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N12 Neurology
Cranial Nerve Deficits
Toronto Notes 2019
Optic chiasm
DDx of CN III Palsy
iCAM
ischemic
Cavernous sinus
Aneurysm (PComm, internal carotid) Midbrain lesion
CN IV is the only cranial nerve that decussates at midline and exits posteriorly
A CN IV lesion may cause a contralateral deficit if lesion affects the nucleus
CN IV is at risk of trauma during neurosurgical procedures involving the midbrain because of its long intracranial course
Distinguishing CN III, IV, and VI Lesions
Pituitary gland III
Dura mater
Sphenoid air sinus
Figure 11. Cavernous sinus (coronal view)
CN IV: Trochlear Nerve
Clinical Features
• verticalandtorsionaldiplopia;defectofintorsionanddepression • patientmaycomplainofdifficultygoingdownstairsorreading
Differential Diagnosis
IV
Internal carotid artery VI
V1 V2
III
Diplopia Oblique Exacerbating Near target
Head Tilt Up and rotated
away
IV
Vertical
Looking down
Down and flexed away
VI
Horizontal Far target
Rotated towards
• common:ischemic(DM,HTN),idiopathic,trauma(TBIorsurgical),congenital • other:cavernoussinuslesion,superiororbitalfissure(tumour,granuloma)
CN V: Trigeminal Nerve
Clinical Features
• ipsilaterallossoffacialsensationandcornealreflex,weaknessofmusclesofmastication(V3only)with pterygoid deviation towards the side of the lesion
Differential Diagnosis
• brainstem:ischemia,tumour,syringobulbia,demyelination
• peripheral:tumour,aneurysm,chronicmeningitis,metastaticinfiltrationofnerve • trigeminalganglion:acousticneuroma,meningioma,fractureofmiddlefossa
• cavernoussinus:carotidaneurysm,meningioma,sinusthrombosis
• trauma
• note:otherCNVlesionsthatcausefacialpain=trigeminalneuralgia,herpeszoster
CN VI: Abducens Nerve
Clinical Features
• restinginwarddeviation(esotropia)
• horizontaldiplopia;defectoflateralgaze
Differential Diagnosis
• pons(infarction,hemorrhage,demyelination,tumour):associatedwithfacialweaknessand contralateral pyramidal signs
• tentorialorifice(compression,meningioma,trauma):falselocalizingsignofincreasedICP
• cavernoussinus:carotidaneurysm,meningioma,sinusthrombosis
• ischemiaofCNVI:DM,temporalarteritis,HTN,atherosclerosis
• congenital:Duane’ssyndrome
CN VII: Facial Nerve
Clinical Features
• LMNlesion:ipsilateralfacialweakness(facialdroop,flatteningofforehead,inabilitytocloseeyes, flattening of nasolabial fold)
• UMN lesion: contralateral facial weakness with forehead sparing (due to bilateral frontalis innervation) • impairedlacrimation,decreasedsalivation,numbnessbehindauricle,hyperacusis,tastedysfunctionof
anterior 2/3 of tongue
Differential Diagnosis
• idiopathic=Bell’spalsy,80-90%ofcases(seeOtolaryngology,OT22)
■ most often related to HSV, but other viruses may be implicated (CMV, herpes zoster, EBV)
• other:temporalbonefracture,EBV,RamsayHunt(VZV),otitismedia/mastoiditis,sarcoidosis,DM mononeuropathy, parotid gland disease, Lyme meningitis, HIV
Jaw deviation is towards the side of a LMN CN V lesion
CN VI has the longest intracranial course and isvulnerabletoincreasedICP,creatingafalse localizing sign
Forehead is spared in a UMN CN VII lesion due to bilateral innervation of CN VII nuclei from cerebral hemispheres for the frontalis
When screening for dysphagia and assessing aspiration risk, the presence of a gag reflex
is insufficient; the correct screening test is to observe the patient drinking water from a cup while observing for any coughing, choking, or “wetness” of voice
© Katerina Tchmoutina 2010