesion (weak SCM). This nerve is famously vulnerable to iatrogenic injury during surgeries in the posterior cervical triangle, such as lymph node biopsies.

The logic of tongue deviation is a classic test of understanding. In a unilateral hypoglossal nerve lesion, the tongue will deviate toward the side of the lesion upon protrusion. This occurs because the genioglossus muscle, a key extrinsic muscle, is paralyzed on the affected side. The genioglossus normally acts to protrude the tongue forward and toward the opposite side. When it is weak, the unopposed action of the healthy contralateral genioglossus pushes the tongue toward the weakened side.

Clinical Vignette: A 65-year-old male presents with a two-week history of slurred speech and difficulty manipulating food in his mouth. On exam, his tongue deviates to the right upon protrusion, and you observe fasciculations and atrophy on the right side of his tongue. Shoulder shrug and SCM strength are normal. Interpretation: This is a classic lower motor neuron sign of the right hypoglossal nerve. The presence of atrophy and fasciculations suggests a progressive process affecting the nerve or its nucleus, such as a skull base tumor near the hypoglossal canal or a motor neuron disease.

Common Causes and Management Considerations

Lesions can occur anywhere along each nerve's pathway, from nucleus to neuromuscular junction.

• Accessory Nerve Lesions: Common causes include iatrogenic injury (neck surgery, carotid endarterectomy), penetrating trauma to the posterior triangle, tumors (e.g., schwannomas, metastatic lymph nodes), and neuralgic amyotrophy. Management focuses on identifying the cause with imaging (MRI of the brain/neck), physical therapy to maintain range of motion and strength, and, in cases of transection, possible surgical repair or nerve grafting.
• Hypoglossal Nerve Lesions: Causes can be medial medullary syndrome (affecting the nucleus), pathologies at the skull base (meningioma, glomus jugulare tumor), or in the neck (due to internal carotid artery dissection or malignancy). Bilateral CN XII palsy is a neurological emergency, often seen in conditions like Guillain-Barré syndrome or amyotrophic lateral sclerosis (ALS), and can cause severe dysarthria and life-threatening airway obstruction due to the tongue falling back. Management is directed at the underlying cause and involves speech and language therapy for dysarthria and swallowing assessments to prevent aspiration.

Common Pitfalls

1. Misinterpreting the Side of a Hypoglossal Lesion: The most frequent error is to think the tongue deviates away from the lesion. Always remember: the tongue is pushed by the healthy genioglossus. If the right genioglossus is weak, the strong left genioglossus pushes the tongue to the right. Deviation is toward the weak side.
2. Confusing SCM Function: When testing head turn, you are palpating the opposite SCM. Weakness in turning the head to the left indicates a problem with the right SCM, and therefore the right CN XI. Mixing this up will incorrectly localize the lesion.
3. Overlooking the Accessory Nerve in Neck Pathology: When a patient presents with a drooping shoulder and neck pain after a seemingly minor procedure, the spinal accessory nerve must be considered. Its superficial course makes it susceptible to compression or stretch injuries that are not immediately apparent.
4. Attributing Tongue Weakness to the Wrong Nerve: Dysarthria or swallowing difficulty often brings CNs IX and X to mind first. A careful tongue exam is required to isolate CN XII function. Remember, the palatoglossus muscle (CN X) elevates the back of the tongue, but protrusion and lateral movement are primarily CN XII.

Summary

• The spinal accessory nerve (CN XI) is a motor nerve with spinal roots (C1-C5) that innervates the sternocleidomastoid and trapezius muscles, controlling head turning and shoulder elevation. It is vulnerable to injury in the posterior cervical triangle.
• The hypoglossal nerve (CN XII) is a somatic motor nerve that innervates all intrinsic and most extrinsic muscles of the tongue, governing its shape and movement.
• A unilateral hypoglossal nerve lesion causes the tongue to deviate toward the side of the lesion upon protrusion due to the unopposed action of the contralateral genioglossus muscle.
• CN XI weakness causes ipsilateral shoulder droop (trapezius) and weakness turning the head away from the lesion (SCM).
• Clinical assessment requires precise knowledge of muscle actions to correctly localize lesions, which can range from brainstem pathology to iatrogenic injury in the neck.