Disorders of the tongue

Embryology

The tongue begins to develop at the end of the fourth gestational week. It arises from the first, third and fourth pharyngeal arches of the pharyngeal apparatus. Initially, the first pharyngeal arch gives rise to a central tuberculum impar (also called the median lingual swelling) and the bilaterally paired lateral lingual swellings. These swellings arise anterior to the foramen cecum of the thyroglossal duct. The lateral lingual swellings outgrow the tuberculum impar, resulting in fusion of the lateral buds in the midline. They subsequently produce the anterior two thirds of the tongue, otherwise called the oral or presulcal tongue.

The second pharyngeal arch gives rise to another midline swelling known as the copula linguae. Caudal to this is the hypobranchial (hypopharyngeal) eminence, which arises from the third and fourth pharyngeal arches. Eventually, the hypobranchial eminence outgrows the copula linguae (which then disappears) and continues to form the posterior third of the tongue, also called the post-sulcal or pharyngeal part.

The muscles of the tongue arise from migrating occipital myotomes. As the primitive myocytes invade the mesenchyme of the pharyngeal apparatus, they take along with it the nerve fibres arising from the hypoglossal nerve (CN XII). As a result, all the muscles of the tongue (except the palatoglossus) are innervated by CN XII.

Both general and special sensory nerve supply to the mucosa of the tongue are dependent on the pharyngeal arches that the region of the tongue arises from. Therefore, the oral part of the tongue, which arises from the first pharyngeal arch, is innervated by the lingual nerve (which is a branch of the mandibular division of the trigeminal nerve [CN V3]; general sensory stimuli) and the chorda tympani branch of the facial nerve (CN VII; special sensory stimuli). Although CN VII is the nerve of the second pharyngeal arch, it innervates the lingual papillae (which are restricted to the presulcal tongue) as they develop in close proximity to the free nerve endings of the chorda tympani.

Similarly, the posterior third of the tongue transmits both general and special innervation via the glossopharyngeal nerve (CN IX), which is the nerve of the third pharyngeal arch. The vagus nerve (CN X) also provides taste innervation to the lingual surface of the epiglottis, which is a derivative of the fourth pharyngeal arch.

Learn more about the nerve supply of the tongue in our article.

Intrinsic tongue muscles

The intrinsic tongue muscles from dorsal to ventral are the:

• Superior longitudinal
• Vertical
• Transverse
• Inferior longitudinal muscles

Extrinsic tongue muscles

The external lingual muscles include:

• genioglossus
• hyoglossus
• styloglossus
• palatoglossus

Mastication and deglutition

Prior to food entering the mouth, the tongue is in a neutral position, with the ventral surface resting on the floor of the oral cavity and the dorsal surface facing superiorly. As food is being placed in the mouth, the tongue is depressed along the median sulcus to act as a receptacle for food. Once placed on the tongue, the food is tossed to either side for grinding by the teeth.

The tongue then twists such that the dorsal surface faces the side where food is being chewed; acting as a medial wall to prevent the food chunks from falling from between the teeth. The tongue also helps to detect whether or not food is adequately chewed and pushes large chunks back between the teeth for further grinding. Once adequately chewed, the buccinators muscle will push the chewed particles medially between the separated rows of teeth. The tongue receives this mixture and blends everything together with the saliva to form a bolus. 

At the beginning of the swallowing phase, the tip of the tongue is elevated and pressed against the incisors and anterior hard palate. It forms a declining slope in an anteroposterior direction that forces the bolus into the oropharynx, where the esophageal phase of swallowing begins. 

Gustatory pathway

The perception of the different taste stimuli is a biochemical process that involves molecular binding, action potential initiation and propagation, signal transduction, and interpretation. Within each taste bud is a mixture of sustentacular and taste cells. The former are modified epithelial cells, while the latter are specialized cells that are frequently replaced every ten days. Taste cells are arranged around the periphery of the taste buds, with the apices of the cells funnelling into the taste pore.

The taste hairs (microvilli) on the apical surface of the cells have numerous receptors for different stimuli. The cells that detect sour taste have ionic receptors that detect hydrogen (H+) ions, which are responsible for the acidic (sour) taste. Ionic receptors are also responsible for detecting the salty taste generated by binding to salt derivatives; especially sodium (Na+) salts. The receptors that detect sweetness are able to bind mostly to organic substances like carbohydrates, aldehydes, esters, and some amino acids. A similar situation arises for detecting bitterness, where a collection of organic molecules can generate this stimulus. However, the alkaloids and long-chain organic molecules are more commonly associated with bitter taste. A fifth taste stimulus, known as umami (savory), is generated by foods containing L-glutamate (levo, the left handed chiral isomer). Glutamate receptors are responsible for detecting these molecules.

Binding of the respective molecules to the taste receptors depolarizes the membrane at the level of the receptor. The potential difference at the membranes results in the initiation of an action potential or the activation of a secondary messenger pathway. In either case, the electrical activity is conducted along the nerve fiber to its respective nucleus, depending on the region of the tongue that is being stimulated. The lingual nerve carries taste from the anterior two thirds of the tongue to the chorda tympani, then to CN VII.

The base of the tongue, circumvallate papillae, and remaining taste receptors in the mouth send their signals via CN IX. CN X also carries some taste sensation from the pharyngeal region as well. All three nerves bring their special afferent impulses to the nucleus tractus solitarius, which passes its second order neurons to the ventral posterior medial nucleus of the thalamus. From the thalamus, the stimuli are sent to the postcentral gyrus of the parietal lobe, in the area of the sylvian fissure.

The nucleus tractus solitarius also participates in intrinsic gustatory reflex pathways. It sends stimuli to the superior and inferior salivary nuclei, which in turn send parasympathetic innervation to the salivary glands in order to promote secretion of saliva. Not only does this initiate digestion, but it moistens the food bolus in preparation for swallowing.

Articulation

Communication is a ubiquitous ability that all organisms are capable of. However, humans have developed a unique ability to speak. This phenomenon requires initiation of a thought process, generation of sound, and modification of sound to form words. Speech enables an individual to communicate emotional and symbolic ideologies and thoughts that are constantly being developed throughout life. Speech is a multilevel process that can be broken up into:

• The linguistic phase, where thoughts are generated and formed. This is regulated by Broca's area on the frontal lobe of the dominant hemisphere. These impulses are transferred via the corticobulbar tracts to the respective ipsilateral and contralateral cranial nerve nuclei (CN IX and CN X). There is also involvement of the cerebellum and the extrapyramidal pathways in regulating the physiological part of speech.

• The physiological phase involves forcing enough air across variably abducted vocal cords (within the larynx). The oro- and nasopharyngeal cavities, along with their bony walls operates as an amphitheatre, which modifies the air to produce varying sounds. The tongue, lips, and teeth add the final touch to the sounds in order to create intelligible words.
• There is a subsequent acoustic phase, where both the listener and the speaker hear the spoken words. These impulses travel via the acoustic pathway to reach the cochlear nucleus, which eventually transmitting impulses to the auditory cortex. 
• Listening and understanding is another linguistic phase in the speech cycle that involves the receptive area of the temporal lobe (which is close to the auditory cortex) and Wernicke’s area in the posterior end of the Sylvian fissure. The speaker can modify the pitch, volume, and pronunciation of their words voluntarily based on what they hear. In the meantime, the listener can integrate and interpret what was said and formulate a response.

Therefore, the tongue is a crucial part of the articulation phase of speech (the other phase being phonation or generating sound). The intricacies of the movements of the tongue during speech far exceeds the scope of this article. However, be cognizant of the fact that the movements of the tongue during speech vary widely depending on the native language of the individual. Also note that in the English language, the tongue is particularly important in articulating consonants. Additionally, flattening of the tongue increases the potential space within the oral cavity, which facilitates the pronunciation of vowels.  

Congenital disorders

Absent or deficient tongue

Complete absence of the tongue – aglossia congenital – or a short and incompletely formed tongue – hypoglossia – have been described as both syndromic (oromandibular limb hypogenesis and hypoglossal-hypodactylia syndromes, respectively) and isolated abnormalities. Both are believed to result from sporadic mutations affecting the formation of the tongue during the 4th to 8th weeks of gestation. These are extremely rare conditions that have serious biological and social consequences. There are also rare cases where the lateral lingual swellings failed to fuse, resulting in a glossoschisis. Although the patient will have a split (bifid) tongue, the deformity seldom extends to the tip and development of the hard palate is usually unaffected.

Ankyloglossia

Ankyloglossia refers to a condition in which the lingual frenulum that attaches the ventral surface of the tongue to the floor of the mouth is too short. Some variations of the disorder are associated with the lingual frenulum extending all the way to the tip of the tongue instead of remaining along the proximal ventral surface. Consequently, the infant would be unable to protrude their tongue fully out of the oral cavity. While the disorder may make breastfeeding challenging for the mother and infant, the frenulum tends to elongate over time. Some individuals may also present later in life with difficulties with articulation. Irrespective of the age of presentation, the defect can be surgically corrected with a frenulectomy (surgical division of the frenulum) if it is causing clinical complications.

Lingual thyroid gland

Ectopic thyroid glands at the base of the tongue have been classified as lingual thyroid glands. This is a type of thyroid dysgenesis where a part of or all of the thyroid gland failed to descend. Consequently, patients may experience obstructive symptoms (i.e. difficulty swallowing, problems with phonation, and difficulty breathing). The ectopic thyroid gland may remain fully functional, but incapable of fully maintaining thyroid hormone levels if the main thyroid gland is excised. Some patients are fortunate to remain asymptomatic. However, females – who are more likely than males to have an ectopic thyroid – are likely to present with symptoms at the beginning or end of their reproductive period. 

Furthermore, failure of the thyroglossal duct to occlude can result in the formation of a thyroglossal duct cyst; which can occur at any point along the thyroglossal duct. An enlarged cyst can (although rarely) be underlying cause of dysphagia and pharyngeal discomfort.

Tongue abnormalities with Down syndrome

Down syndrome is a relatively common polyploidy characterized genetically by an extra copy of chromosome 21, and clinically by a constellation of multi-systemic, morphological abnormalities. While a lot of persons are familiar with the facial, cardiac, and intellectual features associated with Down syndrome, there are other morphological changes that clinicians should be aware of as well.

With respect to the tongue, it is not uncommon for the organ to be enlarged in these patients (a condition known as macroglossia). They may also present with hypertrophied papillae and fissuring of the tongue. The macroglossia accounts for the typical appearance of an individual living with Down syndrome to have their tongue hanging out of their mouth. This may cause significant concern during early life as the enlarged tongue may occlude the upper airway. However, macroglossia is a clinical sign and not a diagnosis by itself. It can be associated with other congenital and acquired disorders such as a vascular malformation, or an anaphylactic reaction.

Acquired disorders

A common mnemonic that can be used to organize the etiology of acquired disorders of the tongue (and any system) is 'TIN CAN BED MIDI'. While individuals have devised numerous variations to this mnemonic, the one most readily recalled is as follows: 

• Trauma
• Inflammation
• Nutritional
• Cardiac
• Autoimmune 
• Neoplasms
• Blood dyscrasias
• Endocrinopathy
• Degenerative disease
• Metabolic Dysfunction
• Idiopathic
• Drugs
• Infection

Trauma

Almost all of these subheadings can be applied to the congenital abnormalities of the tongue. Traumatic injury to the tongue can arise from mechanical injury where the tongue is accidentally bitten during a seizure episode or more commonly while the patient is recovering from an inferior alveolar nerve block following a dental procedure. Thermal injury to the tongue is also possible following attempted ingestion of hot food.

Glossitis

Glossitis refers to inflammation of the tongue associated with loss of the lingual papillae. The pattern of depapillation depends on the underlying cause of the inflammation. Complete loss of the lingual papillae can cause the tongue to appear smooth and glossy; a condition known as atrophic glossitis.

Nutritional deficiencies such as vitamin B12 (also causing a beefy red tongue), folate, and iron are often associated with a variant of glossitis (along with other symptoms and signs). 

Cyanosis

There are numerous cardiac abnormalities that can result in low blood oxygen saturation. This decline in oxygen supply throughout the body will result in tissue hypoxemia and subsequently, hypoxia if left untreated. While this can be detected with the use of pulse oximetry, in the absence of such resources, a change in the color of the tongue and underlying mucosa from the typical pink to blue is indicative of the process. This feature is known as central cyanosis.

Autoimmunity

Autoimmune disorders such as Sjögren syndrome and oral lichen planus can also have manifestations in the mouth. Sjögren is associated with dry mouth (xerostomia), while the purple, polygonal, plaques of lichen planus can develop within the oral cavity. Myasthenia gravis is another autoimmune disorder that may indirectly affect the tongue as well. This disease is characterized clinically by fluctuating muscle weakness that worsens with repeated use and resolves with rest; and pathologically by antibody mediated (anti-AChR ab) destruction of postsynaptic nicotinic acetylcholine receptors (n-AChR) of the skeletal neuromuscular junction. Since this disorder may also affect cranial nerve synapses (i.e. bulbar symptoms of myasthenia gravis), patients may also present with difficulties with articulation (CN IX & X), phonation (CN IX & X), chewing (CN V & IX) , and swallowing (CN IX & X).

Neoplasms

Lingual squamous cell carcinoma is an example of a malignant neoplasm of the tongue. It is more common among males over 50 years old and has been linked to the human papillomavirus (HPV), smoking and excessive alcohol intake. Eight out of ten patients will have a painful ulcer as part of their initial presentation, often with referred pain to the ears. Majority of the lesions occur on the lateral third of the tongue. The next most common site is the posterior third, followed equally by the tip, ventral surface and dorsum.