Eye movements

The normal position of the eyeballs is parallelism of the visual axes when fixating a distant object or their intersection when fixating a close object.

1. Strabismus is an abnormal position of the eyeballs.
2. Orthophoria - the ideal position of the eyeballs (without effort), including in the absence of a stimulus for the fusion reflex, is rare (most people have a slight heterophoria).
3. Heterophoria (phoria) is a tendency of the eyeballs to deviate (latent strabismus). Additional effort is required to maintain the correct position.
4. Heterotropia (tropia) - abnormal position of the eyeballs (manifest form); phoria can become tropia if:
   • Insufficient muscle strength to maintain the correct position of the eyes.
   • The stimulus to the fusion reflex is weakened (monocular blurring of the visual image).
   • The neurogenic mechanisms that coordinate binocularity are disrupted.
5. The prefixes "eso" and "exo" mean the deviation of the eyeball inward and outward, respectively. Exophoria is the tendency of the eyeballs to deviate outward, esotropia is manifest convergent strabismus. The deviation can be vertical (then the prefixes "gityu" - down and "hyper" - up) or torsional.
6. The visual axis (line of sight) connects the fovea with the fixation point, passing through the center of the eyeball. Normally, the visual axes of the two eyes intersect at the fixation point. The fovea is located somewhat temporally to the posterior pole (the geometric center); the visual axis passes somewhat nasally to the center of the cornea.
7. The anatomical axis is a line passing through the posterior pole and the center of the cornea.
8. The angle kappa is the angle between the visual and anatomical axis, usually about 5. A positive angle kappa occurs when the fovea is located temporally to the posterior pole, and a negative angle kappa occurs when it is located more nasal. An abnormal angle kappa can cause false strabismus (see below).

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Ductions

Monocular eye movements around the Fick axes include adduction, abduction, elevation, depression, intorsion, and extorsion. They are assessed under monocular occlusion with the patient following an object in each gaze position.

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Versions

Versions are binocular, simultaneous, conjugate (unidirectional) eye movements.

• Dextroversion and levatorversion (look right, look left), elevation (look up) and depression (look down). These four actions bring the eyeball to the secondary gaze position by rotating around the horizontal (X) or vertical (Z) Fick axes.
• Dextroelevation and dextrodepression (gaze up to the right; gaze down to the right), left elevation and left depression (gaze up to the left and down to the left). These four oblique positions are tertiary gaze positions, into which the eyeball is transferred by rotation around the horizontal and vertical axes.
• Dextrocycloversion and left cycloversion (torsional movements of the superior limbus of both eyes to the right, torsion to the left).

Vergence

These are binocular, simultaneous, disjugate eye movements (oppositely directed). Convergence is a simultaneous adduction (inward rotation). Divergence is an outward rotation from the convergence position. Convergence can be a voluntary reflex consisting of 4 components.

1. Tonic convergence with obligatory innervational tone of the internal rectus muscle when the patient is awake.
2. Proximal convergence is caused by the awareness of the proximity of an object.
3. Fusion convergence is an optomotor reflex that maintains binocular single vision and provides projection of identical images onto corresponding areas of the retina of each eye. The reflex is initiated by bitemporal disparity of the image, and no changes in refraction occur.
4. Accommodative convergence is induced by accommodation and is part of the synkinetic reflex. Each diopter of accommodation is accompanied by an increase in accommodative convergence with a certain ratio of accommodative convergence to accommodation (LC/L). The index is the ratio of the number of prism diopters (D) to the diopters of accommodation (liter). Normally, it is 3-5 D (for 1 D of accommodation there are 3-5 D of accommodative convergence). The pathological index AC/L is important in the development of strabismus.

Gaze positions

1. The six basic positions of gaze are the positions of the eyeball depending on the action of one of the muscles.
   • Dextroversion (right external and left internal muscles).
   • Left-sidedness (left external and right internal muscles).
   • Dextroelevation (right superior rectus and left inferior oblique muscles).
   • Left elevation (left superior rectus and right inferior oblique muscles).
   • Dextrodepression (right inferior rectus and left superior oblique muscles).
   • Levodepression (left inferior rectus and right superior oblique muscles).
2. Nine diagnostic positions of gaze in which the deviation of the eyeball is assessed: six cardinal positions, primary position, elevation and depression (Fig.).

Laws of eye movements

1. Agonist and antagonist - a pair of muscles of one eye, causing it to move in opposite directions. Agonist - the primary muscle causing the eye to move in a certain direction, antagonist - acts in the opposite direction. For example, the right external rectus muscle is the antagonist of the right internal rectus muscle.
2. Synergists are muscles of the same eye that act in the same direction. For example, the superior rectus and inferior oblique muscles of one eye are synergist elevators.
3. Paired muscles are a pair of muscles of different eyes that produce conjugate movements. For example, the paired muscle of the left superior oblique - inferior rectus of the right eye.
4. Sherrington's law of reciprocal innervation (inhibition) states that an increase in the innervation of an extraocular muscle (e.g., the internal rectus muscle of the right eye) is accompanied by a reciprocal decrease in the innervation of the antagonist (the external rectus muscle of the left eye). This means that contraction of the internal rectus muscle is accompanied by relaxation of the external rectus muscle and vice versa. Sherrington's law applies to versions and vergences.
5. Hering's law of equal innervation states that during conjugate eye movements, paired muscles receive an equal impulse simultaneously. In the case of paralytic strabismus, symmetrical innervation to both muscles is determined by the fixating eye, so the angle of strabismus will vary depending on the fixating eye. For example, in case of paresis of the external muscle of the left eye, the fixing eye is the right eye; inward deviation of the left eye occurs due to the tone of the internal rectus muscle in the absence of the function of the antagonist - the paretic external rectus muscle of the left eye. This angle of deviation of the eyeball is called the primary angle. Additional innervation is required to maintain fixation by the paretic eye. However, according to Hering's law, an impulse of the same strength is directed to the internal rectus muscle of the right eye (paired muscle), which leads to its hyperfunction and excessive adduction of the right eye. The angle of deviation between the two eyes is called the secondary angle. In paralytic strabismus, the secondary angle exceeds the primary one.

Anatomy of the eye muscles

The outer and inner walls of the orbit are located at an angle of 45 relative to each other. Thus, the angle between the orbital axis and the lateral and medial walls of the orbit is 11.4, but for simplicity it is equal to 23. When looking directly at a fixation point at the horizon and the head is raised (the primary position of gaze), the visual axis forms an angle of 23 with the orbital axis. The action of the extraocular muscles depends on the position of the eyeball at the moment of muscle contraction.

1. The primary action of a muscle is its main action in the primary position of the eyes.
2. The secondary effect is an additional effect on the position of the eyeball.
3. The Listing plane is an imaginary coronal plane passing through the center of rotation of the eyeball, which rotates about the Fick axis intersecting the Listing plane.
   • Rotate left and right about the vertical Z axis.
   • Movement up and down relative to the horizontal X-axis.
   • Torsional movements relative to the Y-axis, which runs from the anterior to the posterior pole as the visual axis.

Rectus oculi muscles of horizontal action

In the primary position of the eyes, the horizontal rectus muscles perform movement only in the horizontal plane relative to the vertical Z axis, i.e. they are limited by their primary action.

1. The internal rectus muscle originates from the ring of Zinn at the apex of the orbit and inserts into the sclera nasally 5.5 mm posterior to the limbus. Its sole function is adduction.
2. The lateral rectus muscle originates from the ring of Zinn and inserts onto the sclera temporally 6.9 mm from the limbus. Its sole function is abduction.

Rectus oculi muscles of vertical action

The vertical rectus muscles run along the axis of the orbit and are attached to the eyeball anterior to the equator, forming an angle of 23° with the visual axis.

The superior rectus muscle originates from the superior part of the ring of Zinn and inserts 7.7 mm posterior to the superior limbus.

• Primary function is elevation of the eyeball. Secondary actions are adduction and intorsion.
• When the eyeball is abducted by 23, the visual axis and the orbital axis coincide. In this position, the muscle does not have a secondary action and functions as a lift, which makes the abduction position optimal for examining the function of the superior rectus muscle.
• If the eyeball could be adducted to 67, the angle between the visual axis and the orbital axis would be 90, and the superior rectus muscle would act only as an intortor.

The inferior rectus muscle originates from the inferior part of the ring of Zinn and inserts 6.5 mm posterior to the inferior limbus.

• Primary function - eyeball depression. Secondary - adduction and extorsion.
• When the eyeball is abducted to 23, the inferior rectus muscle acts only as a depressor. As with the superior rectus muscle, this is the best position for examining the function of the inferior rectus muscle.
• If the eyeball could be adducted to 67, the inferior rectus muscle would act only as an extortor.

Tillaux spiral

An imaginary line running along the insertions of the rectus muscles is an important anatomical landmark in strabismus surgery. The insertions are moving away from the limbus, the line forms a spiral. The insertion of the internal rectus muscle is closest to the limbus (5.5 mm), followed by the inferior rectus (6.5 mm), external rectus (6.9 mm), and superior rectus (7.7 mm).

Oblique muscles of the eye

The oblique muscles are attached behind the equator; the angle between the muscles and the visual axis is 51.

The superior oblique muscle originates from the superointernal margin of the optic foramen. It crosses the trochlea at the angle between the superior and medial walls of the orbit, then passes posteriorly and laterally, inserting in the posterior superior temporal quadrant of the eyeball.

• Primary function is intorsion. Secondary function is lowering and abduction.
• When the eyeball is in the state of 51 adduction, the visual axis coincides with the line of action of the muscles; the muscle acts only as a depressor, which makes this position optimal for studying the function of the superior oblique muscle.
• When the eyeball is abducted to 39, the visual axis and the superior oblique form an angle of 90. In this position, the superior oblique muscle has only an intortor function.

The inferior oblique muscle originates from a small fossa behind the orbital fissure lateral to the lacrimal sac, passes posteriorly and laterally, and inserts in the posterior inferotemporal quadrant of the eyeball, near the macula.

• The primary function is extorsion, the secondary function is elevation and abduction.
• When the eyeball is in a state of adduction 51, the inferior oblique muscle acts only as a levator.
• When the eye is adducted to 39', the main action is extorsion.

Innervation of the eye muscles

1. The external rectus muscles are innervated by the sixth pair of cranial nerves (abducens nerve - abducens muscle).
2. The superior oblique muscles are innervated by the IV pair of cranial nerves (trochlear nerve - the muscle passes over the trochlea).
3. Other muscles and the levator oculi superioris are innervated by the third pair of cranial nerves (oculomotor).

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