Allen and Ferguson classification is used for research purposes to classify subaxial spine injuries. It is based ofn the mechanism of injury and position of the neck during injury. This classification was proposed by Allen and Ferguson in 19823 and at the time of writing (July 2016) remains the ...
Allergic bronchopulmonary aspergillosis (ABPA) is at the mild end of the spectrum of disease caused by pulmonary aspergillosis and can be classified as an eosinophilic lung disease 2-4.
This entity is most commonly encountered in patients with longstanding asthma, and only occasio...
Allergic fungal sinusitis is the most common form of fungal sinusitis and is common in warm and humid climates. On imaging, it usually presents as opacification and expansion of multiple paranasal sinuses, unilaterally or bilaterally, with content that is centrally hyperdense on CT. MRI shows T2...
Allgrove syndrome (also known as triple A syndrome) is an autosomal recessive condition that consists of three main findings:
Allodynia refers to pain due to a stimulus which does not normally provoke pain. Temperature or physical stimuli can provoke allodynia, and it often occurs after injury to a site.
The word allodynia is derived from the Greek words άλλος (állos) meaning "other" and οδύνη (odýni) meani...
All-trans retinoic acid (ATRA) syndrome (more recently known as differentiation syndrome (DS)8) is a condition that can occur with patients with acute promyelocytic leukaemia who are on therapeutic all-trans-retinoic acid (ATRA).
All-trans-retinoic acid (ATRA) is a normal constituent of plasma....
Alobar holoprosencephaly is a subtype of holoprosencephaly (HPE), and is the most severe of the classical three subtypes, with both semilobar and lobar holoprosencephaly having less severe clinical manifestations.
For a general discussion of epidemiology, clinical presentation, and pathology, p...
Alpha-1-antitrypsin (A1AT) deficiency is a hereditary metabolic disorder and is the most common genetic cause of emphysema and metabolic liver disease in children. It results in the unopposed action of neutrophil elastase and subsequent severe basal pan lobular emphysema and respiratory symptoms...
Alpha angle can refer to two different musculoskeletal measurements:
alpha angle (developmental dysplasia of the hip)
alpha angle (femoroacetabular impingement)
The alpha angle is a measurement used in the ultrasonographic assessment of developmental dysplasia of the hip (DDH).
The angle is formed by the acetabular roof to the vertical cortex of the ilium and thus reflects the depth of the bony acetabular roof. This is a similar measurement to the acet...
All nuclei with the atomic number Z>82, are considered unstable. These are considered “neutron rich” and undergo the decay process by emitting a particle containing two neutrons and two protons.
Alpha decay is the process in which an alpha particle (containing two neutrons and two protons) is e...
Alpha-thalassemia/mental retardation syndrome X-linked (ATRX) is an important genomic marker of gliomas. Loss/mutation of ATRX is almost never seen in patients with 1p/19q co-deletion (i.e. they are essentially mutually exclusive). Oligodendrogliomas will, therefore, have intact ATRX and 1p19q c...
Alport syndrome is an X-linked recessive disease characterised by:
sensory neural hearing loss: typically high frequency 2
anterior lenticonus: most common ocular abnormality; may result in cataracts
perimacular pigmentary changes
flecks around the fovea 2...
Mammographic screening detects early breast cancers and thereby reduces potential mortality. However, its sensitivity is inversely related to breast density 1.
Altered density between two mammograms can arise in a number of situations:
Affecting both breasts:
interval commencement/cessation ...
Alternating radiolucent and radiodense metaphyseal lines can be seen with a number of conditions and the differential diagnosis is wide:
growth arrest lines
rickets: especially those on prolonged treatment, e.g. vitamin D dependent rickets
Alternatives are part of multiple choice questions, comprising the options from which an examinee must choose the correct answer.
Each multiple choice question should have, ideally, 5 alternatives, one of which is the correct answer (the "key"). In some instances, 5 options are not appropriate...
The Alvarado score is a clinical decision rule and predictor of the likelihood of acute appendicitis:
right lower quadrant tenderness (+2)
elevated temperature (37.3°C or 99.1°F) (+1)
rebound tenderness (+1)
migration of pain to the right lower quadrant (+1)
nausea or vomitin...
Alveolar rhabdomyosarcomas are a type of rhabdomyosarcoma and account for 20-40% of all rhabdomyosarcomas 1-2.
Unlike embryonal rhabdomyosarcomas, which are more common, these tumours occur in slightly older individuals, typically 10-25 years of age 1.
Alveolar sarcoidosis is an atypical pulmonary manifestation of sarcoidosis.
This appearance may be apparent in approximately 4% of those with pulmonary sarcoidosis on plain film 1 and up to 15% on CT 2.
This appearance is thought to result from the aggregation of a va...
Alveolar soft part sarcomas are rare, highly vascular, deep soft tissue malignancy that is classically seen in the lower extremities of young adults. They account for <1% of all soft tissue sarcomas.
There is a slight female predilection in patients less than 30 years old 1.
The alveoli are tiny hollow air sacs that comprise the basic unit of respiration.
Alveoli are found within the lung parenchyma and are found at the terminal ends of the respiratory tree, clustered around alveolar sacs and alveolar ducts. Each alveolus is approximately 0.2 mm in ...
Alzheimer disease (AD) is a common neurodegenerative disease, responsible for 60-80% of all dementias, and imposing a significant burden on developed nations. It is the result of accumulation and deposition of cerebral amyloid-β (Aβ) and is the most common cerebral amyloid deposition disease.
Alzheimer type I glia are a type of glial cell. They are large multinucleated astrocytes encountered in glial tumours and progressive multifocal leukoencephalopathy (PML) 1.
Alzheimer type II glia are a type of glial cell. They are a pathological reactive astrocyte seen in the brain, unrelated to Alzheimer disease. They are seen most frequently in Wilson disease, but also in other systemic metabolic disorders, particularly those with elevated ammonia levels, typical...
Amastia is a rare congenital condition characterised by the absence of breast tissue, nipple and areola. This may occur unilaterally or bilaterally.
During embryological development, breasts first appear as ectoderm ridges during the 6th week of gestation. This ridge grows thicker an...
Amaurosis fugax is the transient mono-ocular loss of vision, normally lasting a few seconds to a few minutes, and is secondary to vascular ischaemia/insufficiency. Usually the cause is ascribed to occlusion of the central retinal artery there are a wide number of local and central causes.
Amazia is a rare congenital condition defined by the absence of glandular parenchyma in either one or both of the breasts and a normal nipple and areola complex.
This is a very rare entity and the true prevalence is not known. Although there are strict definition criteria, the di...
The ambient cistern is part of the subarachnoid cisterns, filled with CSF.
The ambient cistern is a thin, sheet-like extension of the quadrigeminal cistern that extends laterally surrounding around the midbrain and posterior to the thalami. It acts as the connection between the q...
Amelia refers to a skeletal dysplasia characterised by the complete absence of upper or lower extremity or all four limbs. It may be associated with other congenital anomalies, i.e. omphalocoele and diaphragmatic hernias 3.
Amelia is a very rare congenital anomalies with incidence...
Ameloblastic fibromas appear as unilocular lucent mandibular lesions, most frequently in the posterior mandible, and are usually associated with impacted teeth, centred on the unerupted crown. They, therefore, appear very similar to unilocular ameloblastomas. They are composed of enamel and embr...
Ameloblastic fibro-odontoma (AFO) is a rare benign mixed odontogenic tumour that usually arises in the maxilla and mandible. According to the 2005 WHO classification of odontogenic tumours, it is defined as a benign tumour that resembles ameloblastic fibroma but contains enamel and dentin.
Ameloblastomas are locally aggressive benign tumours that arise from the mandible, or, less commonly, from the maxilla. Usually present as a slowly but continuously growing hard painless lesion near the angle of the mandible in the 3rd to 5th decades of life, which can be severely disfiguring if...
The American Academy of Orthopedic Surgeons classification of periprosthetic hip fractures divides the femur into three separate regions:
level I: proximal femur distally to the lower extent of the lesser trochanter
level II: 10 cm of femur distal to level I
level III: femur distal to level...
The American Board of Radiology (ABR) is a national certifying board for radiologists in the United States of America. It is a member of the American Board of Specialities.
The ABR currently requires three sets of examination for certification:
maintenance of certif...
The American College of Radiology (ACR) was founded in 1923 by Albert Soiland, an American radiologist 2. Its contemporary core purpose, according to its website, is "To serve patients and society by empowering members to advance the practice, science and professions of radiological care" 1.
The American College of Radiology (ACR) publishes and updates imaging guidelines, sometimes in collaboration with other bodies, on a range of imaging pathologies and reporting issues:
breast lesions: BI-RADS
liver lesions: LI-RADS
prostate lesions: PI-RADS
thyroid nodules: TI-RADS
The American Journal of Roentgenology, also known as AJR, is a peer-reviewed monthly journal published by the American Roentgen Ray Society (ARRS). Its current editor in chief is Dr Thomas H Berquist. Its global circulation is close to 25,000 paying subscribers 16.
Like many of ...
The American Roentgen Ray Society (ARRS) is the first and oldest learned society for radiologists in the United States, it was founded in 1900. Its current President is Bernard F King, Jr.
ARRS publishes the peer-reviewed journal American Journal of Roentgenology (AJR). Its current editor i...
Amiodarone hepatotoxicity is one of the complications that can occur with amiodarone therapy.
In the majority of patients, it is discovered incidentally during routine testing of liver biochemistry and rarely do the hepatic effects develop into symptomatic liver injury o...
Amiodarone lung is an interstitial lung disease seen in patients being administered amiodarone and can manifest in a number of histopathologic patterns.
The reported prevalence of pulmonary toxicity in the patients receiving amiodarone is ~10% (range 2-18%) 8.
Patients are usuall...
Amniocentesis, also known as amniotic fluid testing or AFT, is a diagnostic or therapeutic medical procedure primarily used in prenatal diagnosis of chromosomal abnormalities and fetal infections.
A small amount of amniotic fluid (15-20 ml), which contains fetal tissue, is extracted from the am...
Amnion refers to a membranous structure which covers and protects the embryo. It forms inside the chorion. The amnion usually fuses with the outer chorion by around 14 weeks of gestation.
The amnion can be visualised in most pregnancies before the 12th week of...
An amnioreduction is a procedure where an amniocentesis is performed for intentional reduction of amniotic fluid volume. This is sometimes performed in the context of extreme polyhydramnios (particularly in the recipient twin in twin to twin transfusion syndrome).
Amniotic bands refer to free floating blind ending amnion with an intact chorionic membrane. In certain situations, they lead to the amniotic band syndrome. They should not be confused with amniotic shelves which refer to the presence of amnion folding around pre-existing uterine adhesions. Some...
Amniotic band syndrome (ABS) comprises of a wide spectrum of abnormalities, all of which result from entrapment of various fetal body parts in a disrupted amnion. Due to the randomness of entrapment, each affected individual has the potential to form a unique deficit.
An amniotic fluid discordance is usually defined as a difference in amniotic fluid volumes in a twin pregnancy. It is a predictor of poor fetal outcome in twin pregnancy related complications.
Causes of amniotic fluid discordance include :
twin-twin transfusion syndrome
Amniotic fluid embolism is a special type of pulmonary embolism where the embolus is comprised of amniotic fluid. It can be a highly fatal complication of pregnancy, with an 80% maternal mortality rate.
It is thought to complicate 1/8000-80,000 pregnancies.
The amniotic fluid index (AFI) is an estimate of the amniotic fluid volume in a fetus. It is part of the fetal biophysical profile.
uterus is divided into four imaginary quadrants with linea nigra and umbilicus acting as the vertical and the horizontal axis respectively
Amniotic fluid in the first trimester has been estimated from weeks 7-12. Although the amniotic fluid index (AFI) is calculated in the second trimester, one can get an idea of whether the amount of amniotic fluid is too much or too little at an earlier time point.
The amniotic fluid volume is r...
Amniotic fluid volume (AFV) is a function both of the amount of water transferred to the gestation across the placental membrane, and the flux of water across the amnion.
Change in volume through gestation
The AFV undergoes characteristic changes with gestation. It progressively ri...
Amniotic shelf (also known as an amniotic sheet 4) refers to a sheet like projection that can result from uterine synechiae that has been encompassed by the expanding chorion and amnion. In contrast to amniotic bands, they are not thought to be associated with any fetal deformity.
Amoebic colitis is a type of infectious colitis, more common in tropical and subtropical areas. The causative agent is the trophozoite form of the protozoan Entamoeba histolytica. In most cases of transmission, the cyst form lives in the colon as commensal and remains asymptomatic.
Amoebic hepatic abscesses are a form of hepatic abscess resulting from Entamoeba histolytica infection.
Patients may experience general malaise or present with frank sepsis and right upper quadrant pain. Although the causative pathogen is found worldwide, it is endemic to...
Amorphous or indistinct calcifications are a morphological descriptive term for breast calcification and are defined as having small, hazy, faint calcifications with no clearly defined shape or form.
80-200 micrometer in diameter
small, hazy calcification
Amphiarthroses are a functional class of joint that permit a small amount of movement under normal conditions.
symphyses (secondary cartilaginous joints)
The ampulla of Vater is a conical structure at the confluence of the common bile duct (CBD) and the main pancreatic duct that protrudes at the major duodenal papilla into the medial aspect of the descending duodenum. The entire structure is encased by smooth muscle fibers that compose the sphinc...
Ampullary adenocarcinomas are rare biliary tumours arising from the distal biliary epithelium at the ampulla of Vater.
Although classically presenting on imaging with the double duct sign, the tumour itself may be occult or of limited characterisation imaging.
These are rare tu...
Ampullary ectopic pregnancy is the commonest type of tubal ectopic pregnancy and accounts for ~70% of such cases. According to one study the disruption of the tubal wall was less than as in isthmic ectopic pregnancy 2.
The term ampullary tumour generally refers to either benign or malignant neoplasms that arise from the glandular epithelium of the ampulla of Vater, including 1:
ampullary adenoma (adenoma of ampulla of Vater)
ampullary carcinoma (carcinoma of ampulla of Vater)
According to some authors, ampu...
The term amputation refers to the disconnection of all or part of a limb from the body. Specifically amputation is defined as removal of the structure through a bone. This is in contrast to disarticulation, which is removal of the structure through a joint.
When due to trauma, traumatic amputat...
The Amsterdam criteria are used in the diagnosis hereditary non polyposis colorectal cancer (HNPCC).
Amsterdam Criteria I
Initial description in 1991:
> or equal to 3 relatives with colorectal cancer (CRC)
> or equal to 1 case in a first degree relative
> or equal to 2 successive generation...
The Amsterdam wrist rules are validated clinical decision rules for determining which patients require radiographic imaging (wrist radiography) for acute wrist pain following trauma. The initial study evaluated 882 patients and were published in 2015 1. The decision rules assessed different clin...
Amyand hernia is a rare form of inguinal hernia in which the vermiform appendix is located within the hernial sac. It is seen in less than 1% of inguinal hernia.
It should not be confused with an appendix-containing femoral hernia, known as De Garengeot hernia.
The amygdala is a very well studied part of the limbic system and forms part of the mesial temporal lobe.
The amygdala is a complex structure, located dorsomedially in the temporal lobe, forming the ventral superior, and medial walls of the inferior horn of the lateral ventricle...
Amyloid arthropathy is the extracellular deposition of the fibrous protein amyloid within the skeletal system. It is a skeletal manifestation of amyloidosis and may involve either the axial skeleton (especially the cervical spine) or the appendicular skeleton.
Amyloidosis is a heterogeneous disease or even considered a constellation of diseases resulting in a deposition relatively similar proteins. It has many causes and can affect essentially any organ system.
There may be male predilection. Typically affects middle-aged individuals a...
Amyoplasia congenita is a syndrome characterized by multiple specific congenital joint contractures, associated with substitution of muscular tissue by fibrosis and adipose tissue.
The estimated incidence is at 1: 10000 live births. There may be a higher prevalence with twin preg...
Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig disease or Charcot disease, is the most common form of motor neurone disease 1,4 resulting in progressive weakness and eventual death due to respiratory insufficiency.
ALS typically is diagnosed in middle age. There is ...
Anal atresia, or imperforate anus, refers to a spectrum of anorectal abnormalities ranging from a membranous separation to complete absence of the anus.
The estimated incidence is 1 in 5000 live births.
Clinically there is no anal opening. Subtypes can be classified in...
The anal canal is the terminal part of the gastrointestinal tract. Anatomically, the anal canal is referred to as the terminal alimentary tract between the dentate line and anal verge. However, histologically it extends more proximally and includes the columns of Morgagni and anal sinuses. Surgi...
Anal canal cancer is relativity rare, however, there are several protocols that exist for assessment of various pelvic pathology. One method adopted for optimum assessment for anal cancer is (Auckland-New Zealand)
Overview: whole pelvis
T1 +/- T2FS
Fine 3 mm slices through region of concern (...
MR of the pelvis can demonstrate hidden areas of pelvic infection and secondary extensions which are important to detect prior to the sugary to minimize high rate of recurrence post intervention. Also pelvic MRI assists to delineate the anatomic relationships of the fistula to sphincters which c...
Anal cancer is a relatively uncommon, accounting for less than 2% of large bowel malignancies, and most of the cases are made of squamous cell carcinoma.
It accounts for less than 2% of large bowel malignancies and 1-6% of anorectal tumours (~1.5% of all gastrointestinal tract ma...
The most recent version of TNM staging of anal cancer is as follows:
Primary tumour (T)
TX: primary tumour cannot be assessed
T0: no evidence of primary tumour
Tis: carcinoma in situ (Bowen disease, high-grade squamous intraepithelial lesion [HSIL], anal intraepithelial neoplasia II-III (AIN...
Anal margin or perianal skin is arbitrarily defined as a skin tissue with a radius of 5 cm from the anal verge, consisting of keratinizing squamous epithelial tissue containing hair follicles.
anal margin neoplasms
The anal sphincter is divided into an internal and external anal sphincter. It surrounds the anal canal.
Internal anal sphincter
continuation of inner rectal muscle
thickened, circular muscle fibres, up to 5 mm thick
composed of visceral muscle
External anal sphincter
The anal triangle forms the posterior half of the diamond-shaped perineum. The triangle's three corners are defined by the tip of the coccyx posteriorly and both ischial tuberosities anterolaterally. The anterior border is the transverse perineal muscles and the posterolateral borders are the sa...
The anal verge is part of the anal region and consists of a band of squamous epithelial tissue which lacks hair follicles and extends from the inter-sphincteric groove to the perianal skin.
Anaplastic astrocytomas are WHO grade III lesions, with imaging appearances and prognosis between those of diffuse low-grade astrocytomas (WHO grade II) and glioblastomas (WHO IV), and similarly, they are classified on the basis of IDH mutation as IDH-mutant, IDH-wild-type and NOS (when IDH stat...
Anaplastic ependymomas (WHO grade III ependymomas), in comparison to lower grade ependymomas, are characterised by a higher proliferative rate and a greater tendency to infiltrate surrounding brain or disseminate into cerebrospinal fluid causing drop metastases 1. The relevance of grading ependy...
Anaplastic gangliogliomas are uncommon aggressive variants of the far more common low-grade ganglioglioma (WHO grade I). The aggressive component is the glial (usually astrocytic) component, which demonstrates nuclear pleomorphism, increased mitotic rate and increased cellularity, as well as mic...
Anaplastic meningiomas (also known as malignant meningiomas) are uncommon, accounting for only ~1% of all meningiomas 1. Along with rhabdoid meningioma and papillary meningioma are considered WHO grade III tumours and demonstrate aggressive local growth and high recurrence rate.
It should be n...
Anaplastic oligodendroglioma is a WHO grade III diffuse infiltrating glioma that has histological features of anaplasia, and molecular markers consistent with an oligodendroglioma (1p19q co-deletion and IDH mutation) as per the current (2016) WHO classification of CNS tumours 1. They make up 20-...
Anaplastic oligodendroglioma NOS (not-otherwise-specified) is a diagnosis in the current (2016) WHO classification of CNS tumours denoting a diffuse infiltrating glioma that has histological features of anaplastic oligodendroglioma, but without 1p19q co-deletion status or IDH mutation status bei...
Anaplastic pleomorphic xanthoastrocytomas are a more aggressive and less common version of pleomorphic xanthoastrocytoma (PXA).
In the current (2016) WHO classification of CNS tumours, they are considered WHO grade III lesions (whereas pleomorphic xanthoastrocytomas are WHO grade II tumours) 1...
An anaplastic rhabdomyosarcoma is a subtype of embryonal rhabdomyosarcoma, characterised by extensive anaplastic cells seen throughout the tumour 1.
Anaplastic thyroid carcinoma (ATC) is a highly aggressive form of thyroid cancer and accounts for ~1-2% of primary thyroid malignancies. Of all the subtypes, this carries the worst prognosis.
Typically occurs in the elderly (peak incidence in 6th to 7th decades). A significant pro...
The anatomical snuff box is a surface anatomy feature. It appears as a triangular depression on the lateral surface of the wrist on full extension of the thumb.
medial: tendons of the extensor pollicis longus
lateral: tendons of the
extensor pollicis brevis
Anatomic variants represent the deviations from the accepted standard human anatomy as printed in the classic textbooks (e.g. Gray's Anatomy 1), and taught in universities, dissecting rooms and clinical practice.
The term "normal anatomic variants" is often used interchangeably wit...
The anatomic position, also referred to as the standard anatomic position, is the consistent position of the human body in which positional reference is made for anatomical nomenclature. It is not reliant on whether the patient is standing, supine, prone, sitting, etc.
The position is defined a...
Articles pertaining to normal anatomy require a different structure, and the following subheadings are recommended:
As with all other articles, the introductory paragraph should introduce the anatomical term and aim to ...
As with all other articles, the introductory paragraph should introduce the anatomical term and aim to give an interesting summary. The first sentence should contain the title of the article in bold.
blood supply and innervation:
As with all other articles, the introductory paragraph should introduce the anatomical term and aim to give an interesting summary. The first sentence should contain the title of the article in bold.
ligaments and tendons: