Usually produce weakness on the same side as the damaged cranial nerve.
The localisation framework
Separate the weak side from the direction of deviation.
This is the source of most confusion. In several lesions, the weak structure is ipsilateral to the lesion, but the visible structure may deviate in the opposite direction because the normal side pulls more strongly.
1
Always ask two questions
Which side is weak? Then: which way does the structure move or deviate? Do not use “ipsilateral” and “contralateral” as if they describe the same thing.
Often produce contralateral weakness because corticobulbar fibres cross before reaching the motor nucleus.
Usually reflects the stronger, intact side acting unopposed.
Mechanical—not neurological—but the jaw deviation pattern resembles a V3 motor lesion.
| Lesion | Side actually weak / affected | What you observe |
|---|---|---|
| Facial nerve LMN (CN VII) | Ipsilateral | Entire ipsilateral face weak, including forehead |
| Facial UMN | Contralateral lower face | Forehead relatively spared |
| TMJ injury | Ipsilateral joint | Mandible deviates toward injured side on opening |
| Hypoglossal LMN (CN XII) | Ipsilateral tongue | Tongue deviates toward lesion |
| Hypoglossal UMN | Contralateral tongue weakness | Tongue deviates away from cerebral lesion |
| Trigeminal motor LMN (V3) | Ipsilateral muscles of mastication | Jaw deviates toward lesion on opening |
| Vagus LMN (CN X) | Ipsilateral palate/pharynx weak | Uvula deviates away from lesion |
Exam-safe summary: the lesion side and the direction of deviation are not always the same. CN X is the classic example: the weak palate is ipsilateral, but the uvula moves away.
CN VII
Facial nerve: forehead sparing is the clue.
Facial weakness becomes much easier once you distinguish a peripheral LMN lesion of the facial nerve from a supranuclear UMN lesion.
Facial nerve LMN lesion
A lesion of the facial nerve after the facial nucleus causes weakness of the entire ipsilateral half of the face.
Left CN VII LMN lesion → left forehead + left lower face weak.
Why the forehead is spared in UMN lesions
The upper part of the facial motor nucleus receives substantial bilateral corticobulbar input. Therefore, a unilateral cortical lesion typically weakens the contralateral lower face while the forehead remains relatively preserved.
Think stroke vs Bell's palsy pattern: forehead spared → think UMN; forehead involved → think LMN.
Jaw deviation
V3 motor lesion and TMJ injury point the same way.
The mechanism differs, but the visible result is similar: on opening, the jaw deviates toward the weak or mechanically restricted side.
Trigeminal motor LMN lesion
If the left V3 motor supply is damaged, the left muscles of mastication—especially the lateral pterygoid—are weak. The intact right lateral pterygoid acts relatively unopposed.
Left V3 lesion → jaw deviates LEFT on opening.
TMJ injury / restriction
A damaged or restricted condyle cannot translate normally during mouth opening. The opposite condyle continues to translate forward.
Left TMJ pathology → jaw deviates LEFT on opening.
One useful pairing
Both a unilateral V3 motor lesion and unilateral TMJ restriction produce jaw deviation toward the affected side.
CN X
Vagus: weak side and uvula direction are opposite.
This is the pattern most likely to produce an apparent contradiction unless you explicitly separate weakness from deviation.
Vagus LMN lesion
Suppose the left vagus is damaged. The left soft palate is weak and droops.
Left palate weak → intact right side pulls uvula RIGHT.
The exact wording to use
Do not say “the vagus lesion is contralateral.” The motor deficit is ipsilateral to the damaged nerve. What is contralateral is the direction of uvular deviation.
CN X LMN: palate droops on the lesion side; uvula deviates away from the lesion.
CN XII
Hypoglossal: UMN and LMN point in opposite directions.
Tongue protrusion depends heavily on the genioglossus muscles. Weakness on one side allows the normal side to push the tongue toward the weak side.
Hypoglossal LMN lesion
A left hypoglossal LMN lesion weakens the left genioglossus.
Left CN XII LMN lesion → tongue points LEFT, toward the lesion.
UMN logic
A left corticobulbar lesion produces predominantly right-sided tongue weakness. On protrusion, the tongue therefore deviates to the right—away from the cerebral lesion.
LMN XII: toward lesion. UMN XII: away from cerebral lesion.
Interactive localisation
Which way will it deviate?
Select a structure, lesion level and side. The tool separates the weak side from the observed deviation.
Localisation result
Very high-yield mnemonic
VIILMN face weak SAME side
V / TMJJaw TOWARD lesion
XUvula AWAY
XIITongue TOWARD LMN lesion
Exam traps
Five ways this topic is commonly misread.
Most errors happen because the candidate remembers only a direction without asking whether the question is describing weakness, deviation, or lesion level.
Trap 01
“Vagus lesion is contralateral.”
No. CN X LMN weakness is ipsilateral. The uvula deviates contralaterally because the normal palate pulls it away.
Trap 02
“Facial palsy always means whole-face weakness.”
Whole ipsilateral facial weakness suggests LMN. An UMN lesion classically affects the contralateral lower face with forehead sparing.
Trap 03
“Tongue always points toward the lesion.”
That is the LMN rule. With an UMN corticobulbar lesion, the tongue points away from the cerebral lesion.
Trap 04
“Jaw deviation means V3 lesion.”
Not necessarily. Ipsilateral TMJ restriction produces the same direction of jaw deviation on opening.
Trap 05
“Deviation = weak side for every structure.”
Not true. Uvula deviation is the key exception: weakness is on one side, but deviation is away from it.
JawTOWARD lesion
Tongue LMNTOWARD lesion
UvulaAWAY from lesion
Face UMNOPPOSITE lower face
Retrieval practice
Eight quick localisation questions.
The cases mix weakness and deviation so you have to localise rather than recite a mnemonic.
Question 1 of 8
Exam-safe summary
The whole topic in four lines.
Use these only after you have identified whether the question is asking about an UMN or LMN lesion.
Facial nerve: LMN → entire ipsilateral face weak; UMN → contralateral lower face weak, forehead relatively spared.
Jaw: V3 motor LMN lesion and TMJ pathology → jaw deviates toward affected side on opening.
Vagus: ipsilateral palate weak; uvula deviates away from lesion.
Hypoglossal: LMN → tongue toward lesion; UMN → tongue away from cerebral lesion.
Final mnemonic: tongue points to the lesion; jaw points to the lesion; uvula points away.