Hypoglossal nerve palsy is an uncommon neurological deficit characterized by unilateral weakness and/or atrophy of the tongue that may arise due to trauma, iatrogenic causes (eg. surgery), infections, vascular malformations, or tumors. The clinical examination is the principal method by which hypoglossal nerve palsy can be detected, but imaging studies, either computed tomography or magnetic resonance imaging, must be used to identify the exact site of the lesion and the underlying cause.
The hypoglossal nerve, also known as the XII cranial nerve, arises from the medulla oblongata, carriers only motor fibers and supplies the muscles of the tongue  . Its travel, from the point of origin to the tissue it innervates, is quite long and is generally divided into five segments - medullary, cisternal (intracranial), skull base, nasopharyngeal/oropharyngeal and carotid  . Many pathologies have been described in the literature when it comes to the damage of the hypoglossal nerve in any of the mentioned segments, and it is estimated that tumors, such as nasopharyngeal carcinomas, gliomas, acoustic neuromas, chordomas, and metastases, are responsible for the majority of cases   . Other known causes include trauma , surgery-induced (examples being endotracheal intubation, cervical spine surgery, etc.)   , and vascular changes (ischemia or hemorrhage) . Signs and symptoms of hypoglossal nerve palsy are highly distinctive, the main feature being a unilateral weakness of the tongue . In addition, the tongue may deviate toward the side of the damaged nerve (if the lesion is located inferior to the hypoglossal nerve nucleus) or contralaterally in the case of a supranuclear lesion . Weakness is often accompanied by atrophy of the tongue, as well as fasciculations . Furthermore, a number of patients might suffer from impaired speech (dysarthria) and swallowing difficulties (dysphagia), which may be severely debilitating    .
To make the diagnosis of a hypoglossal nerve palsy, the physician is obliged to perform a detailed physical examination and obtain a thorough patient history that should reveal any preexisting disorders or events that could predispose the patient to this neurological deficit. The unilateral weakness of the tongue can be readily observed during a properly conducted neurological exam and the assessment of cranial nerves. To test the hypoglossal nerve, the patient is asked to protrude his tongue, when the deviation (and possibly atrophy or fasciculations) to one side is apparent    . In order to identify the underlying cause, however, a more detailed investigation is warranted, with imaging studies being the cornerstone. Computed tomography (CT) and magnetic resonance imaging (MRI) are both recommended for the evaluation of hypoglossal nerve palsy. CT is able to visualize the skeletal structures in greater detail, while soft-tissues are better defined using MRI, particularly when gadolinium is employed as a contrast . In fact, either CT or MR angiography are of great value in the diagnosis of vascular insults .