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 occipitomental (OM) or Waters view is an angled AP radiograph of the skull, and can be used to assess for facial fractures as well as the ethmoid and maxillary sinuses.
The occipitotemporal/occipitomastoid suture is the obliquely oriented continuation of lambdoid sutural syndesmosis is seen between the anterior border of the occipital bone and the mastoid portion of the temporal bone. The mastoid foramen can be located in the occipitomastoid suture.
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 is ...
Odontogenic myxomas are rare tumour 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 odonto...
The Roy-Camille classification of fractures of the odontoid process of C2 depends on the direction of the fracture line 1.
The level of fracture line as described by the Anderson and D’Alonzo classification, is not predictive of the degree of instability or the risk of non-union. This classific...
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 oropharynx 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 oropharynx, commences at the lower border of cricoid cartil...
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 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...
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 rounded canal, located in the lesser wing of sphenoid near the base where it joins the body of sphenoid. It transmits the optic nerve and ophthalmic artery.
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 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 pla...
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...
The orbit is a feature of the face and contains the globe.
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 is open anteriorly where it is bound by the orbital septum, which f...
Cavernous venous malformations of the orbit (aka cavernous haemangioma) 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 venous malfo...
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 corresponding to the most common orbital tumour in children. They are typically divided according to whether or not they are deep (within the orbit) or superficial (adjacent to the orbital...
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 can be a commonly encountered entity.
It is important to differentiate between orbital and periorbital cellulitis, as this has therapeutic and prognostic implications:
periorbital cellulitis (preseptal cellulitis) is limited to the soft tissues anterior to the orbital septum ...
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 meningiom...
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...
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.
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 chambers 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 ...
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:
The oropharynx forms part 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 in 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 inner 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 chil...
Osteoarthritis of the temporomandibular joint (TMJ) is the end point of long-standing TMJ dysfunction. It is a common finding incidentally on base of skull imaging, and it should be remembered that TMJ pain does not correlate well with OA changes. Indeed pain from TMJ dysfunction is often self l...
Osteomas are benign mature bony growths, seen almost exclusively in bones preformed 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 "heteropla...
The ostiomeatal complex (or unit), sometimes less correctly spelled as osteomeatal complex, is a common channel that links the frontal sinus, anterior and middle ethmoid sinuses and the maxillary sinus to the middle meatus that allows air flow 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 middle turbinate
oversized ethmoid ...
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 a flat disc-shaped parasympathetic ganglion situated in the infratemporal fossa. It lies immediately below the foramen ovale, medial to the mandibular nerve and supplies secretomotor fibres to the parotid gland.
Its preganglionic parasympathetic fibres originate from the in...
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
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...
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.