Neuro Lecture 8: CN III, IV, VI

What is "OS"?

Ocular sinister (left eye)

What is "OD"?

Ocular dexterous (right eye)

What does visual acuity refer to?

- Sharpness of vision

- Normal is 20/20

When do your eyes converge?

Seeing close objects

When do your eyes diverge?

Seeing distant objects

What does accommodation refer to?

Lens adjustment for refraction

What is conjugate gaze?

All 6 muscles in both eyes act together to allow for smooth movement

What is a dysconjugate gaze?

Eyes do not look at same place at same time when a muscle is affected

What does depth perception refer to?

Binocular vision

What does dissociation refer to?

Eye movements with the head fixed

What does strabismus refer to?

- Eyes not aligned

- Lacking a coordinated gaze

- Constant

- Tropia

What does tropia refer to?

Constant or intermittent visible misalignment when both eyes are focusing on an object

What are the types of tropia? (4)

- Esotropia

- Exotropia

- Hypertropia

- Hypotropia

Is esotropia inward or outward?

Inward

Is exotropia inward or outward?

Outward

Is hypertropia upward or downward?

Upward

Is hyportropia upward or downward?

Downward

What does phoria refer to?

- One eye goes out of alignment when tired

- Not constant

- Daydreaming

When does phoria become visible?

- When binocular vision is disrupted (one eye covered)

- Brain compensates for misalignment

- Cover/cover test

How much of the visual field is covered horizontally?

180-200º

How much of the visual field is covered vertically?

120-135º

What occurs during binocular vision?

- Image falls on same point and same time on bilateral retina

- Brain sees 1 image

How much overlap occurs with binocular vision?

120º

What is the function of binocular vision?

- Depth perception

- 3D vision

What does peripheral vision allow for?

Detection of movement and objects without focus on them

What is higher with central vision?

Visual acuity

What is CN III responsible for generally?

Oculomotor

What is the functional category of CN III?

- Somatic motor

- Parasympathetic

What muscles is the somatic motor aspect of CN III responsible for?

- Levator palpebrae superior

- Superior/inferior/medial rectus

- Inferior oblique

- (Rest of oculomotor muscles except those under CN IV and VI)

What muscles is the parasympathetic aspect of CN III responsible for?

- Parasympathetics to pupil constrictor and ciliary muscles

- Near vision

What is CN IV responsible for generally?

Trochlear

What is the functional category of CN IV?

Somatic motor

What muscles is CN IV responsible for?

- Superior oblique muscle

- Depression (motor) of eye

- Intorsion

What is CN VI responsible for generally?

Abducens

What is the functional category of CN VI?

Somatic motor

What muscles is CN VI responsible for?

- Lateral rectus muscle

- Abduction of eye

What mnemonic can help you remember the muscles of the eye and their corresponding nerve?

LN6SO4R3

What can a CN III injury result in?

- Third nerve (oculomotor) palsy

- Diplopia

- Ptosis

- Blown pupil

What does third nerve (oculomotor) palsy lead to?

- Lateral deviation of eye

- Medial rectus impaired

What does diplopia lead to?

- Double vision

- Dysconjugate gaze

What does ptosis lead to?

- Drooping of upper eyelid

- Levator palpebrae is impaired

What does blown pupil lead to?

- Pupil dilated

- Pupillary constrictors are impaired

What is the function of pupillary light reflex?

- Controls the light reaching the retina and photoreceptors (protective)

- Direct response

- Consensual response

What is a direct response (pupillary light reflex)?

Constriction of pupil that you can shine light onto

What is a consensual response (pupillary light reflex)?

- Constriction of contralateral pupil

- Due to crossing at posterior commissure

What pathway is CN II part of?

Extra geniculate

What is step 1 of the pathway of the pupillary light reflex (CN II and CN III)?

CN II detects stimulus

What is step 2 of the pathway of the pupillary light reflex (CN II and CN III)?

Message sent to bilateral pretectal nucleus

What is step 3 of the pathway of the pupillary light reflex (CN II and CN III)?

- Pretectal nucleus sends bilateral signals to Edinger Westphal nucleus

- Crossing via posterior commissure

What is step 4 of the pathway of the pupillary light reflex (CN II and CN III)?

Edinger Westphal nucleus sends bilateral signals through CN III

What is step 5 of the pathway of the pupillary light reflex (CN II and CN III)?

Pupillary sphincters in iris and ciliary body/muscles receive signals

What happens if light is shown in left eye and left CN II is impaired?

- Pupillary light reflex dysfunction

- No detection of visual stimulation

What happens if light is shown in left eye and left CN III is impaired?

- Pupillary light reflex dysfunction

- No reflex in left eye

- Reflex in right eye

What happens if light is shown in left eye and right CN III is impaired?

- Pupillary light reflex dysfunction

- Reflex in left eye

- No reflex in right eye

What is the function of the accommodation reflex?

Aids in focusing on objects at varying distances

What structures are involved with the accommodation reflex?

- Lens

- Pupillary sphincter apparatus

- CN II (afferent)

- CN III (efferent)

What 3 underlying mechanisms occur with the accommodation reflex?

1. Convergence of both eyes

2. Miosis (pupillary constriction)

3. Accommodation

What muscle is related to the underlying mechanism of convergence?

Medial rectus muscle

What muscles are related to the underlying mechanism of miosis?

- Pupillary muscles

- CN III (parasympathetic)

What occurs with the underlying mechanism of accommodation?

1. Ciliar muscles contract

2. Zonular fibers relax

3. Shape of lens changes to more convex

4. Refractive power of lens increased

What test is done for accommodation?

Test for convergence

What is step 1 of CN IV pathway?

Originates posteriorly (dorsal) in midbrain

What is step 2 of CN IV pathway?

Wraps around cerebral peduncle

What is step 3 of CN IV pathway?

- Into cavernous sinus

- Then into superior orbital fissure

What results from CN IV injury?

- CN IV (trochlear) palsy

- Hypertropia (strabismus)

- Extorsion

- Vertical diplopia

What results from injury to CN VI?

- CN VI palsy

- Horizontal diplopia

- Strabismus (medial)

What occurs with CN VI palsy?

- Eye remains adducted (rests in direction of nose)

- Difficulty abducting eye

What occurs with horizontal diplopia with CN VI injury?

Worse when patient tries to gaze toward affected eye (abduction)

What does vergence refer to?

Fixation of a single point by both eyes as target moves toward/away from viewer

What would you test for vergence?

Test for accommodation (convergence)

What does saccades refer to?

Rapid eye movements to bring the fovea to view target into field of view

What is the purpose of testing for smooth pursuits?

- Tracking a moving object

- Stable viewing of moving target keeping it in sharp focus

What is the visual field?

Area the eye can see when you focus on a single point

What would you test for visual field?

- Confrontation visual field test

- Static perimetry test

- Test each quadrant of eye

What test is done for contrast sensitivity?

Red dot test

What is the function of the tectospinal reflex?

Orienting reflex

What reflexes relate to blinking?

- Blink reflex

- Corneal reflex

- Glabellar reflex

What is the function of the blink reflex?

Protective reflex mediated by brainstem

What nerves mediate the corneal reflex?

- CN V

- CN VII

- Fewer connection

What is the glabellar reflex seen with?

- Habituation with successive tapping

- Dementia and Parkinson's

What is the function of the vestibular ocular reflex?

- Gaze stability

- Head moves in one direction and eyes in another

What is the vestibular ocular reflex mediated by?

Vestibular nuclei in brain stem and cerebellum

How is strabismus seem with children and infants?

- May not experience diplopia as they learn to suppress image from deviating eye

- Not good for development (first 3 months of life)

What conditions affect the visual field? (8)

- Glaucoma

- Macular degeneration

- MS

- Thyroid eye disease

- Pituitary gland disorders

- CNS problems

- Stroke

- Long-term use of certain medications