CRANIAL NERVE XII, a very brief but well informed take

A PPT about the 12th Cranial nerve

1. CRANIAL NERVE XII Hypoglossal nerve Presented by Ore.

2. I NTRODUCTION The hypoglossal nerve is the twelfth paired cranial nerve. Its name is derived from ancient Greek, ‘ hypo ‘ meaning under, and ‘ glossal ‘ meaning tongue. The nerve has a purely somatic motor function , innervating all the extrinsic and intrinsic muscles of the tongue (except the palatoglossus, innervated by vagus nerve). Your hypoglossal nerve is one of your 12 paired cranial nerves. Your hypoglossal nerve starts at the base of your brain. It travels down your neck and branches out, ending at the base and underside of your tongue.

3. ANATOMY The hypoglossal nerve arises from the hypoglossal nucleus in the medulla oblongata of the brainstem. It then passes laterally across the posterior cranial fossa, within the subarachnoid space. The nerve exits the cranium via the hypoglossal canal of the occipital bone. Now extracranial, the nerve receives a branch of the cervical plexus that conducts fibres from C1/C2 spinal nerve roots. These fibres do not combine with the hypoglossal nerve – they merely travel within its sheath. It then passes inferiorly to the angle of the mandible , crossing the internal and external carotid arteries, and moving in an anterior direction to enter the tongue.

4. FUNCTION Your hypoglossal nerve controls muscles that move your tongue, allowing you to: Make noises with your mouth, like clicking sounds. Move substances around in your mouth. Speak. Swallow.

5. Motor Function The hypoglossal nerve is responsible for motor innervation of the vast majority of the muscles of the tongue (except for palatoglossus ). These muscles can be subdivided into two groups: i) Extrinsic muscles * Genioglossus (makes up the bulk of the tongue) * Hyoglossus * Styloglossus * Palatoglossus (innervated by vagus nerve) ii) Intrinsic muscles * Superior longitudinal * Inferior longitudinal * Transverse * Vertical Together, these muscles are responsible for all movements of the tongue.

6. Role of the C1/C2 Roots The C1/C2 roots that travel with the hypoglossal nerve also have a motor function. They branch off to innervate the geniohyoid (elevates the hyoid bone) and thyrohyoid (depresses the hyoid bone) muscles. Another branch containing C1/C2 fibres descends to supply the ansa cervicalis – a loop of nerves that is part of the cervical plexus. From the ansa cervicalis, nerves arise to innervate the omohyoid, sternohyoid and sternothyroid muscles. These muscles all act to depress the hyoid bone

7. Clinical Examination Examining the hypoglossal nerve involves observing the primary innervation target of the nerve, the tongue. The 3 observable aspects of the tongue are Strength , Bulk , and Dexterity . Special attention is given when the tongue is weak , atrophied , moving abnormally , or impaired . The tongue is first observed for position and appearance while resting. Then, the patient is asked to protrude the tongue, move it in and out, side to side, and up and down. The patient does each movement twice: once slowly and the next rapidly. Dexterity is tested by having the patient repeat sounds involving the tongue, such as saying "la la la" or using words with "t" or "d" sounds. Testing strength consists of the patient forcing the tongue to the side against the patient’s cheek while the examiner tries to dislodge the tongue using his/her finger. The tongue’s strength can be checked more accurately using the tongue blade. The strength test is especially useful in testing for unilateral weakness caused by ipsilateral hypoglossal nerve damage.

8. Hypoglossal Nerve Lesions The hypoglossal nerve can be damaged at the hypoglossal nucleus (nuclear) , above the hypoglossal nucleus (supranuclear) , or interrupted at the motor axons (infranuclear) . Such damage causes paralysis, fasciculations (as noted by a scalloped appearance of the tongue), and eventual atrophy of the tongue muscles. When 1 of the 2 nerves is damaged, the tongue, when protruded, deviates towards the damaged nerve because of the overaction of the strong genioglossus muscles. Supranuclear lesions occur at the cerebral cortex, the corticobulbar tract of the internal capsule, cerebral peduncles, or the pons. Supranuclear lesions cause the tongue to protrude away from the nerve because of predominant neural crossing for upper motor neurons. Supranuclear lesions do not tend to cause atrophy but can lead to an uncoordinated tongue with slow but spastic tongue movements. Supranuclear lesions commonly result from strokes but can also be caused by pseudobulbar palsy. Infranuclear and nuclear lesions cause weakness of the tongue but additionally cause ipsilateral atrophy. Lower motor neuron disease can also cause fasciculation when localized to the lower motor neurons.

9. Unilateral lesions are not typically a serious problem for patients, as the remaining hypoglossal nerve partly compensates any impediments. However, bilateral lesions can cause profound difficulty with speech and swallowing, as the patient cannot protrude the tongue for these necessary functions. Hypoglossal nerves can be damaged unilaterally by a multitude of causes, especially tumors , infection , or trauma . The concept of trauma includes surgical trauma, as with Carotid Endarterectomy (surgery to remove the plaques from the carotid artery). Rare bilateral lesions can be the result of radiation therapy.

10. Hypoglossal Nerve in Neurologic Disorders Progressive bulbar palsy and advanced Amyotrophic Lateral Sclerosis (ALS) can cause severe tongue atrophy and an inability for the tongue to protrude, leading to the inertia of the tongue (glossoplegia). Fasciculations frequently occur with atrophy in the case of motor neuron disease. Tremors can also occur on the tongue in the case of alcoholism, parkinsonism, and paresis. Neck-Tongue Syndrome Neck-tongue syndrome is a pain on 1 side of the upper neck or back of the head, usually involving rapid neck rotation. The syndrome also typically involves the tongue, causing the ipsilateral side of the tongue to feel the pain. The syndrome's cause is unknown, although 2 theories have been suggested, and both involve strain/pressure on the C2 nerve. As the C2 nerve route is involved with the hypoglossal nerve route, tongue pain can result. Obstructive Sleep Apnea and Hypoglossal Nerve Stimulation In obstructive sleep apnea (OSA), the decrease in muscle tone of the genioglossus muscle causes the tongue to retract and impede airflow into the trachea. The hypoglossal nerve stimulator is 1 possible treatment if the patient is refractory to Continuous Positive Airway Pressure (CPAP), oral devices, or surgery. Mild hypoglossal nerve stimulation causes the nerve to pull the tongue forward, enabling better airflow.

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