ORBIT & OCULAR ADNEXAE - Extraocular muscles Flashcards
What are the three intraocular muscles?
- ciliary muscle
- sphincter pupillae
- dilator pupillae
What are the three spaces in the orbit?
- extraconal space
- intraoconal space
- subperiosteal space
Where is the origin of the recti muscles?
Annulus of Zinn which encircles the optic foramen/canal and superior orbital fissure.
Which recti have adhesions with the dural sheath of the optic nerve?
What is the clinical importance of this?
superior rectus and medial rectus –> PAINFUL EYE MOVEMENTS IN OPTIC NEURITIS in upgaze and adduction
What is the tenons fascia? What does it fuse with anteriorly and posteriorly?
Thin membrane which envelops the eyeball from optic nerve to corneal limbus
posteriorly - fused with optic nerve sheath
anteriorly - fuses with intermuscular septa 3mm posterior to limbus
What is the structure of the muscle pulleys around the extraocular muscles?
sleeves of connective tissue around rectus muscles as they penetrate tenon capsule - contain collagen, elastin and smooth muscle.
What is the function of the muscle pulleys around the extraocular muscles?
- Suspends muscle from adjacent orbital wall
- redirects muscle - acts as functional origin of muscle - only muscle fibres in front of muscle pulleys get displaced during movement.
- prevents displacement of muscle during movement
What is the angle between the orbital and optic axis?
22.5 degrees
How far apart are the medial walls from each other?
2.5cm
What is the angle betwen the two lateral walls?
What is the angle between the ipsilateral medial and lateral walls?
- 90 degrees
- 45 degrees
What is the angle of the superior and inferior recti muscles compared to the visual axis?
22.5 degrees (they follow the orbital axis)
What is the angle of the medial rectus compared to visual axis?
What is the angle of the lateral rectus compared to the visual axis?
- 0 degrees / parallel
- 45 degrees
What are the distance of insertion of recti from the limbus? What is this phenomenon called?
Spiral of Tillaux
SLIM RULE.
Superior rectus - 7.7mm
Lateral rectus - 6.9mm
Inferior rectus - 6.5mm
Medial rectus - 5.5mm
Where is the sclera the thinnest? How thin?
at insertion of the rectus (0.3mm) –> commonest site for perforation during severe blunt trauma to the globe
Which EOM has the shortest tendon length?
Inferior oblique.
Label the diagram.
What is the relation of LPS and SO compared to the SR?
Above SR - LPS
Below SR (insertion point only) - SO
Which muscles are supplied by the superior division of CNIII?
LPS and SR
Which muscles are supplied by the inferior division of CNIII?
MR, IR and IO.
What is the blood supply to the EOMs?
Anterior ciliary arteries from the lateral and medial muscular arteries of ophthalmic artery
- 2 for each rectus except LR (1 ACA).
What muscles does the medial muscular artery supply?
What muscles does the lateral muscular artery supply?
- medial: MR, IR, IO
- lateral: LR, SR, SO, LPS
Where is the origin of the superior oblique?
Periosteum of lesser wing of sphenoid bone (medial and superior to optic foramen)
Which EOM is the longest?
Which EOM is the thinest?
Superior oblique.
What is the course of the superior oblique?
Starts from sphenoid bone and travels in parallel to medial rectus and engages with the trochlea and travels posteriorly and laterally and then inserts into the posterior aspect of the eye, below the SR muscle behind the equator.
When does the SO muscle become tendinous in relation to the trochlea?
10mm before the trochlea
What is the angle of the trochlea / SO ie how much angle the direction of muscle changes at the trochlea ?
54 degrees
What is the functional origin of the SO?
The trochlea
What is the angle between the insertion of SO and visual axis?
51 degrees.
What movement are the anterior fibres of the insertion fibres of SO associated with?
Intorsion
What movement are the posterior fibres of the insertion fibres of SO associated with?
Depression
How far are the anterior and posterior end of the SO fibres from the limbus?
How far are the anterior and posterior end of the SO fibres from the SR?
LIMBUS
anterior end: 12-14mm
posterior end: 17-19mm
SR
anterior end: 3mm
posterior end: 13mm
What is the primary action of the SO?
What are the secondary actions of SO?
- INTORSION
- DEPRESSION AND ABDUCTION
SO –> IAD
What common action do Obliques cause? medial or lateral movement?
What common action do Recti cause? medial or lateral movement?
obliques act LATERALLY - abduction
rectus act MEDIALLY - adduction
What common action do superior muscles cause?
What common action do inferior muscles cause?
superior: intorsion
inferior: extorsion
What is the longest EOM?
What is the shortest EOM?
longest - SO, SR (recti muscles)
shortest - IO, IR (recti muscles)
Where is the origin of the inferior oblique muscle?
From anterior orbit (only muscle) at orbital surface/orbital floor of maxilla lateral to lacrimal groove.
What is the course of the inferior oblique? Where does it insert?
Travels upwards posteriorly and laterally and then passes inferiorly to the inferior rectus and inserts behind the lateral rectus muscle (10mm) behind posterior to the equator.
Which muscles act in front of the equator? Which muscles act behind the equator?
behind - obliques - pulls posterior part of the eye to its direction so anterior part of eye go opposite direction.
in front - rectii.
What are the actions of the inferior oblique muscle?
- extortion
- elevation
- abduction
IO - EAE
What is the main differences between EOM and other skeletal muscles?
(5)
- higher ratio of nerve axons to muscle fibres (1:3 vs 1:50) which facilitates precise control.
2.Thinner epimysium (muscle sheath)
- Thicker/larger perimysium (connective tissue)
- Highly vascularised and poorly fasciculated
- Round oval fibres which are larger centrally and smaller in periphery
What features are normally associated with myopathy but are normal in EOMs? (5)
- Mononuclear cell infiltrate
- Central nuclei
- Disorganised sarcolemma
- Disorganised/disrupted z-lines
- Mitochondrial clumping
What are the two types of single motor endplates in EOMs?
- Fibrillinstruktur - Type A - thick fast for saccadic movements
- Felderstruktur - Type B - slow/tonic eye movements
Which rectii muscle has a second head and where does it arise from?
Lateral rectus - greater wing of sphenoid lateral to annulus of zinn.
Which EOM is the only one to have a fusiform (spindle shape)
superior oblique
Which EOM has no tendon?
inferior oblique
Which nerves pass near the lateral rectus? (2)
- abducent nerve (CN6) - passes medial aspect of LR
- Lacrimal nerve - passes above LR
Where does the ophthalmic artery travel in relation to the medial rectus?
Where does nasociliary nerve travel in relation to medial rectus
Both Travel above the MR.
Does the inferior division of CN3 travel above or below the inferior rectus?
above the IR.
Which muscle is innvervated bilaterally by a single, unpaired nucleus?
levator palebrae superioris
Where does the LPS originate from?
Lesser wing of the sphenoid, above and anterior to optic canal (blends with origin of superior rectus)
What are the sequelae following development of a squint?
- Secondary overaction of yoke muscle (contralateral synergist) by Hering’s law
- Secondary overaction of ipsilateral unopposed antagonist
- Secondary underaction of contralateral antagonist
Synergist and antagonist table of extraocular muscles.
See table.
At what direction and angle does the inferior rectus cause maximal depression?
At what direction and angle does superior oblique cause maximal depression.
- Inferior rectus : 24 degrees of ABDUCTION
- Inferior oblique: 54 degrees of ADDUCTION
What are the eye movement defects caused by an MLF lesion?
- IPSILATERAL ADDuction deficit
- CONTRAlateral ABDuction nystagumus
Which recti muscle is largest?
Medial Rectus
Which muscle receives contralateral oculomotor nucleus innervation?
Superior rectus
What are the actions of the superior rectus? (3)
Primary: Elevation
Secondary: Intorsion
Tertiaty: Adduction.
What is the size of a fast twitch extraocular muscle fibre?
What is the size of a slow twitch extraocular muscle fibre?
Fast: 10nm
Slow: 15nm
What is the auxillary blood supply of the EOMs? (2)
Lateral rectus: Lacrimal Artery
Inferior oblique/inferior rectus: Infraorbital artery (from ICA)
Type A Fibres are…
Type B Fibres are…
Type C Fibres are….
fast twitch/slow twitch / type of conjugate movement.
Type A Fibres - fast twich - saccades
Type B Fibres - slow twitch - smooth pursuit
Type C Fibres - tonic contractions
