The Evans' index is the ratio of maximum width of the frontal horns of the lateral ventricles and maximal internal diameter of skull at the same level employed in axial CT and MRI images. This ratio varies with the age and sex.
It is useful as a marker of ventricular volume and thus has been pr...
Exencephaly is a lethal congenital fetal brain developmental anomaly (neural tube defect) considered to be a precursor to anencephaly in the acrania-exencephaly-anencephaly sequence.
It is characterised by calvarial absence and loss of fetal brain tissue to variable degrees and is co...
External auditory canal atresia (EACA) is characterised by complete or incomplete bony atresia of the external auditory canal (EAC) and, especially when seen in the setting of an associated syndrome, a dysplastic auricle and an abnormal middle ear cavity.
The incidence is 1 in 10,...
The external capsule is a series of white matter tracts in the brain situated between the putamen and claustrum. It is composed of claustrocortical fibres dorsally and the combined mass of the uncinate fasciculus and inferior frontal occipital fasciculus ventrally.
The putamen s...
The external petrosal nerve is one of the three branches from the geniculate ganglion. It carries sympathetic fibers to the middle meningeal artery.
External ventricular drains (EVDs) are a common neurosurgical procedure used to monitor and treat raised intracranial pressure in the acute setting.
Treatment and prognosis
intracranial haemorrhage (e.g. subdural, intraventricular)
Extra-axial is a descriptive term to denote lesions that are external to the brain parenchyma, in contrast to intra-axial which describes lesions within the brain substance.
Often it is trivially easy to distinguish an intra-axial from an extra-axial mass. In many cases,...
Extra-axial collections are collections of fluid within the skull, but outside the brain parenchyma. They may be comprised of CSF, blood or pus and may exist in the extradural, subdural or subarachnoid space.
This is a summary article; we do not have a more in-depth referenc...
Extra-axial masses of the intracranial compartment have a wide range of differentials, ranging from benign developmental cysts to malignant tumours.
choroid plexus papilloma/carcinoma
cranial nerve schwannoma
Extraconal orbital lesions include lesions which arise from structures within the extraconal orbital space and those extending from adjacent structures into the orbits.
dermoid cyst: most common lesion in paediatrics
lacrimial gland lesions
Extracranial brain herniation refers to herniation of brain tissue external to the calvaria through a skull bone defect, which may be post-traumatic or post-surgical. Unlike encephaloceles, brain herniation is surrounded by the meninges.
The herniated brain tissue requires surgical reduction a...
Extracranial meningiomas, also known as primary extradural meningiomas or ectopic meningioma, are a rare location-specific type of meningioma that arise outside the dural covering of the brain and spinal cord. They are essentially extracranial tumours, most often occurring in the head and neck, ...
Differentiating extradural (EDH) from subdural (SDH) haemorrhage in the head is usually straightforward, but occasionally it can be challenging. SDHs are more common and there are a few distinguishing features which are usually reliable.
History and mechanism of injury
Extradural haematoma (EDH), also known as an epidural haematoma, is a collection of blood that forms between the inner surface of the skull and outer layer of the dura, which is called the endosteal layer. They are usually associated with a history of head trauma and frequently associated skull ...
Extradural haemorrhages (EDH) represent collections of blood in the extradural (epidural) space. The haemorrhage sits between the skull superficially and the dura which overlies the brain parenchyma.
This is a summary article; read more in our article on extradural haemorrhag...
Venous extradural haemorrhages are a relatively uncommon subtype of extradural haemorrhages, differing from arterial extradural haemorrhages not only in aetiology, but also location and prognosis.
They occur as a result of damage to the dural venous sinuses and often result in the displacement...
Extradural neural axis compartment (EDNAC) exists from the tip of the coccyx all the way to the back of the globe, and yet it is relatively unknown as a concept. It is bounded externally by the periosteum of the vertebrae and sacrum inferiorly and the skull superiorly, and the visceral layer of ...
The extradural (epidural) space is a potential space between the cranial bones and the endosteal layer of the dura mater, which is otherwise adherent to the cranial bone.
The extradural space is a potential space inside the cranial vault and is not normally appreciable unless th...
Extraneural spread of primary intracranial neoplasms is distinctly uncommon, occurring far less frequently than CSF spread.
The most frequent neoplsams to do so, in decreasing order of frequency are:
A mnemonic to remember the order in which extraocular muscles are involved in thyroid-associated orbitopathy (TAO) is:
I: inferior rectus
M: medial rectus
S: superior rectus
L: lateral rectus
There is some debate about this however. Some claim superior rectu...
A mnemonic to remember the nerve supply to the extraocular muscles:
LR6SO4O3 (mock 'chemical formula')
The letters represent the extraocular muscles and numbers represent their respective cranial nerve supply:
LR6: lateral rectus, innervated by the 6th (abducens) nerve
Extrapontine myelinolysis (EPM) is one of the complications occurring secondary to rapid correction of hyponatraemia, and is, along with central pontine myelinolysis encompassed by the more recent term osmotic demyelination syndrome.
In the vast majority of cases it is associated with central p...
Extrapulmonary tuberculosis (TB) refers to the haematogenous spread of Mycobacterium tuberculosis.
Extrapulmonary tubercuosis can occur as a primary form of the disease, i.e. direct infection of an extrapulmonary organ without the presence of primary pulmonary tuberculosis or it can ...
The extrapyramidal system is the part of the motor system involved in modulation and regulation of movement. As its name suggests, it is distinct from the motor fibres that are relayed through the pyramids of the medulla oblongata (corticospinal and corticobulbar tracts).
It is composed of nerv...
Extraventricular neurocytomas, previously known as cerebral neurocytomas, are rare WHO grade II primary CNS neoplasms usually arising in the cerebral hemispheres. They are, as the name implies, extraventricular versions of central neurocytomas.
These tumours are reported at essen...
The extreme capsule is a series of white matter tracts in the brain that run between the claustrum and insular cortex.
The eye of the tiger sign refers to abnormal low T2 signal on MRI (due to abnormal accumulation of iron) in the globus pallidus with a longitudinal stripe of high signal (due to gliosis and spongiosis).
The eye of the tiger sign is most classically associated with pantothenate kinase-associate...
Fabry disease is a multisystem disorder which results from an X-linked inborn error of metabolism. The disease is characterised by a deficiency in hydrolase alpha-galactosidase activity with a resultant abnormal accumulation of globotriaosylceramide (Gb3) in various organ systems. In men, the co...
The facial-cavernous anastomoses are the communications of the facial and deep facial veins with the cavernous sinus.
At the medial canthus of the eye there is a communication with the ophthalmic veins, which drain into the cavernous sinus. Blood from the frontal scalp normally f...
The facial colliculus is an elevation on the floor of the fourth ventricle and is not formed by the facial nerve nucleus, but by the fibres of the facial nerve arching backwards around the abducens nerve (CN VI) nucleus before turning forwards once more in the caudal pons.
Facial colliculus syndrome refers to a constellation of neurological signs due to a lesion at the facial colliculus, involving:
abducens nerve (CN VI) nucleus
facial nerve (CN VII) fibres at the genu
medial longitudinal fasciculus
lower motor neuron facial nerve palsy ...
The facial nerve is one of the key cranial nerves with a complex and broad range of functions.
Although at first glance it is the motor nerve of facial expression which begins as a trunk and emerges from the parotid gland as five branches (see facial nerve branches mnemonic), it has taste and p...
There are many mnemonics to recall the branches of the facial nerve (superior to inferior) as they exit the anterior border of the parotid gland. Examples include:
Tall Zulus Bear Many Children
Two Zulus Bit My Cat
Two Zebras Bit My Coccyx
Ten Zebras Buggered My Car
To Zanzibar By Motor Car...
Facial nerve choristomas are rare, being characterised by non-neoplastic proliferation of smooth muscle cells and fibrous tissue. Facial nerve choristomas presumably can occur anywhere along the course of the facial nerve (CN VII), although the only cases reported are in the internal acoustic me...
Facial nerve schwannoma (FNS), also known as facial nerve neuroma/neurilemoma, is a schwannoma that arises from the facial nerve. They are generally uncommon, and when involving the temporal bone, make up less than 1% of all temporal bone tumours.
FNS is a rare tumour 2.
Helpful mnemonics for remembering the segments of the facial nerve include:
I Love Going To Makeover Parties 1
I Love Grinning, Then Making Pouts
both grinning and pouting are performed by muscles which are innervated by the facial nerve
I Must Learn To Make (facial) Expressions
Facial palsy refers to the neurological syndrome of facial paralysis. It can result from a broad range of physiological insults to the facial nerve or its central nervous system origins. The most common causes of this is Bell palsy.
While facial palsy refers to the clinical presen...
Fahr syndrome, also known as bilateral striatopallidodentate calcinosis, is characterised by abnormal vascular calcium deposition, particularly in the basal ganglia, cerebellar dentate nuclei, and white matter, with subsequent atrophy.
It can be either primary (usually autosomal dominant) or se...
The falx cerebelli is a small infolding of the dura in the sagittal plane over the floor of the posterior cranial fossa. It partially separates the two cerebellar hemispheres 1.
The falx cerebelli is attached posteriorly in the midline to the internal occipital crest of the occip...
The falx cerebri is the largest of the four main folds (or septa) of the intracranial dura mater, separating the cerebral hemispheres 1.
The falx cerebri is a double-fold of dura mater that descends through the interhemispheric fissure in the midline of the brain to separate the...
In discussing mineralisation of the falx cerebri, many radiology textbooks use the term falx calcification and make no mention of falx ossification.
Ossification of dural folds is relatively unusual; one study suggested a prevalence of falx ossification of 0.7% 1. Even though, os...
Familial multiple cavernous malformation syndrome(s) are uncommon, accounting for only a minority of cavernous malformations.
It has been more frequently reported in patients of Hispanic descent 1.
The presentation is most commonly with seizures (38-55%) 1 ...
Fatal familial insomnia is an extremely rare autosomally inherited prion disease 1. Unlike other prion diseases, it does not exhibit spongiform changes. The main pathological findings are gliosis in the inferior olivary nuclei and thalami.
Fat embolism syndrome (FES) is a rare clinical condition caused by circulating fat emboli leading to a multisystemic dysfunction. The classical clinical triad consists of:
It occurs in ~2.5% (range 0.5-4%) of th...
A fatty falx cerebri is a common finding, seen (according to one study) 7.3% of patients, and is explained by fat being a normal feature of the extradural neural axis compartment located between the two visceral layers of the falx.
A fatty falx is an incidental finding and should not be mistak...
The Fazekas scale is used to simply quantify the amount of white matter T2 hyperintense lesions usually attributed to chronic small vessel ischaemia, although clearly not all such lesions are due to this.
This classification was proposed by Fazekas et al. in 1987 1 and at the time of writing (l...
FCD may refer to:
focal cortical dysplasia (of the brain)
fibrous cortical defect (of the bone)
Feingold syndrome is characterised by the combination of:
alimentary tract atresias especially oesophageal atresia
Fetal brain tumours are uncommon and tends to have very different pathological spectrum than that observed in adults; in order of decreasing frequency:
fetal intracranial teratoma: most common tumour by far
astrocytoma/glioblastoma: next most common
craniopharyngioma: papillary type
Fetal hydrocephalus often refers to an extension of fetal ventriculomegaly where the ventricular dilatation is more severe. It is usually defined when the fetal lateral ventricular diameter is greater than 15 mm 1.
The estimated incidence is 0.5-3% per 1000 live births. There may ...
A fetal interhemispheric cyst refers to an interhemishperic cyst diagnosed in utero. It is seen as a cystic collection located in the interhemispheric fissure, with or without communication with the ventricular system.
agenesis of the corpus callosum: strong association...
Fetal intracranial calcification refers to intracranial calcification detected in utero. This can arise from a number of pathologies which include:
in utero infections
fetal toxoplasmosis infection: calcification tends to be randomly distributed
fetal cytomegalovirus infection1: calcificatio...
Fetal intracranial cystic lesions can arise number of pathologies, including:
fetal arachnoid cyst
fetal choroid plexus cyst
fetal connatal cyst
fetal porencephalic cyst
fetal interhemispheric cyst
fetal sub ependymal cyst
dorsal cyst of holoprosencephaly
Blake pouch cyst...
Fetal intracranial haemorrhage may occur either within the cerebral ventricles, subdural space or infratentorial fossa.
Haemorrhages can occur in a number of situations:
mechanical trauma, e.g. maternal abdominal blunt or birth trauma
severe fetal hypoxia
background fetal thromboc...
A fetal (origin of the) posterior cerebral artery is a common variant in the posterior cerebral circulation, estimated to occur in 20-30% of individuals 2.
The posterior communicating artery (PCOM) is larger than the P1 segment of the posterior cerebral artery (PCA) and supplies the bulk of the...
Fetal schizencephaly refers to schizencephaly diagnosed in utero. Usually only open lips types can be diagnosed antenatally.
may show a unilateral or bilateral defect extending from the pial surface to the ventricular wall
there may be other feature...
Fetal ventriculomegaly refers to the presence of dilated cerebral ventricles in utero.
Important in itself, it is also associated with other CNS anomalies.
Using the current sonographic cut-off criteria (see radiographic features below), the estimated prevalence may be at ~0.9% o...
Fetal ventriculomegaly (ventricle width >10 mm) is an important finding in itself and it is also associated with other central nervous system abnormalities. For more information, see the main article fetal ventriculomegaly.
Fetal ventriculomegaly can be thought of in ter...
Fibrillary astrocytomas are the most common type of diffuse low grade astrocytoma and as of the 2016 update to WHO classification of CNS tumours it no longer exists as a distinct entity, having been incorporated into the generic term diffuse astrocytoma 6.
Unlike the other variant...
Fibrocartilaginous embolism (also known as nucleus pulposus embolism) is a rare cause of spinal cord ischaemia due to embolisation of nucleus pulposus material from intervertebral disc in a retrograde direction into a spinal artery or vein.
Fibrocartilaginous embolism is a diagnosis of suspicio...
Fibrous meningiomas (also known as fibroblastic meningiomas) are the second most common histological subtype of meningioma, found in ~50% of all meningiomas, usually along with meningothelial histology (40%) or in isolation (7%). They are, for some reason, the most common intraventricular mening...
The following lesions may resemble a figure of eight (sometimes referred to as snowman shaped):
supracardiac variety of total anomalous pulmonary venous return 1
pituitary macroadenoma with suprasellar extension 2
intraspinal neurofibroma with extraspinal extension through neural foramina
The Fisher scale is the initial and best known system of classifying the amount of subarachnoid haemorrhage on CT scans, and is useful in predicting the occurrence and severity of cerebral vasospasm, highest in grade 3 2.
Numerous other scales have been proposed, incorporating various paramete...
The flat floor of fourth ventricle sign is useful in detecting a pontine mass and is a sign of mass effect. The normal floor of the fourth ventricle (remember that the floor is anterior) normally slopes upwards towards the midline, with the facial colliculi visible on either side.
It is a non-...
Flavivirus encephalitis includes a number of entities which characteristically involve the basal ganglia and thalami.
Typically these conditions present with a prodrome which is nonspecific but indicative of a viral infection. Symptoms include fever, myalgia, rash, rigou...
The presence of Flexner-Wintersteiner rosette is characteristic for retinoblastoma but is also seen in pineoblastoma and medulloepitheliomas.
Flow-diverter stents are relatively new and important devices in the management of intracranial aneurysms, especially ones that are large, broad-necked or fusiform.
In a multi-centre study in Italy, Briganti et al. reported an overall morbidity rate of 3.7% and a mortality rate ...
Focal areas of signal intensity (FASI), alternatively called focal abnormal signal intensity or unidentified bright objects (UBO), are bright areas on T2-weighted images commonly identified in the basal ganglia (often the globus pallidus), thalamus, brainstem (pons), cerebellum, and subcortical ...
Focal brainstem gliomas are a relatively uncommon type of brainstem glioma, which carry a more favourable prognosis compared to the more common diffuse brainstem glioma. The name is a reflection of the imaging findings, which demonstrate a sharply demarcated mass with relatively frequent enhance...
Focal calvarial thinning can result from a number of causes. They include:
bilateral thinning of the parietal bones (normal variant) most common
mega cisterna magna
peripherally located tumors (e.g. oligodendroglioma)
Focal cortical dysplasias (FCD) represent a heterogeneous group of disorders of cortical formation, which may demonstrate both architectural and proliferative features. They are one of the most common causes of epilepsy and can be associated with hippocampal sclerosis and cortical glioneuronal n...
The fogging phenomenon is seen on non contrast CT of the brain and represents a transient phase of the evolution of cerebral infarct where the region of cortical infarction regains a near-normal appearance.
During the first week following...
Foix-Alajouanine syndrome refers to presentation of spinal AVMs with progressive neurological deterioration.
Initially, patients have a spastic paraplegia which progresses to flaccidity, loss of sphincter control and ascending sensory level. It is thought to be due to venous hypertension.
The foramen caecum is located in the anterior cranial fossa, anterior to the cribriform plate of the ethmoid bone and posterior to the frontal bone, within the frontoethmoidal suture. It lies at a variable distance from the crista galli.
The foramen caecum is frequently found in infants, uncomm...
Foramen caecum can refer to a number of different anatomical structures:
foramen caecum (tongue)
foramen caecum (anterior cranial fossa)
The foramen magnum is the largest foramen of the skull and is part of the occipital bone 1. It is oval in shape with a large antero-posterior diameter 2.
The foramen magnum is found in the most inferior part of the posterior cranial fossa 3. It is traversed by vital structures in...
The foramen of Magendie (also called median aperture) is one of the foramina in the ventricular system and links the fourth ventricle and the cisterna magna. It is one of the three ways that cerebrospinal fluid (CSF) can leave the fourth ventricle and enter the subarachnoid space. The two other ...
Foramen ovale is an oval shaped opening in the middle cranial fossa located at the posterior base of the greater wing of the sphenoid bone, lateral to the lingula. It transmits the mandibular division of the trigeminal nerve (CN Vc), accessory meningeal artery, emissary veins between the caverno...
The foramen rotundum is located in the middle cranial fossa, inferomedial to the superior orbital fissure at the base of greater wing of the sphenoid bone. Its medial border is formed by lateral wall of sphenoid sinus. It runs downwards and laterally in an oblique path and joins the middle crani...
The foramen spinosum is located in the posteromedial part of greater wing of sphenoid bone posterolateral to foramen ovale which connects the middle cranial fossa with the infratemporal fossa. It transmits the middle meningeal artery, middle meningeal vein, and (usually) the nervus spinosus.
The forceps major, also known as the posterior forceps, is a fibre bundle which connects the occipital lobes and crosses the midline via the splenium of the corpus callosum.
The forceps minor, also known as the anterior forceps, is a fibre bundle which connects the lateral and medial surfaces of the frontal lobes and crosses the midline via the genu of the corpus callosum.
The fornix is the main efferent system of the hippocampus and an important part of the limbic system. It is one of the commissural fibres connecting the cerebral hemispheres.
Roughly C-shaped, the fornix extends from the hippocampus to the mammillary bodies of the hypothalamus an...
The term fornix (plural: fornices) is used for anatomical structures in multiple organ systems that all share an arch-like morphology:
History and etymology
Fornix is Latin for...
The fornix conjunctiva is loose soft tissue lying at the junction between the palpebral conjunctiva (covering the inner surface of the eyelid) and the bulbar conjunctiva (covering the globe). Each eye has two fornices, the superior and inferior fornices. The fornix permits freedom of movement of...
The fossa of Rosenmüller, also known as the posterolateral or pharyngeal recess, is the most common site of origin for nasopharyngeal carcinoma.
It is located superior and posterior to the torus tubarius (the posterior projection of the cartilaginous portion of the eustachian tub...
Foster Kennedy syndrome describes the clinical syndrome of unilateral optic atrophy with contralateral papilloedema caused by an ipsilateral compressive mass lesion.
The syndrome consists of two cardinal features:
ipsilateral optic nerve atrophy presenting with central s...
The fourth ventricle is one of the components of the ventricular system in the brain, along with the lateral and third ventricles. It extends from the cerebral aqueduct (of Sylvius) to the obex and is filled with CSF.
CSF enters the ventricle via the cerebral aqueduct and leaves via one of four...
A fourth ventriculocoele is large posterior fossa cyst which remodels, thins and eventually erodes through the occipital bone to form an occipital encephalocoele.
It may be classified as part of the Dandy-Walker continuum, but this is controversial.
Fracture-a-la-signature (or signature fracture) is another term used to described a depressed skull fracture.
Fracture-a-la-signature derives its name from forensic medicine because the size and shape of a depressed skull fracture may give information on the type of weapon used. It can be a si...
Determining fracture type is really important when looking at a fracture and trying to describe it. Fractures can broadly be split into complete and incomplete fractures.
This is a summary article. For more information, you can read a more in-depth reference articles: complet...
Fragile X-associated tremor/ataxia syndrome (FXTAS) is a progressive degenerative movement disorder resulting from a fragile X “premutation”, defined as 55-200 CGG repeats in the 5’-untranslated region of the FMR1 gene 1. The premutation can expand in subsequent generations (during oogenesis) to...
Friedreich ataxia (FA) is the most common inherited progressive ataxia. It carries an autosomal recessive inheritance 1.
Thought to have an estimated prevalence of ~1:50,000. There is no recognised gender predilection.
Typically present in childhood to adolescence. Those with a h...