Lecture 17 - Extra-ocular muscles Flashcards
Extrinsic muscles
Muscles of the eyelid
- Obicularis oculi (CN VII)
- Levator palpebrae superioris (CN III)
- Superior tarsal muscles - sympathetics
Extra-ocular muscles:
- Superior rectalis
- Inferior rectalis
- Lateral rectalis - CN VI
- Medial rectalis
- Superior oblique - CN IV
- Inferior oblique
Intrinsic muscles
Muscles of the iris:
- Sphincter pupillae - parasympathetics
- Dilator pupillae - sympathetics
Ciliary muscles - controls thickness of lens
Where do the rectalis muscles originate from
Tendinous ring
Where do the oblique muscles originate from
Bony wall of orbit
Innervation of extra-ocular muscles
CN III:
- Superior rectalis
- Inferior rectalis
- Medial rectalis
- Inferior oblique
- Superior oblique - CN IV
- Lateral rectalis - CN VI
Primary resting gaze
- Equal and opposite pull of all the extra-ocular muscles
- Aligned visual axis - no diplopia
- Images reaaching the cortex is fused and seen as one therefore binocular vision - 3D depth perception
How to change position of gaze
Greater pull is exerted by specific extra-ocular muscles with simultaneous relaxation of antagonist
Requires aligned visual axes via conjugate gaze to prevent diplopia
Medial rectus
Origin: tendinous ring in apex of orbit
Insertion: Medial eyeball
Action: Adduction
Innervation: CN III
Lateral rectus
Origin: tendinous ring in apex of orbit
Insertion: Lateral eyeball
Action: abduction
Innervation: Abducens nerve
Superior rectus
Origin: tendinous ring in apex of orbit
Insertion - superior anterolateral surface of eyeball obliquely
Action: Elevate
Slight adduction
Slight internally rotates
Innervation: CN III
Inferior rectus
Origin: tendinous ring in apex of orbit
Insertion: Anteroinferior surface of eyeball obliquely
Action: Depress
Slightly adduct
Slight external rotation
Superior oblique
Origin: Lateral walls of orbit in apex of orbit through trochlear
Insertion: Superoposterior aspect of globe
Action:
Internally rotate
Depress
Slightly abduct
Innervation: CN IV
Inferior oblique
Origin: anteromedial surface of floor of orbit
Insertion: inferoposterior surface of globe
Action:
Elevate
Externally rotate
Slightly abduct
Innervation: CN III
Strabismus
Squint - resting position of eyeball deviates due to lack of antagonising
CN VI lesion
Eye is adducted in resting gaze
Lateral rectus weakness: can’t abduct
CN IV lesion
Eye is held in up and in position at rest:
Elevated
Externally rotated
Adducted
Superior oblique affected therefore cannot:
Depress
Internally rotate
Abduct
Compensate for slight extortion by tilting head
Worsening diplopia when looking down and in - walking downstairs and reading
Clinical examination of eye movements
Isolate action of each muscle to test therefore ‘H’ with eyes
Test CN III, IV and VI
Midline elevation
IO: elevates/externally rotates/abducts
SR: Elevates/ internally rotates adducts
Midline depression
SO: Depress/ internally rotate/ abduct
IR: Depress/ externally rotate/adduct
Lateral stating position
Elevation by superior rectus
Depression by inferior rectus rectus
Medial starting position
Elevation by inferior oblique
Depression by superior oblique
CN III lesion
Complete ptosis - LPS weakness
Down and out position due to weakness of:
- Superior rectus
- Inferior rectus
- Medial rectus - unopposed lateral rectus
- Inferior oblique - Unopposed superior oblique
Pupil dilated - sphincter pupillae weakness (in vascular lesion spared)