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 ...
The Obersteiner-Redlich zone represents the transition point along a nerve where enveloping glial cells are replaced by Schwann cells. Vestibular schwannomas are believed to arise most commonly from this region 1.
Obesity is defined as a body mass index (BMI) greater than 30. It is described as being a "modern epidemic" due to increased rates of metabolic syndrome and other complications in these patients, along with a high and increasing prevalence.
Obesity rates vary around the world but...
The obex is the most caudal point in the fourth ventricle and is therefore considered as part of the ventricular system in the brain. It is the point in the medulla where ventricle narrows to become the central canal of the spinal cord.
It occurs at the level of the foramen magnum and is theref...
The oblique fissures (also called the major fissures or greater fissures) are bilateral structures in both lungs separating the lung lobes.
Right oblique fissure
The superior part of the right oblique fissure separates the right upper lobe from the right lower lobe and the infe...
Oblique fractures are complete fractures that occur at a plane oblique to the long axis of the bone. Like transverse fractures, the term is predominantly used in the context of describing a fracture in a long bone.
Oblique fractures are particularly prone to angulation in the plane of the fract...
The oblique pericardial sinus is a blind-ending pericardial cul-de-sac behind the heart which opens into the pericardial space proper inferiorly.
It's boundaries are:
right (in ascending order): inferior vena cava, right inferior pulmonary vein and right superior pulmonary vein
Obliteration of the retrosternal airspace is seen in any cause of an anterior mediastinal mass.
Obliterative bronchiolitis (OB), also known as bronchiolitis obliterans or constrictive bronchiolitis, is a type of bronchiolitis and refers to bronchiolar inflammation with submucosal peribronchial fibrosis associated with luminal stenosis and occlusions. OB should not be confused with bronchio...
A mnemonic to remember the causes of obliterative bronchiolitis is:
C: cryptogenic organising pneumonia (COP)/BOOP
R: rheumatoid arthritis
I: infectious: post-viral/post-atypical infection (e.g. Mycoplasma pneumonia)
T: transplant: heart/lung/haematopoetic stem cell transpl...
The obstetric curriculum is one of our curriculum articles and aims to be a collection of articles that represent the core obstetric knowledge.
Topics pertaining to the normal and abnormal pregnancy, the gravid uterus and fetal environment, placenta, normal fetal development and fet...
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 ...
Obstructive bronchopulmonary aspergillosis is an uncommon manifestation of non-invasive aspergillosis seen in patients with AIDS. It is typically caused by Aspergillus fumigatus and represents marked overgrowth of intraluminal of the fungus.
Obstructive cystic renal dysplasia, or Potter type IV cystic renal disease, is a potential complication that can occur from prolonged obstruction of the bladder outlet or urethra during gestation.
Ureteric obstruction during active nephrogenesis results in cystic renal dysplasia; th...
Obstructive hydrocephalus is a term usually used to denote obstructive non-communicating hydrocephalus.
It is actually a term that causes confusion as used in the above sense implies that communicating hydrocephalus does not have an obstruction to CSF flow / absorption; this is not true as the...
Obstructive jaundice represents a set of conditions that cause jaundice by obstructing the flow of bile into the duodenum anywhere along the intrahepatic or extrahepatic biliary tree.
This is a summary article; read more in our article on jaundice.
An obstructive uropathy is a catch-all term encompassing any cause of complete or partial, congenital or acquired and permanent or intermittent obstruction to the urinary tract. Depending on the severity of obstruction and extent, it may result in permanent change in both the collecting system p...
The obturator artery is a branch of the anterior division of the internal iliac artery. It provides vascular supply within the pelvis and lower limb.
origin: anterior division of the internal iliac artery
location: pelvis and lower limb
supply: pelvic muscles, ilium, head of femur, ...
The obturator canal is a small opening in the superior aspect of the obturator foramen that connects the pelvis to the medial compartment of the thigh. The obturator foramen is otherwise covered by the obturator membrane.
The contents of the obturator canal are the obtu...
The obturator externus is a flat, triangular muscle, which covers the outer surface of the anterior wall of the pelvis.
origin: external surface of obturator membrane and adjacent bone (inferior pubic ramus and the ramus of the ischium)
insertion: trochanteric fossa of femur
The obturator foramen is the large, obliquely oriented, ovoid aperture located at the anterior aspect of both sides of the pelvis, bounded by parts of the ischium and pubis. It is covered by thin fibrous membrane, the obturator membrane, which is covered on the internal and external surfaces by ...
Obturator hernias are a rare type of abdominal hernia, and can be clinically very difficult to diagnose.
Typically obturator hernias occur in elderly women 3-4 or patients with chronically raised intra-abdominal pressure (e.g. ascites, COPD, chronic cough)4. It has been suggested...
The obturator internus muscle, like the piriformis muscle, is a muscle of the pelvic wall and of the gluteal region.
anterolateral wall of true pelvis
deep surface of obturator membrane and surrounding bone
insertion: medial side of greater trochanter of femur
The obturator nerve is a large nerve arising from the lumbar plexus and the nerve of the medial compartment of the thigh. It arises from the anterior divisions of L2-4 in the lumbar plexus.
The nerve descends medial to psoas major to the obturator canal where it ...
Obturator nerve neuropathy is an entrapment neuropathy that occurs when the obturator nerve is compressed by surrounding structures.
It occurs as the nerve dives under the superior pubic ramus to enter the obturator foramen: the anterior division lies between adductor brevis, ...
The obturator vein travels between the adductor region of thigh and internal iliac vein entering the pelvis through the obturator foramen (along with the obturator artery and obturator nerve).
Course and termination
Along the pelvic side wall the vein travels between the ureter and the intern...
The obtuse marginal (OM) arteries sometimes referred to as lateral branches are branch coronary arteries that come off the circumflex artery. There can be one or more obtuse marginal arteries. It typically traverses along the left margin of heart towards the apex. The first obtuse marginal arter...
Occam's razor (also known as the KISS principle or lex parsimoniae), an often cited principle of parsimony, economy, or succinctness used in problem-solving.
It states that among competing hypotheses, the one with the fewest assumptions should be selected. Although other, more complicated solut...
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 condylar fractures result from high-energy blunt trauma and is a specific and localized type of basilar skull fracture.
The exact incidence of these fractures is unknown but are reported to occur in 3-4% patients with moderate-severe traumatic brain injuries 3.
Occipital horn syndrome is a rare X-linked disorder of copper metabolism resulting in connective tissue abnormalities. The Menkes kinky hair syndrome is the severe form of this disease, with patients rarely surviving past 3 years of age. Occipital horn syndrome (also known as X-linked cutis lax...
Occipitalis originates from tendinous fibres from the lateral two thirds of the superior nuchal line (of the occipital bone) and the mastoid part of the temporal bone. It inserts into the epicranial aponeurosis.
It is supplied by the posterior auricular and occipital arteries.
It is innervat...
The occipital lobe is the smallest of the four lobes of the brain. It sits posterior to the temporal lobe and parietal lobes, underlying the occipital bone and overlying the tentorium cerebelli. Its most notable functional component is the primary and secondary visual cortex.
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 pole is an anatomical landmark that corresponds to the posterior portion of the occipital lobe. It is formed by the convergence of the superior and inferior occipital gyri in the majority of individuals; the middle occipital gyrus also contributes when it is present 1.
The occipital sinus is the smallest of the dural venous sinuses and lies, as its name suggests, on the inner surface of the occipital bone. Tributaries from the margins of the foramen magnum, some of which connect with both the sigmoid sinus and internal vertebral plexus, coalesce to pass in the...
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...
anterior: posterior border of sternocleidomastoid
posterior: anterior border of trapezius
inferior: inferior belly of omohyoid
spinal accessory nerve
cutaneous and muscular branches of the cervical plexus
upper most part of brachial plexus
Occipital vertebrae is a very rare anatomical variant and results from incomplete or aberrant fusion of occipital bone ossification centres. There is a broad spectrum of occipital vertebrae variations and the four most common include:
third condyle (condylus tertius)
The occipitotemporal sulcus, also known as the lateral occipitotemporal sulcus, runs anteroposteriorly on the inferior surface of the temporal lobe, separating the inferior temporal gyrus laterally from the fusiform gyrus medially. It runs parallel and lateral to the collateral sulcus (which is ...
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...
Occult fractures are those that are not visible on imaging, most commonly plain radiographs and sometimes CT, either due to lack of displacement or limitations of the imaging study. There may be signs of a fracture without one actually being seen. MRI or nuclear medicine studies are sometimes re...
Occult intrasacral meningocoele is a rare congenital lesion characterised by the presence of a cyst within the sacral thecal sac. It is an extradural sacral arachnoid cyst, not a true meningocoele, since meninges are not involved.
It is associated with spinal dysraphism, tethered cord syndrome ...
Ochronosis, or alkaptonuria, is a rare multisystem autosomal recessive metabolic disorder.
On imaging, the most particular presentation is on the spine, with osteoporotic bones and dense disc calcifications.
The term ochronosis usually refers to the bluish black discoloration of ...
An octreoscan is a scinitigraphic study that uses 111In labelled octreotide which is a somatostatin analog. It is particularly useful for assessment of neuroendocrine tumours.
glomus jugulare tumour
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...
O'Donoghue unhappy triad or terrible triad often occurs in contact sports, such as basketball, football, or rugby, when there is a lateral force applied to the knee while the foot is fixated on the ground. This produces the "pivot shift" mechanism.
The O'Donoghue unhappy triad comprises three t...
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...
Odontoid process fracture, also known as the peg or dens fracture, occurs where there is a fracture through the odontoid process of C2.
The mechanism of injury is variable, and can occur both during flexion or extension with or without compression 5.
There are two cl...
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 OEIS complex refers to the combined occurence of:
bladder exstrophy / cloacal exstrophy
an imperforate anus
and spinal anomalies: e.g
The estimated occurrence is at around 1-200,000 to 400,000 live births 1,8.
An oesophageal atresia refers to an absence in contiguity of the oesophagus due to an inappropriate division of the primitive foregut into the trachea and oesophagus. This is the most common congenital anomaly of the oesophagus.
It is thought to occur in ~1:3000-4500 live births ...
Oesophageal atresia is closely related to tracheo-oesophageal fistula and can be divided into1:
type A: isolated oesophageal atresia (8%)
type B: proximal fistula with distal atresia (1%)
type C: proximal atresia with distal fistula (85%)
type D: double fistula with intervening atresia (1%)
Oesophageal bronchus refers to the rare occurrence where a bronchus arises directly from the oesophagus.
It is more common in females with a M:F of 1:2 2.
Oesophageal bronchi may be the main bronchus, which gives rise to oesophageal lung, or may be a lobar bronchu...
Oesophageal cancer staging can depend slightly on whether the tumour is squamous cell or adenocarcinoma subtype. Due to the lack of a serosal layer, oesophageal cancer often tends to have mediastinal spread at the time of diagnosis.
Tx: primary tumour cannot be...
Oesophageal cancer is a relatively uncommon tumour that occurs within the oesophagus of affected individuals. Patients present with symptoms of increasing dysphagia that progress from solid foods to liquids.
This is a summary article; read more in our article on oesophageal c...
Oesophageal carcinoma is relatively uncommon. It tends to present with increasing dysphagia, initially to solids and progressing to liquids as the tumour increases in size, obstructing the lumen of the oesophagus.
Oesophageal cancer is responsible for <1% of all cancers and 4-10% ...
Oesophageal diverticula are sac or pouch projections arising from the oesophagus.
They can occur in all ages but more frequent in adults and elderly people.
Oesophageal diverticula are either:
true diverticula: include all oesophageal layers
false diverticula: contai...
Duplication of the oesophagus has a range of macroscopic appearances, ranging from complete (which is very rare) to partial cystic duplication (oesophageal duplication cyst). It is the second most common gastrointestinal tract duplication after that of the ileum.
A complete duplic...
Oesophageal duplication cysts are a type of congenital foregut duplication cyst.
Less common compared to other foregut duplication cysts. There may be an increased male predilection 5.
Patients are generally asymptomatic but may complain of dysphagia due to...
Oesophageal dysmotility refers to the pathological disruption of the normal sequential and coordinated muscle motion of the oesophagus to transport food from the oropharynx to the stomach. It is an umbrella term used to refer to the common pathophysiological endpoint of dysmotility that can be c...
Oesophageal fibrovascular polyps are benign intraluminal submucosal pedunculated tumours that can grow significantly and cause dysphagia symptoms. They usually occur in the upper third of the oesophagus, at the level of the upper oesophageal sphincter.
They were previously denomin...
Oesophageal food impaction (or steakhouse syndrome) refers to a food bolus acutely obstructing the oesophagus.
The main symptom is usually of acute dysphagia.
Depending on the composition of the ingested content, the bolus may be ...
The oesophageal hiatus is the opening in the diaphragm through which the oesophagus passes from the thoracic to abdominal cavity. It is one of three apertures in the diaphragm and is located in the right crus.
It is situated in the muscular part of the diaphragm at the level of T10 and is ellip...
Oesophageal leiomyoma is a benign smooth-muscle neoplasm of the oesophagus. It is the most common benign tumour of the oesophagus.
It is most frequently presents in young and middle age groups (20-50 years). The overall incidence is around 8-43 per 10,000 autopsy series 4.
Oesophageal leiomyomatosis is rare benign condition.
It usually presents at childhood. There is a recognised increased female predilection.
It is considered a hamartomatous condition and is associated with abnormal diffuse proliferation of smooth muscle fibres in dista...
Oesophageal leiomyosarcoma is a rare malignant tumour of the oesophagus of smooth muscle origin.
The first case of oesophageal leiomyosarcoma was described in 1905. Since then, there have been over 164 cases described in the literature.
Fluoroscopy - Bari...
Oesophageal lipomas are rare fat containing intrathoracic lesion.
They may account for approximately 0.4% of the benign tumours of the alimentary tract 1. There may be greater male predilection. The average age of presentation at around 50 years.
As with all lipomas, ...
A mnemonic to remember the causes of an oesophageal mass is:
CALL the MVP
Oesophageal myotomy (or Heller myotomy) is a procedure that can be performed to treat a lower oesophageal sphincter (LES) that fails to relax (e.g. achalasia). The procedure involves a longitudinal incision of the distal oesophageal musculature to break the sphincter tone.
A fundoplication wrap...
Oesophageal perforation is a rare but serious medical emergency with a very high mortality rate, especially if the diagnosis is delayed.
Most patients are in their 60s with a slight male predominance 5.
If a perforation is not detected during the procedure...
An oesophageal-pleural fistula refers to an abnormal connection between the oesophagus and pleura.
They can arise from a number of underlying pathologies which can result in an oesophageal rupture 3:
post oesophageal dilatation
secondary to a ...
Oesophageal intramural pseudodiverticulosis is an uncommon condition in which there are numerous small outpouchings within the oesophageal wall.
It is a rare condition, found in <1% of oesophagrams. It may occur at any age, but is more common between ages 50 and 70. There is a sl...
Oesophageal squamous papilloma is an uncommon finding on oesophagography (barium swallow). It is a benign lesion, but it is difficult to differentiate it from osophageal carcinoma on oesophagography and the diagnosis is usually made with endoscopic biopsy.
Oesophageal stents are a treatment option in patients with oesophageal strictures. It is most commonly used for symptomatic relief in those with dysphagia secondary to malignancy. The stent is typically covered in nature and inserted endoscopically or fluoroscopically. The distal oesophagus is t...
Oesophageal strictures refer to any persistent intrinsic narrowing of the oesophagus.
The most common causes are fibrosis induced by inflammatory and neoplastic processes. Because radiographic findings are not reliable in differentiating benign from malignant strictures, ...
Oesophageal webs refer to an oesophageal constriction caused by a thin mucosal membrane projecting into the lumen.
Oesophageal webs tend to affect middle-aged females.
Patients are usually asymptomatic and the finding may be incidental and unimportant. Howe...
Oesophagectomy (or esophagectomy) is a surgical procedure that involves excision of the majority of the oesophagus and part of the proximal stomach, usually as a treatment for oesophageal carcinoma or carcinoma of the gastric cardia, although benign conditions (e.g. stricture) can be treated wit...
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 term off-ended is used by some orthopaedic surgeons and radiologists to describe a long bone fracture that is displaced by more than the width of the bone. An off-ended fracture is often shortened due to muscle contraction.
Oil cysts in breast imaging refer to benign breast lesions where an area of focal fat necrosis becomes walled off by fibrous tissue.
Non-tender palpable lump
Fat debris from ruptured lipocytes tends to conglomerate to form a macroscopic pool of ...
The O'Kelly-Marotta (OKM) grading scale is a proposed method of assessing the degree of angiographic filling and contrast stasis in the setting of intracranial aneurysms treated by flow diversion. Taking into account the dynamic nature of the contrast stasis, it is designed for use with cerebral...