Hypoglossal Nerve Palsy: Description, Causes and Risk Factors:
Hypoglossal nerve exits the medulla oblongata, extends through the skull base, and traverses the suprahyoid neck before ramifying to supply the tongue musculature. The fibers of the hypoglossal nerve arise in the hypoglossal nuclei, which extend through the medulla oblongata in a paramedian location. This is the medullary segment. The fibers course anteriorly, lateral to the medial lemniscus, to exit the medulla in the preolivary sulcus. The rootlets of the hypoglossal nerve lie posterolateral to the vertebral artery within the premedullary cistern; this is the cisternal segment of the nerve. The rootlets then merge to form the hypoglossal nerve within the hypoglossal (anterior condylar) canal of the cipital hone; this is the skull base segment. Emerging from the hypoglossal canal, the hypoglossal nerve enters the nasopharyngeal carotid space. At this point, the nerve lies deep to the internal jugular vein, internal carotid artery, and glossopharyngeal and vagus nerves. As the nerve passes intcriorly, it comes to lie between the internal carotid artery and internal jugular vein, superficial to the vagus nerve. At the level of the angle of the mandible, the nerve deviates from the path of these other lower cranial nerves. It loops anteriorly around the root of the occipital artery, lying inferior to the posterior belly of the digastric muscle, where it becomes superficial. At the level of the hyoid hone, the nerve crosses the lingual artery and curves anteriorly to run along the surface of the hyoglossus muscle, deep to the mylohyoid sling. This
segment of the nerve lies within the sublingual space. As it passes anteriorly, the nerve lies on the surface of the genioglossus muscle before penetrating that muscle.
Hypoglossal nerve palsy is relatively uncommon. Damage to the 12th cranial nerve (hypoglossal nerve or cranial nerve XII) produces characteristic clinical manifestations. Nerve dysfunction may occur in isolation; more commonly, it may he associated with complex palsy of the lower nerve. To design the most efficient imaging strategy in the presence of such symptoms, one must be familiar with the anatomy of the hypoglossal nerve and the diseases that affect the nerve throughout its course from the medulla oblongata to the tongue.
Dysfunction of the hypoglossal nerve may be a consequence of supranuclear, nuclear, or infranuclear disease. The nuclear and infranuclear hypoglossal nerve can be divided into five segments: the medullary, cisternal, skull base, nasopharyngeal/oropharyngeal carotid space, and sublingual segments. Such anatomy segmental analysis allows the radiologist to develop a systematic approach to a case of hypoglossal nerve palsy. Knowledge of the most common pathologic conditions occurring in each segment will permit the radiologist to produce a more sharply focused differential diagnosis. The segmental approach to the anatomy and pathologic conditions of the hypoglossal nerve is used to clarify the differential diagnosis.
Tumours account for over half of the reported cases of hypoglossal nerve palsy. The most common are metastatic carcinomas, chordomas, nasopharyngeal carcinomas, gliomas, and acoustic neuromas. In the only large series, 100 cases reported by Keane, the next most common category was trauma — usually penetrating injuries. Uncommon causes include multiple sclerosis (MS) and idiopathic isolated hypoglossal nerve palsy, which is a diagnosis of exclusion. Acute onset of pain with tongue weakness points to dissection of the extracranial internal carotid artery, a medical emergency. Bilateral tongue wasting is usually due to neurological disorders such as motor neurone disease and Kennedy's disease. Even with bilateral involvement it is uncommon for patients to complain specifically about tongue function. Occasionally we see psychogenic or hysterical 12th nerve palsy in which the tongue may be deviated to the side opposite to that of the limb paralysis.
Symptoms of hypoglossal nerve palsy typically include unilateral or bilateral tongue weakness with deviation towards the affected side on tongue protrusion, tongue atrophy (with scalloping or accentuation of the midline groove), fasciculation of the tongue at rest, tongue flaccidity, or the inability to move the tongue rapidly from side to side or vertically.
Both CT and MR imaging are used in assessment of dysfunction
of the hypoglossal nerve. While MR imaging has the advantage of superior soft-tissue contrast, CT delineates cortical bone with exquisite detail. At the skull base, the multiplanar capability of MR imaging can be extremely useful. The high signal intensity of fat on Ti-weighted MR images allows bone marrow replacement to be identified at an early stage. Enhancement with gadolinium
may mask this finding unless fat saturation techniques are employed. Below the skull base, use of either contrast-enhanced CT or MR imaging has been recommended. The patient's clinical status and the radiologist's preference for use of CT or MR imaging in the suprahyoid neck will influence the choice.
We now know that the hypoglossal nerve is responsible for the many and most important functions of our tongue. We can't even dream of living without being able to eat or talk. Amazing that the gift of words and sounds is safely wrapped by one of these nervous networks!
Surgery can be prescribed if there are penetrating wounds to the nerve.
NOTE: The above information is for processing purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition.
DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.
- Exercises for the palate and the pharynx, if there is difficulty in speaking (dysarthria), for any injury caused by blunt trauma.
- A treatment to improve communication skills and behavioral impairments like dysphagia (disorder for difficulty in swallowing food) through various specialist teams is also advised.
- Practice of various conducive strategies for implementing safe and apt intake of food.