Neuroanatomy Flashcards
Special things about CN4
Only CN that exits dorsally
Longest intracranial course
Decussates so the right nucleus of CN4 innervates the left SO
Causes of CN4 palsy:
#1: contrecoup injury from free tentorial edge
#2: ischemia
#3: idiopathic palsies
Other: hydrocephalus, vascular loops, tumors
CN6 palsy
CN6 nucleus contains interneurons which end in contra/L MR subnucleus of CN3 (conjugate eye movements)
Also means that CN6 nucleus lesion leads to ipsi/L GAZE palsy, not just ABduction deficit
MCC CN6 palsy = ischemic mononeuropathy (unless < 50 yo). No imaging if motility resolves w/in 3 mo
CN3 nuclei anatomy
CN3 subnuclei (located in midbrain) SR subnuclei crossed. Ex: Left SR subnucleus fibers cross to R SR Upper lids nuclei (single midline structure) innervates the levator OU Pupillary constrictor (parasympathetic innervation)& accommodation muscles --> under control of ipsi/L subnucleus
SO fissure divides to SR/levator (superior) and inferior (MR, IR, IO, parasympathetic axons to ciliary ganglion)
Pupillary fibers of CN3
Pupillary fibers are on the PERIPHERY of the nerve. Therefore, pupil is NOT usually involved in infarction (i.e. from DM, HTN) which affects the center.
(Therefore compressive CN3 palsy with pupil involvement is often 2/2 tumor/aneurysm. Often PAINFUL, and usually 2/2 PCOM aneurysm).
PERIPHERAL PUPILLARY FIBERS compressed by ANEURYSM. PAINFUL PCOM (MCC).
But pain is not good Sx to distinguish between compression vs. infarction.
CN3 path
1) CN3 leaves midbrain in brainstem from cerebral peduncle –> enters subarachnoid space –>
vascular supplied by terminal portion of basilar artery (at origin of superior cerebellar and posterior cerebral)
passes through these two arteries
2) travels medially along the PCOM artery and lateral to internal carotid artery
3) nerve enters cavernous sinus (lies in lateral wall superior to CN4) receives vascular supply from branches off ICA, ophthalmic artery, sup cerebellar, PCA
and then goes to SO fissure (where it divides into superior/inferior division)
4) superior division (SR and LP) and inferior (rest of EOM and Parasympathetic fibers from ciliary ganglion)
Rule of the pupil
In COMPLETE CN3 of the pupil, pupil is NORMAL if microvascular infarction. BUT, in PCOM aneurysm, anisocoria can develop at any time within 5 days of the start of diplopia
CN3 Nucleus lesions
Very rare. Ipsi/L Cn3 palsy. Ptosis OU contra/L SR palsy. OR CN3 palsy OU without ptosis OR ptosis OU alone OR any isolated palsy of muscle innervated by CN3
R CN3 nucleus innervates the RIGHT IO, IR, MR and LEFT (SR).
centrally located common CN3r subnucleus innervates levator on BOTH sides
Therefore CN3 palsy should have contralateral & ipsilateral palsy of the superior rectus and contralaterallevator palpebrae superioris.
Why lesions of CN3 nucleus also affect the ipsilateral SR considering it is innervated by the contralateral 3rd nerve nucleus?
B/C fibers from the SR subnucleus decussate and pass adjacent to the contralateral 3rd nerve nucleus. Therefore, virtually all lesions affecting the 3rd nerve nucleus will also hit fibers from the contralateral superior rectus subnucleus as they pass by in the brainstem.
Weber syndrome
Weber is WEAK
CN3 fasicle syndrome: Involvement of cerebral peduncles
ipsi/L CN3 paresis with contra/L hemiparesis
Nothnagel syndrome
Claude Nothnagel is CLUMSY
Nothnagel: CN3 fasicle syndrome: involvement of superior cerebellar peduncle
Ipsi/L CN3 paresis and cerebellar ataxia
(can also see this in Claude’s syndrome only Claude’s affects the midbrain, and ipsi/L and CONTRA/L ataxia and dysdiadochokinesis [dancing like movements])
Benedikt syndrome
CN3 fasicle syndrome: involvement of red nucleus
ipsi/L CN3 paresis with contra/L hemitremor
PCA and MCA VF defect
2/2 anatomy and vascular supply of the optic pathways, only lesions of the occipital lobe can produce a macular sparing homonymous hemianopia.
2 vascular supplies to the primary visual cortex of the occipital lobe
1) posterior cerebral artery supplies the majority of the primary visual cortex responsible for the contralateral visual hemifield
2) middle cerebral artery supplies the very posterior tip of the primary visual cortex of the occipital lobe responsible for the contralateral macular portion of the hemifield.
Therefore, a stroke of the PCA affecting the occipital lobe is the most likely cause of a macular-sparing homonymous hemianopia.
ON
intraorbital ON - some slack (none in intracranial portion) 125-1017 1 = intraocular 25 = intraorbital 10 = intracanalicular 17 = intracranial
Average optic nerve: 1.5 mm
Exits via the lamina cribosa and becomes myelinated by oligodendrocytes (NOT schwann cells as in peripheral nerves), the optic nerve size doubles to 30 mm in diameter.
sympathetic chain
Involves 3 interconnected neuron segments (1st, 2nd, and 3rd order). Throughout the entire sympathetic pathway, axons never decussate meaning that causative lesions are always SAME SIDE AS the Horner syndrome.
1st order neurons: cell bodies in the POSTERIOR LATERAL HYPOTHALAMUS descend through the brainstem to the INTERMEDIOLATERAL CELL COLUMN b/t C7 and T2 where they synapse with cell bodies of the second order neurons.
1ST ORDER HORNER CAUSES: Wallenberg (or lateral medullary syndrome) usually arises from a stroke. Other causes include: thalamic hemorrhage / stroke and tumors, cervical disc disease, demyelination, and other pathology of the cervical spinal cord.
2nd order neurons project from the INTERMEDIOLATERAL CELL COLUMN b/t C7 and T2 to the paravertebral sympathetic chain where fibers ascend to the SUPERIOR CERVICAL GANGLION and synapse with third order neurons.
2ND ORDER HONER CAUSES: apical lung malignancies (“Pancoast syndrome”), thoracic aortic aneurysms, brachial plexus injuries, and thoracic surgery with disruption of the superior portion of the paravertebral sympathetic chain.
3RD ORDER neuron cell bodies lie in the SUPERIOR CERVICAL GANGLION and pass superiorly into the cranial cavity with the INTERNAL CAROTID ARTERTY (ICA) in a fine web of nerves along the wall of the artery known as the carotid plexus. The ocular sympathetics continue with the internal carotid as it passes through the CAVERNOUS SINUS. While in the cavernous sinus, the third order neuron axons travel from the ICA to the abducens nerve before joining the NASOCILIARY BRANCH OF V1. The sympathetics then travel with the nasociliary branch of V1 into the orbit and through the CILIARY GANGLION (without synapsing) to extend to the dilator muscle of the eye.
3RD ORDER HORNER CAUSES: internal carotid artery pathology (including dissection), cavernous sinus compressive / inflammatory lesions, and trigeminal autonomic cephalgia type headaches (e.g. cluster headache, SUNCT, etc).
APD
optic tract lesion often results in an APD of the contralateral eye.
2/2 more crossed than uncrossed pupillary fibers in the optic tract.
R optic tract lesion results in decreased afferent input from both retinas but relatively-speaking there is a more marked reduction of afferent input from the contralateral eye.
ON axons
approximately 1-1.2 million ganglion cells which make up the optic nerve.
Optociliary shunt vessels
(aka retinochoroidal collaterals) are a dilation of the naturally-occurring veins that drain from the peripapillary retinal circulation into the choroidal circulation.
Similar to hemorrhoids or esophageal varices, optociliary shunt vessels evolve in response to chronically-poor drainage of the central retinal vein.
DDX: optic nerve sheath meningiomas, low-grade optic nerve gliomas, chronic papilledema, chronic glaucoma, and old central retinal vein occlusions.
Crarinopharyngioma vs pituitary adenoma VF defects
Craniopharyngiomas classically arise from ABOVE the optic chiasm and therefore classically (but not always) compress the SUPERIOR OPTIC CHIASM first. The retinal ganglion cell axons first compromised by such compression would be the superonasal fibers supplying the INFEROTEMPORAL VF in both eyes. With increasing compression more fibers become involved and the deficits may progress to a complete hemianopia.
IN CONTRAST, a compressive pituitary adenoma which classically arises from BELOW the optic chiasm and would typically START WITH SUPEROTEMPORAL VF deficits in both eyes.
dural venous sinus thrombosis (DVST)
On MRV: a DVST with a flow void (a filling defect in the venous sinus where hyperintense flow should be seen) in the transverse sinus.
DVST is one of the more common mimickers of pseudotumor cerebri and often presents acutely with one or more of the following: headache, blurred vision, papilledema, transient visual obscurations, and unilateral / bilateral sixth nerve palsy. More rarely, DVST can cause intracranial hemorrhage and stroke. Before the advent of non-invasive venography (MRV and CTV), many patients with DVST were misdiagnosed as having pseudotumor cerebri. In fact, an MRV or CTV (in addition to a brain CT or MRI) should be performed in any patient with bilateral papilledema who does not fit the typical pseudotumor cerebri profile (overweight female of childbearing age).
DVST, similar to deep venous thrombosis or pulmonary embolism, can be precipitated by acquired or hereditary hypercoaguability.
Common DVST-associated hypercoaguable states: pregnancy (and the peripartum period), OCP use, lupus, inherited coagulopathies, infection, dehydration, and acute head injury (not a remote history).
Any infection will increase the risk for DVST although middle / inner ear infections and meningitis are among the more common precipitating infections. Patients with DVST but without a clear hypercoaguable risk factor should be considered for a hypercoaguable laboratory evaluation and/or referral to a hematologist.
What structure upstream of the cavernous sinus leads to arterialization of conjunctival vessels in both low flow dural sinus fistulas (DSF) and high flow carotid cavernous fistulas (CCF)?
Elevated pressure and reduced drainage of the superior ophthalmic vein leads to arterialization of the bulbar conjunctiva.
This is an important fact to remember because one very helpful radiographic sign of a CCF or DSF is a dilated superior ophthalmic vein. This is not a specific finding of CCF / DSF, however, and can occur in other processes impeding venous drainage such as a cavernous sinus thrombosis.
Idiopathic or so called “microvascular” cranial mononeuropathies
Idiopathic or so called “microvascular” cranial mononeuropathies of cranial nerve 3, 4, or 6 are thought to be due to microvascular disease of the VASA NERVORUM, or blood supply to the peripheral nerves. Eye or periorbital pain is very common in these cases. In theory, this infarct of the nerve results in focal demyelination without permanent damage to the nerve axons allowing for full resolution with time.
MC etiology for acute onset cranial mononeuropathies of cranial nerve 3, 4, or 6 in patients with microvascular risk factors. Such risk factors include HTN, DM, peripheral vascular disease, coronary artery disease, hypercholesterolemia, smoking, and age.
Classic teaching: patients over the age of 50 (or 55 depending on the source) who develop an acute onset, isolated peripheral cranial nerve 3, 4, or 6 palsy can be observed without requiring an extensive work-up. Any cranial mononeuropathy in patients less than 50 (or 55) must be imaged and worked .
CN3 palsy - what factors to observe vs not observe
in order to observe a 3rd nerve palsy there must be NO PUPIL INVOLVEMENT AND A COMPLETE CN3 PALSY.
The definition of pupil involvement varies but includes a larger ipsilateral pupil that responds poorly to light due to impaired parasympathetic innervation. Any anisocoria is concerning while anisocoria greater than 2 mm is extremely concerning for a compressive mass of the 3rd nerve (most often an aneurysm of the posterior communicating artery).
A complete CN3 palsy means that all muscles innervated by the 3rd cranial nerve are maximally affected. Microvascular cranial mononeuropathies are either resolved or mostly resolved by 3 months. The onset is usually acute although there can be progression of the motility deficit / alignment over the first 7 days after initial onset. A progressive worsening of diplopia, motility, or alignment after the first week of symptoms is certainly an indication for a complete work-up including neuro-imaging.
MRI transverse section - midbrain
Midbrain - “mickey mouse sign” - and other structures seen well here:
Superior Ophthalmic vein starts superonasaly and passes posterolaterally (SUPERIOR to ON and INFERIOR to SR) –> exits through SO fissure and drains into cavernous sinus.
Can also see ON, Optic tract
Pupil involving CN3
Pupil involving = ASSUME acute 3rd nerve palsy from aneurysm. While saccular aneurysms can occur virtually anywhere in the cerebral circulation, aneurysms at the junction of the POSTERIOR COMMUNICATING ARTERY (PCOM) and the internal carotid artery (INTERNAL CAROTID ARTERY) often present with a pupil involving 3rd nerve palsy by compressing the 3rd nerve as it passes by in the subarachnoid space.
The parasympathetic fibers responsible for contraction of the pupillary sphincter muscle (and therefore miosis) travel in the peripheral segment of the nerve and are therefore very vulnerable to external compression.
Microvascular 3rd nerve palsies can have pupil involvement but it is a diagnosis of exclusion made only after a thorough evaluation to rule out an aneurysm.
Strokes and seizures
Hemispheric strokes can affect the afferent and efferent function of the eyes in multiple ways. General rule of stroke is that infarcted neurons do not recover function in adults.
Exception = patients with a gaze deviation towards the side of the infarct due to dysfunction of the frontal eye field (Brodmann area 8) recover quickly.
Frontal eye field exerts supranuclear control over lateral gaze towards the opposite side so an infarct of the right frontal lobe will result in a temporary right gaze deviation. A seizure, however, in the region of the frontal eye field will cause an episodic deviation towards the contralateral side.
Quick recovery from supranuclear gaze deviations caused by hemispheric strokes (usually a matter of days) hypothesis:
1) 2/2 brain compensation via other viable neurons to serve the same function.
2) brain simply “recalibrates” ocular alignment shortly after infarction of the frontal eye field.
