The membranous labyrinth or endolymphatic labyrinth is the part of the inner ear housed in the osseous labyrinth. It comprises 3 distinct, but joined, sensory sacs and ducts supplied by the vestibulocochlear nerve (CN VIII):
cochlear duct in the cochlea, responsible for hearing
utricle and sac...
Ménière disease (or idiopathic endolymphatic hydrops) is an inner ear disorder and as such can affect balance and hearing.
One or both ears can be affected. The chief symptoms are:
vertigo (often attacks which can be incapacitating)
a sensation o...
The mental artery is a terminal branch of the inferior alveolar artery which itself is a branch of the first part of the maxillary artery. It emerges onto the face from the mandibular canal with the mental nerve at the mental foramen, and supplies muscles and skin in the chin region. The mental ...
The mental foramen is a small foramen on the anterior surface of the mandible, typically adjacent to the root of the mandibular second premolar tooth. Normal variants have the forman located anywhere between the canine and 1st molar.
The mental nerve, a terminal branch of inferior alveolar nerv...
The mental nerve is one of the two terminal branches of the inferior alveolar nerve, a branch of the posterior division of the mandibular division of the trigeminal nerve. It emerges from the mandibular canal anteriorly through the mental foramen and supplies the labial gingiva of the lower lip ...
A mesiodens is the most common supernumerary tooth and is located in the palatal midline between the two maxillary central incisors.
It is rare with an estimated prevalence of ~1% (range 0.09 to 2.2%) 3. There is an increased male predilection with a M F ratio of ~2.5:1.
The mesotympanum forms the main compartment of the tympanic cavity and contains most of the important structures of the middle ear, including most of the ossicular chain.
The mesotympanum is found in the middle ear.
superiorly it is arbitrarily separated f...
Metastases to the thyroid are an uncommon cause of thyroid malignancy.
Metastases to the thyroid represent 1.4-3% of all malignancies 5. In autopsy series, the incidence is ~10% (range 2-24%) 1,5.
The most common sites of primary malignancy include (note these will v...
Meth mouth is the name given to the overt dental disease that is one of the signs of methamphetamine use.
Clinical examination often reveals blackened, stained, rotting or crumbling teeth. Serial studies only a few years apart may show a striking deterioration in the pati...
The metopic suture (also known as the frontal, interfrontal, or median frontal suture) is a type of calvarial suture. It is often associated with frontal sinus agenesis or hypoplasia 7.
This suture runs through the midline across the frontal bone from the nasion to the bregma, a...
MIBG scan is a scintigraphic study that uses metaiodobenzylguanidine labeled to iodine-123 or iodine-131. It is indicated in the investigation of phaeochromocytoma.
MIBG is positive in:
olfactory neuroblastoma 1
carcinoid tumor 4
Michel aplasia, also known as complete labyrinthine aplasia (CLA), is a congenital abnormality of the inner ear and is characterized by bilateral absence of differentiated inner ear structures with resultant anacusis. It should not be confused with Michel dysplasia.
Michel aplasia i...
Microdontia is where a tooth appears smaller than normal. It most commonly affects the maxillary lateral incisor and the third molars.
Microgenia is a term meaning a small chin. It is somewhat related to but distinct from the term micrognathia which means a small mandible.
Microgenia can be isolated or be associated with a numer of anomalies which include
camptomelic dysplasia 1
Microphthalmia essentially means small eyes. It is characterized by a small eye within a small orbit. It can be unilateral or bilateral. It is sometimes defined as a globe with a total axial length (TAL) that is at least two standard deviations below the mean for age.
Microtia refers to a small pinna of the ear, when is it completely absent it is termed anotia.
The estimated incidence at around 1:9000 live births. It is more common in males and there is a recognized right sided predilection.
Microtia can be associated ...
The middle cranial fossa is a butterfly-shaped depression of the skull base, which is narrow in the middle and wider laterally. It houses the temporal lobes of the cerebrum.
The middle cranial fossa can be divided into medial and lateral parts. In the medial part, the following s...
The middle ear (a.k.a. tympanic cavity or tympanum) is an air-filled chamber in the petrous part of the temporal bone. It is separated from the external ear by the tympanic membrane, and from the inner ear by the medial wall of the tympanic cavity. It contains the three auditory ossicles whose p...
Middle ear effusions are frequent in children due to prominent adenoids and horizontal Eustachian tubes. These do not require imaging, and can be treated expectantly / medically / surgically with gromits. Eustachian tube dysfunction is the accepted etiology, with resorption of air and extravasat...
Development of granulation tissue in the middle ear cavity is a generalized response to injury/inflammation, chronic otomastoiditis. It can either be typical or go down the pathway of becoming a cholesterol granuloma.
Typical granulation tissue is common, more so than cho...
There are three ossicles (from lateral to medial):
Their role is to mechanically amplify the vibrations of the tympanic membrane and transmit them to the cochlea where they can be interpreted as sound. They are located in the middle ear cavity and articulate with each ot...
There are a range of middle ear tumors, which are more likely to be benign than malignant.
The three most common middle ear tumors are (not in any particular order as there are differences in the literature) 1-3:
glomus tympanicum paraganglioma
The middle meatus is an air passage of the lateral nasal cavity located between the middle nasal concha and lateral nasal wall. The anterior ethmoid air cells, maxillary and frontal sinuses all drain into the middle meatus.
The middle meningeal artery branches off the first part of the maxillary artery. It passes vertically through the roots of the auriculotemporal nerve and enters the middle cranial fossa via the foramen spinosum. Here it gives off two branches - superior tympanic branch and ganglionic branch - be...
The middle nasal conchae or turbinates are one of the pairs of conchae in the nose.
The middle nasal concha consists of the medial surface of the labyrinth of ethmoid which is a thin lamella that descends from the undersurface of the cribriform plate and ends in a free, convolute...
The middle pharyngeal constrictor muscle is one of the pharyngeal constrictor muscles. Its primary action is constricting of the pharynx (in coordination with the superior pharyngeal constrictor and the inferior pharyngeal constrictor muscles) to deliver a bolus of food into the esophagus.
The middle superior alveolar nerve, also known as the middle superior dental nerve, is the second branch of the infra-orbital nerve, from the maxillary division of the trigeminal nerve. It is the smallest of the superior alveolar nerves and contributes to the superior dental plexus.
The middle thyroid vein is a tributary of the internal jugular vein.
Origin and course
The middle thyroid vein arises from the lateral surface of the thyroid. It traverses laterally to the internal jugular vein, passing anterior to the common carotid artery and posterior to the ...
The middle meningeal nerve, also known as the meningeal branch of the maxillary nerve, is a branch of the maxillary division of the trigeminal nerve. It supplies the dura of the middle cranial fossa.
location: branch of maxillary nerve (CN V2)
origin and course: originates from CN V2 ...
Midfacial hypoplasia is a type of congenital facial anomaly and can be feature of a number of congenital syndromic anomalies. These include:
chromosome 1p36 deletion syndrome 2
chromosome 18q deletion syndrome
fetal alcohol syndrome
fetal valproate s...
A variety of congenital midface anomalies occur in children. Although rare, these disorders are clinically important because of their potential for connection to the central nervous system. Lesions presenting as a midline nasal mass include:
nasal dermoid cyst...
Midline neck masses have a relatively narrow differential, as few structures are present in the midline. Dividing the causes according to structure of origin is a useful schema.
lymph node(s): Delphian node(s)
thyroglossal duct cyst
Mikulicz syndrome was considered a form of Sjögren syndrome (type 1), however, more recently it is considered in IgG4 related disease spectrum.
It may also be referred to by numerous other eponymous names as well as the catchy: sicca syndrome without a connective tissue disorder 3...
Minor salivary glands are a subset of the salivary glands. They are small and unnamed salivary glandular tissue scattered throughout the oral cavity and oropharynx as well as more widely in the mucosa of the aerodigestive tract.
They number in the hundreds and are important because salivary gl...
Minor salivary gland tumors are a subtype of salivary gland tumors originating from the minor salivary glands, which are found in the lips, tongue, hard palate, and the mucosa of the oropharynx and larynx, among other locations.
MSGTs account for ~15% of all salivary gland tumors....
Mixed temporal bone fractures are a combination of longitudinal and transverse fracture types, and are probably the most common type. They frequently involve the otic capsule, and are associated with both conductive and sensorineural hearing loss.
The modified Memphis criteria are a set of screening criteria for blunt cerebrovascular injury (BCVI) in trauma. The presence of one or more of these criteria makes necessary a complementary CTA or DSA study to exclude a BCVI.
The screening protocol criteria for BCVI are:
base of skull fractur...
The modified Stenvers view is an oblique radiographic projection used to demonstrate the petrous temporal bone, IAM and bony labyrinth. It is primarily used to assess electrode placement following the insertion of a cochlear implant. Specifically it assesses the:
integrity, positioning, and dep...
The cochlear modiolus is a conical-shaped structure that consists of spongy (porous) bone located in the center of the cochlea which contains the spiral ganglion.
The spiral lamina projects from the modiolus.
Abnormality of the modiolus results in sensorineural hearing loss.
History and etymo...
Mondini malformation is a historical term used to describe an incomplete partition type II anomaly with large vestibular aqueduct.
The term is often used inappropriately to describe any cochlear abnormality, rather than a specific type of cochlear hypoplasia. Thus, most would favor...
Morning glory disc anomaly (MGDA),also known as morning glory syndrome, is a rare congenital malformation of the optic nerve which is frequently associated with midline abnormalities of the brain and skull 1.
Morning glory disc anomaly is rare and is more commonly found in females...
A mucocele simply refers to accumulation and expansion of a structure by mucus. It occurs in a variety of locations which are discussed separately:
paranasal sinus mucocele
oral cavity e.g. ranula, mucous retention cysts
mucocele of the appendix 1
mucocele of the gallbladder
mucocele of the...
Mucoepidermoid carcinoma is a tumor that usually occurs in the salivary glands. It can mimic most other tumors of the glands, and therefore is often considered in the differential.
Mucoepidermoid carcinomas are seen throughout all adult age groups, but are most common in middle a...
Multinodular goiter (commonly abbreviated to MNG) is defined as an enlarged thyroid gland (i.e. goiter) due to multiple nodules which may have normal, decreased or increased function.
When increased activity and hyperthyroidism are present then the condition is referred to a toxic...
Multiphase CT angiography is an evolving imaging technique in acute ischemic stroke. The technique aims to quickly and reliably identify brain which is potentially salvageable with intervention. Brain tissue viability depends on many factors, with this technique assessing collateral leptomeninge...
The differential diagnosis for multiple cystic neck lesions is different to that for a solitary cystic neck mass.
Cystic neck lesions are seen in:
necrotic metastatic SCC nodes: older patient, M>F
papillary thyroid carcinoma metastases: usually a younger patient, F>M
Multiple endocrine neoplasia (MEN) syndromes are a collection of syndromes characterized by the presence of, as the name would suggest, multiple endocrine tumors. They are autosomal dominant in inheritance.
MEN1 (Wermer syndrome)
MEN2 (multiple endocrine adenomatosis)
MEN2a (Sipple syndrome)
Multiple endocrine neoplasia type I (MEN1), also known as Wermer syndrome, is an autosomal dominant genetic disease that results in proliferative lesions in multiple endocrine organs, particularly the pituitary gland, islet cells of the pancreas and parathyroid glands.
There are other multiple...
Multiple endocrine neoplasia type II (MEN2) is also known as mucosal neuroma syndrome or multiple endocrine adenomatosis. It is a collection of syndromes characterized by the presence of multiple endocrine tumors.
They are autosomal dominant in inheritance, and share medullary thyroid carcinom...
Multiple endocrine neoplasia (MEN) type IIa, also known as Sipple syndrome, accounts for most cases of MEN2 and is characterized by:
pheochromocytomas: in 50% of patients, often bilateral, and can be extra-adrenal
medullary thyroid cancer: 100% of patients, aggressive, and may secrete calciton...
Multiple endocrine neoplasia (MEN) type IIb, also known as MEN type 3 (MEN3) 3 or mucosal neuroma syndrome 2, accounts for only 5% cases of MEN2 and is characterized by:
pheochromocytoma(s): in 50% of patients, often bilateral, and can be extra-adrenal
medullary thyroid cancer: 100% of patient...
Multiple endocrine neoplasia type IV (MEN4), previously known as MENX, is a rare type of MEN with clinical overlap with MEN1 but is due to a CKDN1B mutation. It is characterized by 1,2:
primary hyperparathyroidism in 80%
anterior pituitary adenoma
neuroendocrine tumors (...
Mumps is a (usually) self-limited viral infection that often occurs in epidemics among children. Its incidence has markedly diminished in countries with childhood vaccination programs, but can still be seen in patient who have not received vaccinations or who have waning immunity.
The muscle of the uvula or musculus uvulae is one of the 5 paired muscles of the soft palate.
origin: posterior border of the hard palate and the posterior nasal spine of the horizontal plate of the palatine bone
insertion: palatine aponeurosis and mucosa of the uvula
action: alters ...
The muscles of mastication can be divided into primary and secondary groups according to whether they connect the mandible directly to the skull or if they attach to other structures in the neck, e.g. hyoid, thyroid cartilage.
medial pterygoid muscle...
The muscles of the nose form a distinct subgroup within the muscles of facial expression. Some of the muscles, e.g. orbicularis oris, are in more than one subgroup.
depressor septi nasalis
levator labii su...
There are five paired muscles of the soft palate that are collectively referred to as the muscles of the soft palate or the palatal muscles:
tensor veli palatini
levator veli palatini
muscle of the uvula
All of them are innervated by the pharyngeal plexus ap...
The muscles of the tongue are divided into 2 groups each comprising 4 muscles. They are classified as intrinsic (to the tongue) and extrinsic muscles. They allow for the complex movements of the tongue and are all innervated by the hypoglossal nerve (CN XII) except one:
intrinsic muscles of th...
A handy mnemonic to recall the muscle attachments of the hyoid bone. The first sentence is for six muscles attaching superiorly, the second sentence is for three muscles attaching inferiorly. Both sentences are in order from lateral to medial:
Christopher, He Didn't See Girls Much. That's Obvio...
The muscular triangle is one of the paired triangles in the anterior triangle of the neck. The triangles of the neck are surgically focused, first described from early dissection-based anatomical studies which predated cross-sectional anatomical description based on imaging (see deep spaces of t...
Myeloid sarcomas, also called granulocytic sarcomas, chloromas, or extramedullary myeloid tumors, are rare extramedullary masses comprised of myeloid precursor cells. Most commonly, these tumors happen in association with acute myeloid leukemia (AML), but they may also occur with other myeloprol...
Mylohyoid boutonniere is a normal focal discontinuity in the mylohyoid muscle, which may permit the sublingual salivary gland, fat or vessels - or a combination thereof - to protrude out from the sublingual space into the submandibular space.
The finding has been observed in up to...
The mylohyoid muscles form a paired muscular sling that forms part of the floor of the mouth. It also separates the sublingual space (and oral cavity) from the submandibular space.
origin: mylohyoid line/ridge on the medial surface of the mandible
insertion: midline raphe that extend...
Myoepitheliomas and malignant myoepitheliomas were considered a variant of pleomorphic adenomas until 1991. However, these are also found in breast and bronchus and have gained recognition as separate entities. They account for less than 1% of salivary gland tumors, only rarely undergo malignant...
The myofascial cone is a structure within the orbit involved in the movement and support of the globe, or more simply the eye. It also serves as the physical division between the intraconal and extraconal compartments, but the cone itself is included in the contents of the intraconal space 1.
Myokymia refers to a fine undulating fascicular tremor of facial musculature which should be distinguished from hemifacial spasm which involves entire muscles rather than individual fascicles. The two conditions may co-exist.
Myrtiformis muscle (MM) (also known as depressor alae nasi) is one of the muscles of the nose, a subset of the muscles of facial expression.
Myrtiformis is often omitted from classic anatomy tomes. Some texts describe this muscle as two separate components, the medial fibers formi...
The nasal ala (plural alae) forms the lateral part (wing) of the nasal vestibule. The alar cartilage on each side provides a skeleton for the other soft tissue components.
The nasal bones are small paired oblong upper central facial bones placed side by side between the frontal processes of the maxilla, jointly forming the nasal ridge.
The nasal bone has two surfaces:
external surface attaches to the procerus and nasalis muscles
internal, which i...
Nasal bone fractures are the most common type of facial fractures, accounting for ~45% of facial fractures, and are often missed when significant facial swelling is present.
Unsurprisingly, nasal bone fractures occur when the nose impacts against a solid object (e.g. fis...
The nasal cartilages make up the skeleton of the external nose, along with the nasal bones, maxilla, frontal bone and bony nasal septum. They are hyaline cartilage responsible for maintaining the shape of the flexible part of the external nose and the vestibule and consists of 5 cartilages:
The nasal cavity forms part of the aerodigestive tract.
The nasal cavity is formed by 1:
anteriorly: anterior nares
laterally: inferior, middle and superior nasal conchae (turbinates)
superiorly: cribriform plate of the ethmoid bone
inferiorly: palatal processes of the maxill...
The nasal conchae or turbinates are long, narrow curled shelves of bone that protrude into the nasal cavity. The superior, middle and inferior conchae divide the nasal cavity into four groove-like air passages. Their position and relationship to other important anatomic landmarks are extremely i...
The nasal cycle is a normal physiological process that occurs in the nasal cavity characterized by alternating partial congestion and decongestion of the nasal venous sinusoids of the nasal turbinates.
The nasal cycle is often seen on CT scan of the paranasal sinuses ...
Nasal encephaloceles are in most cases a form of neural tube defect particularly common in Southeast Asia. They are are herniation of cranial content through a bony defect in the anterior skull base into the nasal area.
The term is variably used but generally encompasses both frontoethmoidal en...
Nasal gliomas, also know as nasal glial heterotopia, are a rare congenital lesion composed of dysplastic glial cells which have lost their intracranial connections and present as an extranasal or intranasal mass.
Nasal gliomas are rare congenital lesions. These masses occur spora...
Nasalis is one of the muscles of the nose, a subset of the muscles of facial expression, consisting of two main parts:
These two parts have opposing actions, the compressor narrows the nostrils and dilatator flares them. Both parts are supplied by the buccal br...
The nasal meatuses are distinct air passages of the lateral nasal cavity located inferior to each nasal conchae.
The plural of meatus is meatus (Latin noun of the fourth declension) or meatuses. Meati is incorrect.
There are three main nasal meatuses:
Nasal septal hematomas arise from ruptures in the small blood vessels in the nasal septum and are largely secondary to trauma. The nasal septum has a rich vascular supply which sources from both the internal and external carotid arteries.
Septal hematomas appear as i...
Nasal septal perforation may affect either the bony, or cartilaginous septum. Most commonly it affects the anterior septal cartilaginous area although with syphilis it characteristically affects the bony septum.
Symptoms include a nasal discharge, nasal congestion (loss o...
The nasal septum (Latin: septum nasi) separates the left and right nasal cavities. It extends from the nares anteriorly to the choanae posteriorly and is covered by squamous epithelium.
The vertical midline nasal septum is comprised primarily of a single nasal cartilage from the exte...
A not-very-useful mnemonic for the causes of nasal septum perforation is:
Say Water Coke Syrup Sugar Lemonade or say Nothing
W: Wegener granulomatosis (now known as granulomatosis with polyangiitis)
The nasal sill is the soft tissue ridge forming the posterior margin of the anterior naris. It also forms the caudal margin of the nasal vestibule.
The nasal sill forms part of the nasal base, which is defined as the inferior third of the nose when viewing the nose from below with the neck exte...
The nasal vestibule forms the small mildly dilated area of the most anterior portion of the nasal cavity, just inside the anterior naris. It is formed by the columella medially, the nasal ala laterally and anteriorly and the nasal sill posteriorly.
The nasion is the midline bony depression between eyes where the frontal and two nasal bones meet, just below the glabella. It is also known as the bridge of the nose.
It is one of the skull landmarks, craniometric points for radiological or anthropological skull measurement.
The nasociliary nerve is the intermediate branch of the ophthalmic division of the trigeminal nerve.
The nasociliary nerve divides off the ophthalmic division just before entering the orbit through the superior orbital fissure and through the tendinous ring between the inferior a...
Nasolabial cyst (also known as nasoalveolar cyst or Klestadt`s cyst) is a rare non-odontogenic, soft-tissue, developmental cyst occurring inferior to the nasal alar region. The cyst is derived from epithelial cells retained in the mesenchyme after fusion of the medial and lateral nasal processes...
The nasolacrimal (drainage) apparatus consists of:
Tears produced by the lacrimal gland, accessory lacrimal glands of Krause, Wolfring and Zeis, and Meibomian glands track medially along the eyelid margins and collect at the lacrimal lake at...
The nasolacrimal duct (NLD) is the terminal part of the nasolacrimal apparatus.
The nasolacrimal duct is the inferior continuation of the lacrimal sac and is ~17 mm in length in total. There are two parts to the nasolacrimal duct:
intraosseous part (12 mm): enters the lacrimal g...
Nasolacrimal tumors, in other words tumors involving the nasolacrimal drainage apparatus, are uncommon, and have a variety of histologies.
Clinical presentation of nasolacrimal tumors are typically fairly non-specific, often resulting in delayed diagnosis 1. Typical prese...
This suture forms the fissure between the frontal process of maxilla and the lateral border of the nasal bone. The nasomaxillary sutures are paried.
Naso-orbitoethmoid (NOE) fractures (also known as orbitoethmoid or nasoethmoidal complex fractures) are fractures which involve the central upper midface.
Naso-orbitoethmoid fractures are caused by a high-impact force applied anteriorly to the nose and transmitted posteriorly through...
The naso-orbitoethmoid (NOE) region or interorbital region simply refers to the space between the orbits.
anterior: frontal process of the maxilla, nasal process of frontal bone, nasal bone
posterior: sphenoid sinus
lateral: medial orbital wall