The obelion refers to the skull landmark along the sagittal suture intersected by an imaginary line connecting the two parietal foramina, where both are present. It is said to represent the site of commencement of sagittal suture closure 1.
It is one of the skull landmarks, craniometric points ...
Obstruction of nasolacrimal drainage apparatus results in epiphora and can be primary or secondary, congenital or acquired. Obstruction can occur in canaliculi, lacrimal sac, or nasolacrimal duct (post saccular) levels.
Congenital obstruction is usually secondary to persistence of the membrane ...
The occipital artery, a posterior branch of the external carotid artery, opposite the external maxillary, near the lower margin of the posterior belly of the digastic muscle, and ends in the posterior part of the scalp.
origin: branch of the external carotid artery at the level of the ...
The occipital bone is a trapezoid skull bone that contributes to the posteroinferior part of the cranial vault. It is pierced by the foramen magnum, permitting communication from the cranial cavity to the vertebral canal
The occipital bone is composed of four parts:
Occipital plagiocephaly is a type of plagiocephaly. It is used to describe the shape of the skull which is a result of an early fusion of the lambdoid suture. The premature fusion may occur either on one side or both sides of the suture.
It is one of the rarest craniosynostosis, a...
The occipital triangle is one of the paired triangles in the posterior triangle of the neck. The triangles of the neck are surgically focussed, first described from early dissection-based anatomical studies which predated cross-sectional anatomical description based on imaging (see deep spaces o...
The occipitotemporal/occipitomastoid suture is the obliquely oriented articulation of the anterior border of the squamous occipital bone and the mastoid portion of the temporal bone. A mastoid foramen is occasionally located near or in it.
The occipitotemporal suture and the parietotemporal sut...
The ocular abductors are the extra-ocular muscles responsible for abduction of the globe:
lateral rectus (primary function)
superior oblique (tertiary function)
inferior oblique (tertiary function)
The ocular adductors are the extra-ocular muscles responsible of adduction of the globe:
medial rectus (primary function)
inferior rectus (tertiary function)
superior rectus (tertiary function)
The ocular depressors are the extra-ocular muscles responsible for depression of the globe:
inferior rectus (primary function)
superior oblique (secondary function)
The ocular elevators are the extra-ocular muscles responsible for elevation of the globe:
superior rectus (primary function)
inferior oblique (secondary function)
The ocular external (lateral) rotators are the extra-ocular muscles responsible for external rotation (extorsion) of the globe:
inferior oblique (primary function)
inferior rectus (secondary function)
The ocular internal (medial) rotators are the extra-ocular muscles responsible for internal rotation (intorsion) of the globe:
superior oblique (primary function)
superior rectus (secondary function)
Ocular metastases, also termed uveal metastases, account for over 80% of all ocular pathology, and need to be distinguished from extraocular metastasis, which are a quite different group of tumours.
This article will discuss metastatic lesions affecting the orbits. For other intracranial metast...
Ocular pathology covers a wide range of conditions and therefore represents the cause of a wide range of symptoms, signs and radiographic features.
Ocular metastases account for over 80% of all ocular pathology. With regard to the remainder of ocular lesions, the primary differentiating factor ...
The oculomotor nerve is the third of the cranial nerves and arises from the midbrain. It is responsible for the movements of four of the six extra-ocular muscles, the other two being innervated by the trochlear and abducens nerves.
Nucleus and cisternal portion
The oculomotor nu...
An oculomotor nerve palsy results in weakness of the superior rectus, inferior rectus, medial rectus, inferior oblique, and levator palpebrae, leading to an eye that is "down and out".
It has numerous possible aetiologies which can be divided according to which portion of the nerve i...
Odontogenic myxomas are rare tumours that involve the mandible or maxilla and account for 3-6% of odontogenic tumours 2.
Typically seen in the 2nd to 3rd decades of life (slightly earlier than ameloblastomas). They are usually not painful.
Arises from mesenchymal odont...
Odontohypophosphatasia is the mildest form of hypophosphatasia that manifests as tooth dysplasia and/or early loss of deciduous or permanent teeth.
As with all forms of hypophosphatasia, the underlying abnormality is a mutation in the ALPL gene that encodes for tissue non-specific al...
Odontomas are one of the most common of mandibular lesions encountered and the most common odontogenic tumours of the mandible. They account for up to two-thirds of all such tumours; the next most common are ameloblastomas, making up the majority of the remaining one-third.
The oesophagus is a muscular tube that conveys food and fluids from the pharynx to the stomach.
The oesophagus is 23-37 cm long with a diameter of 1-2 cm and is divided into three parts:
cervical: continuous with the hypopharynx, commences at the lower border of cricoid cartilag...
The olfactory fossae, grooves or recesses of the cribriform plate are shallow depressions located within the ethmoid bone in the anterior cranial fossa that contains the olfactory bulb of the olfactory nerve. The floor is formed by the medial lamella of the cribriform plate, bordered laterally b...
Olfactory neuroblastomas, also known as esthesioneuroblastomas, are tumours arising from the basal layer of the olfactory epithelium in the superior recess of the nasal cavity.
They usually present as a soft tissue mass in the superior olfactory recess involving the anterior and middle ethmoid ...
Olfactory neuroblastomas (or esthesioneuroblastomas) are most frequently staged using a system proposed by Kadish et al in 1976 1.
group a: tumour is limited to the nasal cavity
group b: tumour is limited to the nasal cavity and paranasal sinuses
group c: tumour extends beyond the nasal cavit...
Olfactory neuroepithelioma (9523/3) is a rare tumour which mimics olfactory neuroblastomas (9500/3) in presentation and imaging features and is often used synonymously with the latter 1-2.
Olfactory neuroepitheliomas differ from olfactory neuroblastomas in a number of ways 1:
The omega sign can refer to a number of different anatomical structures or signs:
omega sign (epiglottitis)
omega sign (hand bump on the precentral gyrus)
The omega sign refers to the thickened aryepiglottic folds and epiglottis seen in epiglottitis, when the larynx is seen endoscopically or via laryngoscope, and not to the appearance on lateral plain films. CT would show the finding, but placing a child with epiglottitis supine to CT their neck i...
Ommaya reservoir, also known as Ommaya shunt, is a device consisting of an intraventricular catheter connected to a reservoir (port) implanted beneath the scalp. It is used for intrathecal administration of medication such as chemotherapy (mainly in the treatment of meningeal lymphomas, but also...
The omohyoid muscle is an infrahyoid muscle of the neck that is innervated by the ansa cervicalis of the cervical plexus receiving fibres from the ventral rami of C1–C3 spinal nerves. The omohyoid is a paired, flat strap of muscle that is made up of superior and inferior bellies joined by an int...
An oncocytic papillary cystadenoma is a rare lesion that can occur as a result of oncocytic cell proliferation. They typically occur in the upper respiratory tract (predominantly larynx but occasionally the nasopharynx and very rarely in the parotid glands 2).
Oncocytomas of the salivary glands are rare benign epithelial tumours.
They typically present in older individuals (6th to 8th decades of life). There may be a slight increased female predilection.
While they can affect any of salivary glands. They most commo...
Ophthalmoplegia describes the abnormal eye movement that occurs because of paralysis of one or more of the six extraocular muscles involved in eye movements. Classification can be based on the cause of the ophthalmoplegia or the directions of the affected movements.
There are numerous causes of...
The opisthion is the median (midline) point of the posterior margin of the foramen magnum.
It is one of the skull landmarks, craniometric points for radiological or anthropological skull measurement.
Various lines and measurements using the opisthion are made in the diagnosi...
The optic canal is a cylindrical canal running obliquely through the lesser wing of sphenoid bone near the base where it joins the body of sphenoid. It transmits the optic nerve and ophthalmic artery. The optic canal opens into the skull base at the optic foramen.
Optic disc drusen (ODD) are also known as hyaline bodies and are a relatively common entity. They are usually found incidentally on CT or on follow-up of abnormal fundoscopy.
Optic disc drusen are identified radiographically in up to 0.3-3.7% of the population and are frequently b...
The optic nerve is the second cranial nerve which along with the olfactory nerve (CN I) is really an extension of the central nervous system, not surrounded by Schwann cells with first sensory bipolar cell body located peripherally in the retina. Their central processes synapse on ganglion cells...
Enlargement of the optic nerves is uncommon and has a surprisingly broad differential:
optic nerve glioma
optic nerve meningioma
Optic nerve meningiomas are benign tumours arising from the arachnoid cap cells of the optic nerve sheath, and represent ~20% of all orbital meningiomas, the majority of which are direct extensions from intracranial meningiomas.
These tumours typically appear as masses within the optic nerve, ...
Optic neuritis merely denotes inflammation of the optic nerve, and is one of the more common causes of optic neuropathy. It can be thought of as broadly divided into infectious and noninfectious causes, although the latter is far more frequent. On imaging, optic neuritis is most easily identif...
The optic radiation (or geniculocalcarine tract) is part of the visual pathway, forming the connection between the lateral geniculate nucleus of the pulvinar of the thalamus with the primary visual cortex of the occipital lobe.
This pathway carries third order neurones correlating to the contr...
The ora serrata is the most anterior extent of the retina. This serrated transitional zone lies between the anterior margin of the retina and the ciliary bodies.
Ora serrata cannot be seen on conventional imaging but is an inferred reference point. On the axial plane...
The oral cavity is the most proximal portion of the aerodigestive tract, and is continuous posteriorly with the oropharynx.
The oral cavity includes the lips anteriorly and extends to a ring of structures (circumvallate papillae, anterior tonsillar pillars, soft palate) that di...
T staging of oral cavity tumours is as follows:
The anterior border of the oral cavity is the junction of the skin and vermilionborder of the lip. The posterior border is formed by the junction of the hard and soft palates superiorly, the circumvallate papillae inferiorly, and the ...
Orbicularis oculi are facial muscles, a ring-like muscle functioning in a number of eyelid movements.
The orbicularis oculi are made up of three parts the orbital, palpebral and lacrimal parts. Each has defined actions.
The orbicularis oculi are secured to the medial and latera...
The orbit is a feature of the face and contains the globe and it's supporting structures, as well as many nerves and vessels.
In the adult, the orbit has a volume of approximately 30 mL, of which the globe occupies 6.5 mL. It has a roof, floor, medial and lateral wall. The orbit ...
Orbital blow-in fractures occur when there is displacement of bone fragments towards the orbits.
blow-in fracture effectively reduces the volume of the orbit
associated intraorbital injuries include extraocular muscle entrapment and optic nerve injury
as an isolated (pure) orbital ...
Cavernous venous malformations of the orbit, also known as cavernous haemangiomas, are the most common vascular lesion of the orbit in adults.
It is important to note that according to newer nomenclature (ISSVA classification of vascular anomalies) these lesions are merely known as slow flow ve...
Several cystic and cyst-like orbital lesions may be encountered in imaging of the orbits:
developmental orbital cysts
dermoid: commonest benign orbital tumour in childhood
congenital cystic eye
Orbital dermoid cysts are congenital lesions representing closed sacs lined by an ectodermal epithelium and comprising the most common orbital tumour in children. They are typically divided into deep (within the orbit) and superficial (adjacent to the orbital rim).
They comprise ~...
Orbital emphysema is the presence of gas within the orbital soft tissues. It is usually due to orbital fractures communicating with the paranasal sinuses but can be caused by penetrating trauma and infection. It is a common finding also after orbital or ocular surgery.
Orbital infection is a relatively commonly encountered pathology.
It comprises of three main clinical entities with the most important distinction between that of orbital and periorbital cellulitis:
periorbital cellulitis (preseptal cellulitis) is limited to the soft tissues anterior to the or...
The differential diagnosis of orbital inflammatory diseases (including orbital pseudotumours) can be divided based on their location into:
dacryoadenitis of lacrimal glands
myositis of extraocular muscles
perineuritis of optic nerve
Primary lymphoma of the orbit corresponds to one of the commonest orbital tumours and accounts for as much as half of all orbital malignancies. It is a B-cell non-Hodgkin lymphoma, and in most cases arises from mucosa-associated lymphoid tissue.
Orbital lymphomas account for only...
Orbital lymphoproliferative lesions compromise a wide spectrum of diseases ranging from benign to malignant.
Patient's may present with a palpable mass or proptosis. Pain is an uncommon symptom unlike orbital pseudotumour, which manifests with acute pain.
An orbital mass carries a relatively wide differential:
lacrimal gland or duct tumours
rhabdomyosarcoma of the orbit
optic nerve meningioma
optic nerve glioma
optic nerve schwannoma
developmental orbital cysts 3:
Meningiomas of the orbit are uncommon. They can be divided into two broad categories:
primary orbital meningioma
almost all are optic nerve meningiomas
rarely ectopic rests of arachnoid cells give rise to orbital meningiomas separate from the optic nerve sheath 1
secondary orbital meningioma...
Orbital metastases are relatively uncommon, but some primary tumours do have a predilection to metastasise to the orbit.
This article concerns itself with extraocular metastases, rather than intraocular tumours or direct extension of tumours from neighbouring regions. For a discussion of intrao...
There are numerous nerves of the orbit with have varying functions. There are best described divided into groups based on how they enter the orbit.
optic nerve (CN II): special sensory nerve of the globe (vision)
Superior orbital fissure
superior division of the oculomotor nerve ...
Orbital pathology covers a variety of diverse diseases that affect the orbit. The complicating factor is that the orbit is composed of a large number of different tissues which each have a plethora of pathologies that can affect them.
For simplification, they can be separated i...
Orbital pseudotumour is an idiopathic inflammatory condition that most commonly involves the extraocular muscles. Less commonly there is inflammatory change involving the uvea, sclera, lacrimal gland, and retrobulbar soft tissues. The exact aetiology is not known but an association with many inf...
The orbital septum is a thin sheet of fibrous tissue that originates from the orbital rim periosteum and blends with the tendon of the levator palpebrae superioris superiorly and inserts into the tarsal plate inferiorly.
The orbital septum separates the intra-orbital fat from eyelid fat and orb...
The orbital spaces are important when considering different pathologies:
subdivided into anterior and posterior segments by the lens
optic nerve-sheath complex
central retinal artery and vein
surrounding sheath of meninges as an extension of the cerebr...
Orbital vascular lesions may be difficult to distinguish on imaging. However, the following conditions have been described:
lymphangioma / lymphangiovenous malformation / venolymphatic malformation
orbital venous malform...
Orbital venous varix (OVV) is an uncommon vascular malformation which is composed of enlarged single or multiple tubular venous channels with direct communication to the systemic venous system.
Orbital venous varices are divided into primary and secondary. Primary orbital varices ...
The organ of Corti, also known as spiral organ, is the receptor organ for hearing, located in the cochlea (housed inside the scalae mediae). It is a strip of sensory epithelium made of hair cells which act as the sensory receptors of the inner ear.
This is a vulgarisation of the compl...
Oro-antral fistula (OAF) is a pathological communication between the oral cavity and the maxillary sinus (antrum).
There is a slightly greater male predilection 2.
Oro-antral fistulas may be subtyped by location into:
T staging of oropharyngeal tumours is as follows:
The oropharynx includes the base of the tongue, the inferior surface of the soft palate and uvula, the anterior and posterior tonsillar pillars, the glossotonsillar sulci, the pharyngeal tonsils, and the lateral and posterior pharyng...
The oropharynx forms part of the pharynx, being the continuation of oral cavity and nasopharynx superiorly, and the larynx and hypopharynx inferiorly.
The oropharynx is the posterior continuation of the oral cavity and the inferior continuation of the nasopharynx.
Oscillopsia (also known as Dandy syndrome or jumbling of the panorama) is the inability to maintain horizon while walking.
Since our heads bob up and down while walking, the otolithic system controls eye movement to maintain a constant horizon when walking. When there is bilateral absent vestib...
Ossicular chain disruption (or ossicular discontinuity) is loss of normal alignment between the three middle ear ossicles. The condition is a cause of conductive hearing loss.
Exact incidence and prevalence are not known. Hearing loss associated with temporal bone fractures in chi...
Ossicular chain fixation refers to the presence of fibrous tissue (chronic adhesive otitis media) which usually appears as noncalcified, soft-tissue density encasing some or all of the ossicular chain.
It may be present in the meso- and epitympanum yet it usually occurs in the niche of oval win...
Osteomas are benign mature bony growths, seen almost exclusively in bones formed in membrane (e.g. the skull).
When they arise from bone they may be referred to as a "homoplastic osteoma", and when they arise in soft tissue they may be referred to as a "heteroplastic osteoma".
The ostiomeatal complex (OMC) (or ostiomeatal unit (OMU)), sometimes less correctly spelled as osteomeatal complex, is a common channel that links the frontal sinus, anterior ethmoid air cells and the maxillary sinus to the middle meatus allowing airflow and mucociliary drainage.
A useful mnemonic to remember the five parts of the ostiomeatal complex is:
H: hiatus semilunaris
E: ethmoidal bulla
M: maxillary ostium
U: uncinate process
Emus have large beaks, and hence could have large ostiomeatal complexes (if they had paranasal sinu...
Some ostiomeatal complex anatomical variations, which do not characterise disease by itself, can promote narrowing and even obstruction 1:
concha bullosa ~10% (range 4-15%) - aerated middle turbinate
intralamellar cell: air cell within vertical portion of the middle turbinate
The Ostmann fat pads (also somewhat lewdly known as Ostmann fatty bodies) are located inferomedial to the Eustachian tubes and is thought to be important in normal closure, preventing transmission of nasopharyngeal pressure to the middle ear. Failure to visualize this thin triangular fat pad is ...
The otic capsule refers to the dense osseous labyrinth of the inner ear that surrounds the cochlea, the vestibule and the semicircular canals. It is surrounded by the less dense / pneumatised petrous apex and mastoid part of the temporal bone.
petrous temporal bone fractures...
The otic ganglion is one of four parasympathetic ganglia of the head and neck. It receives parasympathetic fibres from the glossopharyngeal nerve.
small and disc shaped
located in the infratemporal fossa
it lies immediately below the foramen ovale, medial to the mandibular divi...
Otitis externa refers to inflammation of the external ear.
It is quite a common condition and may affect up to 10% of people during their lifetime.
It can present in several forms:
acute otitis externa
chronic otitis externa
necrotising otitis externa
Otitis media refers to inflammation or infection of the middle ear cavity. It is commonly seen in children and is termed otomastoiditis when inflammation spreads to involve the mastoid.
Common complaints include otalgia, otorrhoea, headache, fever, and systemic upset. In ...
Otosclerosis (also known as otospongiosis) is an idiopathic slowly progressive primary disorder of the bony labyrinth. It is one of the leading causes of deafness in adults.
The term otosclerosis is somewhat of a misnomer. Much of the clinical course is characterised by lucent rat...
Otosyphyilis, otic syphilis or syphilitic labyrinthitis is a manifestation of neurosyphilis affecting the inner ear.
For a general discussion, and for links to other system specific manifestations, please refer to the article on syphilis.
The pathological features of syphilitic lab...
Oval window atresia or congenital absence of oval window shows absent cleavage plane between the lateral semicircular canal above and cochlear promontory below associated with anomalous stapes and malpositioned facial nerve (cranial nerve VII).
Profound conductive hearin...
Oxalosis results in supersaturation of calcium oxalate in the urine (hyperoxaluria), which in turn results in nephrolithiasis and cortical nephrocalcinosis.
This article focus on the secondary oxalosis, please refer to primary oxalosis for a specific discussion on this entity.