DIFFERENCES BETWEEN ANATOMICAL DESCRIPTIONS OF ACTIONS OF EXTRAOCULAR

DIFFERENCES BETWEEN ANATOMICAL DESCRIPTIONS OF ACTIONS OF EXTRAOCULAR MUSCLES AND CLINICAL TESTS OF THEIR FUNCTION SUMMARY: The anatomical description of the actions of eye muscles delineates the movements each muscle produces. The clinical exam of eye movements shows directions the patient is asked to move the eye to test for deficits in muscles and nerves. The clinical tests are empirically derived. Differences in the descriptions reflect the fact that movements of elevation and depression are produced by combinations of muscles.

DIFFERENCES BETWEEN ANATOMICAL DESCRIPTIONS OF ACTIONS OF EXTRAOCULAR MUSCLES AND CLINICAL TESTS OF THEIR FUNCTION 1 - In a person, movements of the eye in elevation and depression (raising and lowering the eye) result from the concerted actions of multiple eye muscles. 2 - ANATOMICAL: The anatomical descriptions of eye movements is based upon the actions of individual muscles. 3 - CLINICAL: Clinical tests of function of the muscles are based upon evaluations of patients' abilities after nerve or muscle lesions.

ANATOMICAL DIAGRAM OF EYE MOVEMENTS References: This diagram is also found in Gray’s Anatomy, any British edition after 1974.

DIFFERENCES BETWEEN ANATOMICAL DESCRIPTIONS OF ACTIONS OF EXTRAOCULAR MUSCLES AND CLINICAL TESTS OF THEIR FUNCTION In clinical tests, two effects apparently predominate in movements of elevation/depression (raise/lower): 1 - Muscles work best when stretched 2 - When looking straight ahead, the SR, IR, SO and IO muscles all insert at an angle. When looking to the side muscle orientations change so that some pull more directly relative to line of sight.

OUTLINE I. EYE LOOKS LATERALLY II. EYE LOOKS MEDIALLLY III. SUMMARY IV. OTHER CLINICAL DESCRIPTIONS: TROCHLEAR NERVE DAMAGE AND TEST

EXPLANATION FOR DIFFERENCE BETWEEN EYE MOVEMENTS CHART AND CARDINAL POINTS OF GAZE: AN EXAMPLE: Consider patient's right eye SR IO Both SR and IO act to raise eye when eyes are looking straight ahead RIGHT EYE NOSE ANATOMICAL ACTION

EXPLANATION FOR DIFFERENCE BETWEEN EYE MOVEMENTS CHART AND CARDINAL POINTS OF GAZE: EYE LOOKS LATERALLY Muscles work best when somewhat stretched, poorly when short IO SR eye looks laterally NOSE ANATOMICAL ACTION 1) both SR and IO act to raise eye 2) if have patient look laterally IO becomes short 3) if then have patient raise eye (look up); IO is too short but SR is long 4) eye is then raised by SR Note: orientation of SR contributes to this effect: muscle pull is most direct when eye is abducted

EXPLANATION FOR DIFFERENCE BETWEEN EYE MOVEMENTS CHART AND CARDINAL POINTS OF GAZE: EFFECT OF SR DAMAGE Muscles work best when somewhat stretched, poorly when short IO SR If SR is damaged, patient cannot raise eye when looking laterally eye looks laterally NOSE ANATOMICAL ACTION

CONSIDER MOVEMENT OF PATIENT'S LEFT EYE IO SR SR IO both SR and IO act to raise eye LEFT EYE NOSE ANATOMICAL ACTION

EXPLANATION FOR DIFFERENCE BETWEEN EYE MOVEMENTS CHART AND CARDINAL POINTS OF GAZE: EYE LOOKS MEDIALLY Muscles work best when somewhat stretched, poorly when short IO SR SR IO eye looks medially NOSE ANATOMICAL ACTION 1) both SR and IO act to raise eye 2) if have patient look medially SR becomes short 3) if then have patient raise eye (look up); SR is too short but IO is long 4) eye is then raised by IO Note: orientation of IO contributes to this effect: muscle pull is most direct when eye is adducted

EXPLANATION FOR DIFFERENCE BETWEEN EYE MOVEMENTS CHART AND CARDINAL POINTS OF GAZE: EFFECT OF IO DAMAGE Muscles work best when somewhat stretched, poorly when short IO SR SR IO if IO is damaged, patient cannot raise eye when looking medially eye looks medially NOSE ANATOMICAL ACTION

SAME EFFECTS WORK FOR IR AND SO: if arrange arrows so that they show the direction the patient is asked to look, get points of gaze SR IR IO SO CLINICAL TEST: CARDINAL POINTS OF GAZE IO SO NOSE SR IR THIS DIAGRAM SHOWS DIRECTION PHYSICIAN ASKS PATIENT TO LOOK, NOT DIRECTION OF PULL OF MUSCLE Note: MR and LR are not different in diagrams as their pull is direct

EXPLANATION FOR DIFFERENCE BETWEEN EYE MOVEMENTS CHART AND CARDINAL POINTS OF GAZE: if arrange arrows so that show direction ask patient to look, get points of gaze CLINICAL TEST: CARDINAL POINTS OF GAZE

Note: The above is a reasonable explanation for the clinical tests (See also Snell, Clinical Anatomy, 7 th Ed. , pp. 826 -829). Other descriptions of effects of nerve lesions more closely follow the anatomical basis of their action. The next slide is a description of the effects of Trochlear nerve lesion based upon illustrations from a lecturer at Yale University. At rest, the effects of Superior Oblique paralysis are due to the unopposed lateral rotation of intact muscles (like Medial Strabismus from damage to the Lateral Rectus). Patient tilts his head to compensate for chronic rotation of one eye. Also, the patient has an inability to look down when the eye is adducted, even though the anatomical action of the Superior Oblique is to abduct the eye.

TROCHLEAR NERVE DAMAGE PARALYZE SUP. RIGHT EYE ROOBLIQUE IN RIGHT EYE TATED LATERALLY TILT HEAD SO BOTH EYES ROTATED COMPENSATION: TILTING HEAD ROTATES LEFT EYE MEDIALLY SYMPTOMS: Extortion (outward rotation) of the affected eye due to the unopposed action of the inferior oblique muscle. Vertical diplopia (double vision) due to the extorted eye. Weakness of downward gaze most noticeable on medially directed eye. This is often reported as difficulty in descending stairs. Head tilt: patient will often tilt his head opposite the side of the affected eye in an attempt to compensate for the outwardly rotated eye. However, anatomical action is still to pull eye down and out and rotate medially. source: Yale University