Movements of the Eye

Introduction

Eye movements maintain precise alignment of the visual axes, enabling clear binocular vision, accurate fixation, and smooth tracking of moving objects. Understanding the actions and nerve supply of the extraocular muscles is essential for recognizing disorders such as diplopia and nystagmus.

EXTRAOCULAR MUSCLES

• The extraocular muscles are six skeletal muscles that originate from the walls of the orbit and insert into the sclera, producing precise movements of the eyeball.
• They include the medial rectus, lateral rectus, superior rectus, inferior rectus, superior oblique, and inferior oblique muscles.

Nerve Supply of EOM

• The lateral rectus is supplied by the abducens nerve.
• The superior oblique is supplied by the trochlear nerve.
• The remaining four extraocular muscles are innervated by the oculomotor nerve.

Action of EOM

• Under normal conditions, the eye can move approximately 50 degrees medially, 30 degrees upward, and 50 degrees downward.
• In the abducted position, the superior rectus elevates the eye, whereas the inferior rectus depresses it.
• In the adducted position, the inferior oblique elevates the eye and the superior oblique depresses it.
• From the primary position, the medial rectus produces adduction and the lateral rectus produces abduction.

Cardinal Movements

Cardinal eye movements are the standard horizontal and vertical gaze movements used to evaluate coordinated function of the extraocular muscles and their cranial nerve supply..

Conjugate Movement

• Conjugate eye movement is the coordinated, symmetrical movement of both eyes in the same direction so that the visual axes remain aligned on a single target.
• It depends on integrated activity of the oculomotor, trochlear, and abducens nuclei through brainstem pathways, ensuring binocular vision and accurate gaze alignment.

Types of Ocular Movements

• Ocular movements maintain clear vision by accurately positioning the eyes on a visual target through coordinated activity of the extraocular muscles.
• These movements require an intact visual cortex and include saccades, smooth pursuit movements, vergence movements, and vestibulo-ocular movements. The first three occur with the head remaining stationary.

Table 148.1: Innervation, function and effect of paralysis of external ocular muscles.

Extraocular Muscle	Motor Nerve Supply	Primary Action	Effect of Paralysis
Lateral rectus	Abducens nerve (Cranial Nerve VI)	Abducts the eyeball (moves it laterally).	The affected eye cannot abduct, resulting in horizontal diplopia that worsens on looking toward the affected side. Unopposed action of the medial rectus causes convergent strabismus (esotropia).
Superior oblique	Trochlear nerve (Cranial Nerve IV)	Depresses the adducted eye and also contributes to intorsion.	Difficulty depressing the adducted eye causes vertical diplopia, especially while reading or walking downstairs. Patients often adopt a compensatory head tilt.
Medial rectus	Oculomotor nerve (Cranial Nerve III)	Adducts the eyeball (moves it medially).	Isolated paralysis is uncommon. Adduction is impaired, producing horizontal diplopia. In complete oculomotor nerve palsy, the eye deviates downward and laterally because of the unopposed actions of the lateral rectus and superior oblique.
Superior rectus	Oculomotor nerve (Cranial Nerve III)	Elevates the abducted eye.	Elevation of the abducted eye is impaired, producing vertical diplopia during upward gaze.
Inferior rectus	Oculomotor nerve (Cranial Nerve III)	Depresses the abducted eye.	Depression of the abducted eye is impaired, causing difficulty looking downward and vertical diplopia.
Inferior oblique	Oculomotor nerve (Cranial Nerve III)	Elevates the adducted eye and contributes to extorsion.	Elevation of the adducted eye is impaired, resulting in vertical diplopia during upward and medial gaze.

Saccades

• Saccades are rapid, conjugate eye movements that shift the line of sight quickly from one object to another.
• They reposition the image of a new target onto the fovea, ensuring clear central vision.
• These movements reduce visual fading caused by continuous fixation and prevent prolonged contraction of the extraocular muscles, thereby minimizing muscle fatigue.
• Saccades are generated mainly by the frontal eye field and coordinated with the superior colliculus and brainstem gaze centers.

Smooth Pursuit Movements

• Smooth pursuit movements are slow, continuous eye movements that allow the eyes to follow a moving object while maintaining stable fixation.
• They keep the image of the moving target on the fovea, preserving clear vision during motion.
• These movements depend on an intact visual cortex and are coordinated by the cerebellum and brainstem pathways.
• During observation of moving objects, smooth pursuit is often followed by a corrective saccade when the eye reaches its limit of movement.

Vergence

• Vergence movements are disconjugate eye movements that adjust the visual axes according to the distance of the object.
• When an object moves closer, both eyes rotate medially, producing convergence.
• When an object moves farther away, both eyes rotate laterally, producing divergence.
• Vergence movements maintain single binocular vision and accurate depth perception during changes in viewing distance.

Vestibular Movements

• Vestibulo-ocular movements stabilize vision during head movements.
• Signals from the semicircular canals of the vestibular apparatus produce compensatory eye movements opposite to the direction of head movement.
• This reflex helps maintain fixation on a stationary object despite movement of the head or body.

Nystagmus

• Nystagmus is an involuntary, rhythmic, oscillatory movement of the eyes consisting of alternating slow and fast phases.
• It may occur physiologically during vestibular stimulation or optokinetic testing, or pathologically in disorders affecting the vestibular system, cerebellum, brainstem, or visual pathways.
• Persistent pathological nystagmus may impair visual stability and reduce visual acuity.

Physiological Nystagmus

• Physiological nystagmus consists of very small, involuntary oscillatory eye movements that occur even during steady fixation.
• These minute movements continuously shift the retinal image over adjacent photoreceptors, preventing prolonged stimulation of the same retinal neurons.
• Continuous image displacement reduces neuronal adaptation, allowing sustained perception of a stationary object.
• Neural elements of the visual pathway adapt more rapidly than photoreceptors; therefore, an unchanging retinal image gradually fades from perception if eye movements are absent.
• Physiological nystagmus helps maintain clear and continuous vision during prolonged fixation and is also referred to as fixation eye movement.
• Optokinetic nystagmus occurs when the eyes follow a rapidly moving visual scene.
• It consists of a slow pursuit movement that tracks the moving object, followed by a rapid saccadic movement that returns the eyes to their initial position, enabling continuous tracking of successive moving targets.

Pathological Nystagmus

Pathological nystagmus is a visible, involuntary oscillatory eye movement caused by disorders of the vestibular system, brainstem, cerebellum, or ocular motor pathways.

Abnormalities of Eye Movements

• Disorders affecting the extraocular muscles, cranial nerves, or ocular motor pathways interfere with normal eye movements and binocular vision.
• Common abnormalities include diplopia, strabismus, and impaired accommodation, depending on the site of the lesion.
• Diplopia occurs when the visual axes are not properly aligned, causing images to fall on non-corresponding retinal points.
• Strabismus is persistent misalignment of the visual axes and may result from paralysis or weakness of one or more extraocular muscles.
• Lesions involving the oculomotor nerve may also impair accommodation because of paralysis of the ciliary muscle.

Depth Perception

• Depth perception is the ability to judge the relative distance of objects in three-dimensional space.
• Because the two eyes are separated horizontally, each retina receives a slightly different image of the same object.
• These images are fused in the visual cortex, producing a single image while simultaneously providing stereoscopic vision.
• Accurate depth perception requires normal binocular vision, proper ocular alignment, and intact cortical processing.
• Loss of vision in one eye markedly reduces stereoscopic depth perception.

Electro-oculography

• Electro-oculography records changes in the resting electrical potential generated mainly by the retinal pigment epithelium during light and dark adaptation.
• Surface electrodes are placed near the medial and lateral canthi, and eye movements generate measurable changes in electrical potential.
• The resting potential across the retinal pigment epithelium is approximately 6 millivolts.
• Electro-oculography is useful for evaluating disorders affecting the retinal pigment epithelium, including inherited retinal dystrophies such as retinitis pigmentosa.

Electroretinography

• Electroretinography records the electrical responses of retinal cells following a brief flash of light.
• The test is performed after pupillary dilation and topical anesthesia using a recording electrode placed on the cornea and reference electrodes on the skin.
• The electroretinogram consists of a negative a-wave, a larger positive b-wave, and a smaller positive c-wave.
• The a-wave originates mainly from photoreceptors, the b-wave reflects activity of bipolar cells and Müller cells, and the c-wave arises from the retinal pigment epithelium.
• Electroretinography assesses the functional integrity of the retina, particularly rod-mediated responses.
• It is valuable in diagnosing retinal detachment, inherited retinal dystrophies, vitamin A deficiency, and other retinal disorders affecting photoreceptor function.

Visual Evoked Potentials

• Visual evoked potentials record the electrical responses generated along the visual pathway, from the retina to the visual cortex, following visual stimulation.
• The test is performed by placing recording electrodes over the occipital scalp while presenting light flashes or patterned visual stimuli.
• The most clinically important waveform is the P100 wave, a positive peak occurring approximately 100 milliseconds after stimulation.
• Changes in the latency or amplitude of visual evoked potentials help detect disorders such as optic neuritis, optic atrophy, demyelinating diseases, refractive errors, and other abnormalities affecting the visual pathway.