Eoms & ocular motility

Extraocular muscles: anatomy, physiology and ocular motility

1. Anatomy of EOMs & Ocular Motility 1 By: Dr Henok Samuel (R1) Moderator: Prof Abebe Bejiga (professor of Ophthalmology, pediatrics and strabismus surgeon)

2. Contents Introduction Recap on Embryology & microscopic anatomy Gross Anatomy of EOM Origin, Nerve and blood supply Action of EOM Basic Kinematics, Mechanics Ocular Movements Fundamental Laws governing ocular motility 2

3. Why do we move our eyes? To acquire objects for central viewing Saccadic eye movements B. To maintain objects in foveal view Pursuit eye movements C. To stabilize the world on the retina 3

4. Extraocular Muscles 4 EOMS are: Striated voluntary muscles Among muscles with fastest but also most sustained contraction Get high blood flow exceeded only by myocardium Have got high innervation ratio

5. Extraocular Muscles 5 Play a vital role in Stereopsis Conjugate eye movements Maintenance of primary gaze position Motor fusion – maintaining corresponding visual elements within the binocular field on corresponding retinal loci. Following of moving objects (smooth pursuit) Accomplish rapid changes in fixation (saccades).

6. Embryology The EOMs are derived from three primordial condensations - paraxial and prechordal mesoderm. A pair of premandibular mesodermal condensation – 26 th day Those muscles innervated by CNIII MR, SR, IR & IO . Two maxilomandibular condensations – 27th Superior oblique . Lateral rectus . Associated periorbital CT and smooth muscle are derived from the neural crest. 6

7. Embryology Their development begins at 3–4 weeks gestation. All of the extraocular muscles and their surrounding tissues are present and in their final anatomic positions by 6 months gestation. The tendon insertions initially extend from the limbus to the equator. Continue to change until 18 months to 2 years after birth - 2–3.0 mm narrower in infants At about 1 month, the nerves to the extraocular muscles reach their respective destinations in the sequence oculomotor, abducens, and trochlear. 7

8. Microscopic Anatomy of EOMs Different from other skeletal muscles by their Diameter of its fibers is small Contain both slow and fast fibers denser connective tissue & blood supply Contain an enormous amount of fibroelastic tissue connective tissue sheaths are more delicate & richer in elastic fibers Have a large nerve to muscle fiber ratio; about 1:5 to 1:10 compared to 1:100 or more in other skeletal muscles 8

9. Microscopic Anatomy of EOMs Components of a muscle fiber Sarcolemma : the plasma cell membrane surrounding each muscle fiber. Transverse tubules (T tubules): a series of invaginations of the sarcolemma into the cell. Sarcoplasm : is the cell cytoplasm & contains normal cellular structures and special muscle fibers, the myofibrils . 9

10. Microscopic Anatomy of EOMs Components of a myofibril; two types Thick myofilaments Thin myofilaments 10

11. Microscopic Anatomy of EOMs Twitch single innervated fibers ( thick ):- contain little mitochondria ( i.e. anaerobic metabolism ) → rapid response to the stimulus with rapid contraction of high amplitude & short duration. responsible for saccadic eye movement & help fixation & pursuit movement. Tonic multiple innervated fibers ( thin ):- contain numerous mitochondria ( i.e. aerobic metabolism ) → slow response to the stimulus with slow contraction of low amplitude & long duration. responsible for gaze in all positions including 1ry position. 11

12. Bony orbit 12

13. Extraocular Muscles (EOMs) Vertical rectus - Superior rectus - Inferior rectus Horizontal rectus - Medial rectus - Lateral rectus Obliques - Superior oblique - Inferior oblique 13

14. Origin of EOMs The four recti muscles have their origin on the common tendinous ring (annulus of Zinn). The area enclosed by the tendinous ring is called the oculomotor foramen Superior Oblique originates from the lesser wing of sphenoid I nferior Oblique originates from the maxillary bone 14

15. Several blood vessels & nerves pass through oculomotor foramen The optic nerve Ophthalmic artery Abducens nerve Oculomotor nerve Upper and lower divisions Nasociliary branch of the ophthalmic nerve 15

16. Insertion Scleral insertions are by tendons whose fibres are parallel to the long axis of the muscle. The tendon fibres enter the superficial sclera and quickly merge into it. The EOMs penetrate it ≈10 mm posterior to their insertions 16 MUSCLE LENGTH OF TENDON IN mm MEDIAL RECTUS 3.7 LATERAL RECTUS 8.8 SUPERIOR RECTUS 5.8 INFERIOR RECTUS 5.5 SUPERIOR OBLIQUE 25 INFERIOR OBLIQUE 1

17. Arterial supply Three main sources Muscular branches of ophthalmic artery Upper (lateral) branch Lower (medial) branch Infraorbital artery from ECA Lacrimal artery 17

18. Arterial supply The muscular branches give rise to the anterior ciliary arteries accompanying the rectus muscles; each rectus muscle has 1–4 anterior ciliary arteries. Anterior segment circulation is most dependent on arteries from vertical rectus muscles and least dependent on arteries from the lateral rectus muscle Surgical manipulation of the rectus muscles permanently disrupts the anterior ciliary arteries. 18

19. Venous drainage 19 Follow the arterial path EOMs drain into two veins: Superior ophthalmic vein and Inferior ophthalmic vein

20. Nerve 20 Without neural activity, the visual axes are usually mildly to moderately divergent. The major tonic input to ocular motility is supplied CNs III, IV, and VI

21. Superior rectus Origin Insertion Nerve supply Blood supply 21

22. Inferior rectus Origin Insertion Nerve supply Blood supply 22

23. Medial rectus Origin Insertion Nerve supply Blood supply 23

24. Lateral rectus Origin Insertion Nerve supply Blood supply 24

25. Spiral of tillaux 25 Imaginary line joining the insertions of the 4 recti and is an important anatomical landmark when performing surgery.

26. Superior Oblique Origin Insertion Nerve supply Blood supply 26

27. Inferior Oblique Origin Insertion Nerve supply Blood supply 27

28. Introduction Obliques are inserted behind equator & form an angle of 51° with visual axis. 28

29. Orbital Connective Tissue Tenon's capsule (fascia bulbi ) a layer of delicate connective tissue that completely envelops the globe - EOMs penetrate it ≈10 mm posterior to their insertions The fascial sheath of the SR muscle closely adheres to the sheath of LPS of upper lid in front of the equator which accounts for the cooperation in elevation of the eye. The sheath of the IO muscle fuses with the sheath of the IR and forms the suspensory ligament of lockwood . 29

30. Pulleys of the EOMs Are about 2mm in length & located near the equator Function as a mechanical origins of the EOMs Reduce sideslip of the extraocular muscles during globe rotation and help to determine the effective direction of pull 30

31. Pulleys of the EOMs They are Composed of Elastic fibers Collagenous The smooth muscles The most prominent smooth muscle is the inframedial orbital muscle extending between the MR and IR pulleys. Anteriorly, these sleeves thin to form slings known as the intermuscular septum . 31

32. Muscle Capsule Each rectus muscle has a surrounding fascial capsule Extends with the muscle from its origin to its insertion Its smooth avascular surface allows the muscles to slide smoothly over the globe. 32

33. Action of extraocular muscles Types of Eye Movements Ductions refer to monocular movements of each eye. 2. Versions refer to binocular conjugate movements of both eyes. 3. Vergences refer to binocular disjunctive movements. 33

34. Uniocular movements Ductions – only one eye is open, the other covered/closed tested by asking the pt. to follow a target in each direction of gaze. Types of ductions:- 1. Adduction 2. Abduction 3. Supraduction 4. Infraduction 5. Incycloduction 6. excycloduction 34

35. Three Axes of Eye Rotations Fick’s Axis Listing Plane 35 Uniocular movements

36. Binocular movements 36 Versions :- both eyes open, attempting to fixate a target & moving in same direction. Binocular, simultaneous, conjugate movements in same direction. Abduction of one eye accompanied by adduction of other eye is called conjugate movements.

37. Introduction Types of versions:- Dextroversion & laevo version Supraversion & Infraversion Dextro elevation & dextro depression Laevo elevation & laevo depression 37

38. Introduction Torsional movements/righting reflexes:- When you tilt head to maintain upright image. Vergences:- binocular,simultaneous,disjugate /disjunctive movements ( opp direction) Convergence– simultaneous adduction Divergence– outward movement from convergent position 38

39. Actions of EOMs The anterior pole of the globe is the reference point used in the description of any eye movement Eye movements are described based on the movement of the muscle insertion towards its origin . The primary action of a muscle is its major effect when the eye is in the primary position. Subsidiary/secondary actions are the additional effects & depend on the position of the eye The point at which the center of the muscle or its tendon first touches the globe is called the tangential point and indicates the direction of the pull. Muscles exert force in proportion to their cross-sectional area and length. For normal amplitude of rotation (45-50 degree ) 10mm change in muscle length is required in each direction 39

40. Positions of Gaze Primary position : defined as position of the eye with Both head & body erect B oth eyes are looking straight ahead Object of regard is at infinity T he eye located at the intersection of the sagittal plane of the head and the horizontal plane passing through the centers of rotation of bot eyes 40

41. Positions of Gaze Secondary position are rotations around either the vertical axis or the horizontal axis. Adduction Abduction Elevation depression 41

42. Positions of Gaze Teritiary position : are rotations around both the vertical & horizontal axis Dextroelevation Dextrodepression Levoelevation Levodepression 42

43. Positions of Gaze 43

44. Medial Rectus Lies parallel to the sagittal axis & perpendicular to the vertical axis As a result has only one action, which is rotation around the vertical axis medially => ADDUCTION 44

45. Lateral Rectus Lies parallel to the sagittal axis & perpendicular to the vertical axis Contraction causes rotation in a temporal direction abduction 45

46. Superior Rectus Run in line with the orbital axis and are inserted in front of the equator. Forms an angle of 23° with the visual axis. With the insertion above the origin and on the anterior globe, movement around the horizontal axis causes elevation – primary action. 46

47. The muscle insertion is lateral to the origin , so movement around the vertical axis causes adduction. Its oblique insertion with the nasal side closer to the limbus than the temporal side on the superior surface of the globe causes intorsion on contraction. 47 Superior Rectus

48. Inferior Rectus Its insertion is below its origin and on the anterior globe, so movement around the horizontal axis causes depression 48

49. Inferior Rectus The muscle insertion is lateral to the origin, so movement around the vertical axis causes adduction. Its oblique insertion on the inferior surface of the globe causes extorsion on contraction. 49

50. Superior Oblique The oblique insertion on the posterosuperior lateral aspect of the globe causes rotation of the eye around the sagittal axis causing intorsion. 50

51. Superior Oblique The insertion is posterior and inferior to the physiologic origin; contraction of the muscle pulls the back of the eye up, and the anterior pole moves down- depression. 51

52. Superior Oblique Because the insertion is lateral to the trochlea, contraction pulls the back of the globe medially, thus moving the anterior pole laterally – abduction . 52

53. Inferior Oblique Because the muscle wraps around the lower portion of the globe and the insertion is superior and lateral to the origin contraction causes extorsion . 53

54. Inferior Oblique Because the insertion is on the posterior eye and above the origin, contraction pulls the back of the eye down, elevating the front – elevation. 54

55. Inferior Oblique because the insertion on the back of the eye is pulled toward the medial side; thus the anterior pole is moved laterally in abduction 55

56. Actions of the EOMs from secondary position Horizontal rectus muscles: lr & MR If the eye is elevated, contraction of the LR & MR no longer causes strictly adduction or abduction but also causes a slight elevation if the eye is depressed, contraction causes further depression 56

57. vertical rectus muscles : SR & IR 57 When the globe is abducted 23°, the visual and orbital axes coincide. In this position they have no secondary actions The SR can act only as an elevator & IR only as a depressor

58. vertical rectus muscles : SR & IR When the eye is adducted 67 the plane of the vertical rectus muscles is at a right angle to the sagittal axis and thus parallel to the horizontal axis. The superior rectus could only act as an intortor & the inferior rectus could act only as an extortor . 58

59. The oblique muscles : SO & Io The obliques form an angle of 51° with the visual axis So if the eye is adducted 51 , the plane of the oblique muscles becomes parallel to the sagittal axis and perpendicular to the horizontal axis. Thus contraction of the SO will cause only depression, and the IO will cause only elevation 59

60. The oblique muscles : SO & Io When the eye is abducted 39 , the plane of the oblique muscles makes a right angle with the sagittal axis & parallels the horizontal axis, The superior oblique can cause only intorsion & the IO extorsion 60

61. Actions of EOMs 61

62. Fundamental Laws governing Ocular Motility Synergists :- refers to muscles having same primary action in same eye. Ex:- sup.rectus & inf.oblique ----elevators inf.rectus & sup.oblique -----depressors Antagonists :- muscles having opposite action in same eye Ex:- sup. & inf. Recti sup. & inf.oblique 62

63. Fundamental Laws governing Ocular Motility Yoke muscle (contralateral synergists):- Ref. to pair of muscles (one from each eye) which contract simultaneously during version movements. Ex :- In dextroversion RLR &LMR Contralateral antagonist:- pair of muscle (one from each eye) having an opposite action. Ex:-In dextroversion RLR & LLR 63

64. Fundamental Laws governing Ocular Motility 64 DONDER’S LAW To each positon of line of sight belongs a definite orientation of the vertical and horizontal retinal meridians relative to the coordinates of space. The orientation of the retinal meridians pertaining to a particular position of globe is achieved irrespective of the path the eye has taken to reach that position. In short, it implies that there is one and only one orientation of the retinal meridians with each position of the eyes.

65. Fundamental Laws governing Ocular Motility Hering’s Law of Equal Innervation Also known as Hering’s law of motor correspondence States ‘ equal and simultanous innervation flows from the brain to a pair of muscles of both eyes (yoke muscles) which contract simultaneously in different binocular movements.’ Eg : Right Lateral R and Left Medial rectus: Dextroversion Both Medial Rectus : Convergence Right IR and Left Superior Oblique: Dextrodepression 65

66. Fundamental Laws governing Ocular Motility Sherrington’s Law of Reciprocal Innervation ‘During ocular motility, an increased flow of innervation to the contracting agonist muscle is accompanied by a decreased flow of innervation to the relaxing antagonist muscle’. Eg. During Dextroversion , Increased innervation – Right LR and Left MR Decreased innervation – Left LR and Right MR 66

67. References Wolf anatomy of the eye and orbit Clinical anatomy and physiology of the visual system Duane’s Ophthalmology 2007 edition parts 1 and 2 Kanski clinical ophthalmology 7 th edition chapter18 BCSC 2019, section 2 and 7 67

68. 68 Thank You!