Brainstem and Cranial Nerve Anatomy

Rule of 4. 1

Brainstem Diagrams. 1

Examination – screening. 1

CN I – Olfactory. 1

Eye Examination - Eye Examination. 1

CN II – Optic. 1

CN III – Oculomotor 1

CN IV – Trochlear 1

CN VI – Abducens. 1

CN V – Trigeminal 1

CN VIII – Vestibulocochlear 1

CN IX – Glossopharyngeal 1

CN X – Vagus. 1

CN XI – Accessory. 1

CN XII – Hypoglossal 1

Rule of 4

4 Rules:

1. There are 4 structures in the midline beginning with M

a. Motor pathway (contralateral corticospinal weakness)

b. Medial lemniscus (contralateral dorsal column sensory loss)

c. Medial Longitudinal fasciculus (ipsilateral INO)

d. Motor nuclei

2. There are 4 lateral structures beginning with S

a. Spinocerebellar pathways (ipsilateral ataxia)

b. Spinothalamic (contralateral pain and temperature loss)

c. Sensory nucleus of the 5^th CN

d. Sympathetic pathway (ipsilateral horner’s syndrome)

3. There are 4 cranial nerves in the medulla, 4 in the pons, and 4 above the pons

4. The 4 midline motor nuclei divide into 12 (3, 4, 6, 12) the others (5, 7, 9, 11) are in the lateral brainstem.

Brainstem Diagrams

Examination – screening

CNI

· Best to just ask if patient has problem with smell

CNII

· Acuity

• Fields +/- blind spot
• Fundoscopy

CNIII, IV, VI

· Eye movements

• Accommodation
• Saccades

CNV

· Sensation in three divisions

• Pain and light touch
• Corneal reflex

o Efferent CN V

o Afferent CN VII

• Motor

o Masseter contraction

o Jaw opening

o +/- Jaw jerk

CNVII

· Facial strength

• Taste anterior two thirds of tongue

CNVIII

· Hearing

CN IX, X

· Palate symmetry

• Uvala asymmetry
• Voice

CN XI

· Head rotation

• Shoulder Shrug

CN XII

· Tongue protrusion

Multiple Cranial Neuropathies

Malignant

Carcinomatous or lymphomatous meningitis

Metastases

Local tumour invasion – nasopharyngeal tumour, sarcoma, cordoma

Perineural invasion – SCC, BCC

Infections

Radiculitis/meningeal infections - TB, fungal, syphilis, lyme

Direct neural infection – Listeria, HIV, CMV, Herpes zoster

Botulism

Inflammatory

Sarcoid

Wegener granulomatosis

Sjogren syndrome

Mixed connective tissue disease

(Myasthenia gravis)

Idiopathic

Tolosa-Hunt like syndrome

Melkersson-Rosenthal syndrome

Idiopathic pachymeningitis

Post-infectious – GBS type

Vascular

Carotid dissection or Jugular occlusion at the skull base

Other

Trauma

Paget disease of skull base

Arnold Chiari malformation

CN I – Olfactory

Course

· Cribriform plate to medial temporal lobe on same side

Examination

· Loss of smell (anosmia)

· Test each nostril with familiar smells

Lesions

· Causes of anosmia – most are bilateral

· URTI

· Smoking

· Age

· Ethmoid tumours

· Basal skull fractures or frontal fracture

· Post-pituitary surgery

· Congenital

· Frontal lobe base disease

o Meningioma of the olfactory groove

o Post basal meningitis

o Sarcoidosis

Eye Examination - Eye Examination

CN II – Optic

Course

· Retina – optic nerve – optic chiasm

· Optic tract – lateral geniculate body

· Optic radiation – visual cortex

· Light reflex fibres – branch off optic tract to superior colliculus (and synapse with fibres of third nerve)

Examination

· Acuity

· Fields

o Hat pin

o Glasses off

o Fields then map scotoma

· Fundoscopy

CN III – Oculomotor

Anatomy

· Nuclei - peri-aqueductal grey matter in mid-brain at level of superior colliculus

· Nerve exits anterior midbrain near cerebral peduncle

· Travels between superior cerebrallar artery and PCA

· Travel along PCOM and lateral to ICA

· Through cavernous sinus and superior orbital fissure.

· Visceral (Edinger-Westphal nucleus)

o Lies dorsal to somatic nucleus

o Supplies cilliary dilation and cilliary muscles (accommodation)

Pupils

- Third nerve supplies parasympathetic fibres to the pupil via Edinger-Wetphal nucleus – cilliary dilation and papillary accommodation.

- Sympathetic supply comes via ascending sympathetic fibres from the spinal cord (C8-T2)

Eye movements

- Supplies all muscles except lateral rectus and superior oblique

Other

- Levator palpebrae – elevates eyelid

Examination

Lesions

Third nerve lesions

· Ipsilateral weakness of :

1. Adduction (Medial rectus)

2. Elevation

3. Depression

· Dilated pupil – unreactive to light (direct or consensual) or accommodation

Causes

· Central

o Vascular, tumour demyelination

· Peripheral

o Compressive lesions

o Tumour

o Basal meningitis

o Nasopharyngeal or orbital tumours

o Ischaemia

o DM

o Cavernous sinus disease (see above – painful opthalmoplegia)

CN IV – Trochlear

Anatomy

· Nucleus, caudal to CNIII nucleus – periaqueductal grey matter at level of inferior colliculus in midbrain.

· The only cranial nerve to emerge dorsally from the brainstem

· Crossed over after emerging

· Travels ventrally and thus has longest course of all cranial nerves (75mm)

· Passes b/n superior cerebellar artery and PCA

· Travels through cavernous sinus

· Supplies superior oblique – intorts the eye, also depression and abducts

Clinical

· Diplopia is common, especially when looking down (going down stairs and reading)

o Vertical or oblique diplopia

· May walk with head tilted to side opposite lesion to maintain binocular vision

Examination

· May be head tilt away from side of lesion

· Elevation (hypertropia) of the affected eye

o Worse when looking away from the affected side

· Unable to move eye in and down

· Examination findings are greater with head tilted to side of lesion.

· In patients with CNII lesion it may be hard to tell if IV is intact

o Ask patient to abduct eye affected with CNII palsy and then watch for subtle intorsion as they try to look down

Lesions

· Most common causes

o Trauma

o Decompensation of congenital CN IV lesion

o Microvascular ischaemia

· Other causes by location

Location of lesion	Associated Sx	Causes
Nucleus (midbrain)	Contralateral Sup Oblique weakness and Ipsilateral Horners	Trauma Infarction Neoplasm
Fascicle	Rare Contralateral ataxia	Superior cerebellar peduncle pathology
Subarachnoid space	Isolated	Trauma Microvascular Meningitis Tumour
Cavernous sinus	See Cavernous sinus syndrome
Orbital apex	See orbital apex syndromes

CN VI – Abducens

Anatomy

· Nucleus – in the centre of the pons, beneath the floor of the 4^th ventricle, adjacent to CNVII

· Exits brainstem anteriorly at ponto-medullary junction

· Runs up the front of the brainstem

· Past basilar artery, over crest of petrous part of temporal bone (point of compression in raised ICP)

· Through cavernous sinus, superior orbital fissure

· Supplies lateral rectus

Examination

· Inward deviation of eye (esotropia)

· Can sometimes appear comitant

· One and a half syndrome

o CN VI and ipsilateral MLF lesion

o Either eye unable to look to side of lesion, only contralateral eye able to look away from lesion.

Lesions

· Common and specific causes

o Raised ICP

o Low ICP – especially with spinal CSF leak which may cause downward pressure.

o Microvascular

o Congenital (Duane or Mobius Syndrome)

· Bilateral disease

o Raised ICP

o Meningitis

· Mimics

o Thalamic esotropia

o Convergence spasm

Causes by location

Site	Associated Sx	Causes
Nucleus (pons)	Conjugate gaze palsy (One and a half syndrome) Ipsilateral CNVII	Stroke Neoplasm
Fascicle (pons)	Contralateral hemiparesis +/- other signs	Stroke Neoplasm Demyelination
Subarachnoid space	Isolated	Microvascular Raised ICP (or low ICP) Meningitis Trauma Tumour Petrous apex infection Vertebral/Basilar dilation/aneurysm Chiari malformation
Cavernous sinus	See cavernous sinus syndrome
Orbital Apex	See Orbital apex syndromes

CN V – Trigeminal

Examination

- Sensation

- Corneal reflex

- Masseter contraction

- Jaw opening – deviates to side of lesion

CNVII

Functions

- Muscles

- Taste on the anterior two-thirds of the tongue

- Lacrimal and salivary glands

Examination

- Muscle function

- Taste on tongue

- Lacrimation (schirmers test)

Lesions

CN VIII – Vestibulocochlear

CN IX – Glossopharyngeal

Nuclei

- Medulla

Course

- Formed by rootlets from groove between olive and inferior cerebellar peduncle

- Travels closely with vagus and exits at jugular foramen

Function

- Taste from the posterior third of the tongue

- Motor and sensory to pharynx and larynx with vagus

- Stylopharyngeus is the only skeletal muscle innervated

- Carotid baroreceptors

Clinical

- Dysphagia, choking

- Often with hoarseness due to vagus nerve damage

Lesions

- Medulla infarction

- Tumour near jugular foramen – will affect vagus as well

- Internal carotid aneurysm or jugular bulb thrombosis

- Vincristine toxicity – jaw pain

Examination

- Elevation of pharynx (vagus and glossopharyngeal)

- Decreased saliva production (never actually tested)

- Test for bilateral sensation to the pharynx and posterior tongue, this may also elicit a gag reflex

- Asymmetry of gag reflex is most indicative of pathology (sensory component of reflex is glossopharyngeal, motor is vagus)

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CN X – Vagus

Nuclei

• Motor – nucleus ambiguus of the medulla
• Parasympathetic – dorsal motor nucleus of the vagus in the brainstem

Course

• Rootlets attach to lateral aspect of medulla, caudal to glossopharyngeal
• The branchial motor fibers leave the vagus nerve as three major branches:
• Pharyngeal branch
• Superior laryngeal nerve
• Recurrent laryngeal nerve

Function

• Sensation
• Pharynx
• Larynx
• Oesophagus
• Tymanic membrane, external auditory meatus
• Chemoreceptors in aortic bodies
• Baroreceptors in aortic arch
• Motor
• Soft palate
• Palatoglossus muscle of the tongue
• Pharynx
• Larynx – except stylopharyngeus muscle (CN IX) and the tensor veli palatini muscle (CN V).
• Upper oesophagus
• Parasympathetic
• Wide distribution – CVS, GIT, Respiratory

Clinical

Examination

• Elevation of palate
• Deviation of uvula
• Gag reflex (efferent arm)
• Voice – hoarse
• Cough – bovine

CN XI – Accessory

CN XII – Hypoglossal

Examination

- Inspect tongue for wasting and fasciculations (lower motor neurone lesion)

- Protrude tongue – deviates towards weaker side

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