Ocular Motor System

Classes of eye movements – 5 principal movements

(1) Vestibular and optokinetic – hold images steady upon the retina during rotational head perturbations (i.e. during locomotion); therefore refers to intermittent rotation of the eye when the subject is seeing a moving object

(2) Saccadic (jerky) eye movements – rapidly change the line of sight bringing the object of interest to the fovea (where it can be seen best) – i.e., reading

(3) Smooth pursuit – enables the image of a moving object to be held close to the fovea

(4) Vergence eye movements – brings the image of a single object simultaneously to both foveas making single, binocular vision possible

(5) Eye and head movements – eye movement commands, head movements, and the vestibulo-ocular reflex interact to change the line of sight (gaze)

Definitions

Saccade – small, rapid, jerky eye movement as it moves from one point to another (e.g., reading)

viscous resistance of orbit demands a phasic extraocular muscle contraction that is achieved by pulse of innervation to drive the eyes quickly (i.e. in a saccade); if this pulse is not generated properly, saccades will be slow

Horizontal saccades – generated by "burst neurons" that are housed in the PONS (Pontine Reticular formation)

Vertical saccades – "burst neurons " in the MIDBRAIN (Midbrain reticular formation)

Nystagmus – rapid involuntary motion of the eyeballs, often to accommodate for dizziness

the brain programs a tonic contraction of innervation to the muscles of the eye to hold it at an eccentric position (against the elastic forces that restore the eye to a central position); a defect in this system will cause the eye to drift back to center (after looking to one side) in resetting saccades (quick phases) causing gaze-evoked nystagmus

Abducens Nucleus – control of horizontal gaze. Contains:

(1) abducens motorneurons – innervate the ipsilateral lateral rectus muscle

(2) abducens internuclear neurons – project to the contralateral medial rectus motorneurons (via the contralateral medial longitudinal Fasiculus)

Lesion of abducens nucleus Þ ipsilateral gaze palsy (i.e., cut right nucleus VI Þ cannot look to right)

Lesion of MLF – paralyzes the ipsilateral medial rectus muscle (internuclear opthalmoplegia)

Parinoud’s syndrome – lesion of the posterior commissure (i.e. pineal tumor) causes a paralysis of all upward eye movements (upgaze palsy)

Localization of ocular motor function in the cerebellum

Vestibulocerebellum (flocculonodular lobe) – important for smooth eye movements. Lesion causes:

(1) impaired smooth pursuit
(2) impaired gaze-holding (gaze-evoked nystagmus)
(3) rebound nystagmus
(4) downbeat nystagmus
(5) inappropriately "calibrated" vestibulo-ocular reflex

Flocculus and paraflocculus: pursuit, gaze-holding and VOR gain adjustment

Nodulus: VOR phase (temporal) adjustment

Dorsal vermis (Latin: worm) of cerebellum – important for saccadic accuracy. Lesion causes saccadic dysmetria

Parallel pathways – involved in voluntary control of saccades

Pathway descends from the frontal eye fields and parietal-occiptal cortex to control saccadic eye movements

Pathway passes directly to the saccadic burst neurons or synapse at Superior Colliculis

Lesion of frontal eye field or superior colliculus alone Þ mild deficit in voluntary control of saccades

Combine lesion of both Þ permanent and severe loss of voluntary saccades

Testing

Sequence of examination tests: vergence, saccades, smooth pursuit, the vestibulo-ocular reflex and its suppression (combined eye-head tracking)

range of motion and cover test; muscle weakness results in diplopia when that muscle is used

Medial Rectus

Primary Action: Adduction

Lateral Rectus (CN VI)

Primary Action: Abduction

Superior Rectus

Primary Action: Elevation

Secondary Action: Intorsion

Tertiary Action: Adduction

Inferior Rectus

Primary Action: Depression

Secondary Action: Extorsion

Tertiary Action: Adduction

Superior Oblique (CN IV) – "Down and Out"

Primary Action: Intorsion – "SIN" Sup oblique – Intorsion

Secondary Action: Depression

Tertiary Action: Abduction

Inferior Oblique

Primary Action: Extorsion

Secondary Action: Elevation

Tertiary Action: Abduction

Disorders

Sixth nerve (abducens) palsy – often following: spinal tap, myelography, v/a shunt. Why? increased intracranial pressure can ‘snag’ nerve temporarily causing double vision.

abducents palsy in children – Benign form: follows febrile illness, immunization
Convergence spasm- pupillary constriction and dilation when abducting eye
Differential diagnosis for sixth nerve palsy: tensilon, thiamine, trauma

Trochlear nerve palsy – head trauma, "medical", neoplasm, craniotomy. Have to test with head tilt test the eye at the side of the lesion will drift down after being covered

Third nerve palsy – eye lid will droop (proptosis), muscles won’t work, pupil will be dilated

Bilateral internuclear opthalmoplegia

Etiology: lesions of the medial longitudinal fasciculi (MLF) – typically multiple sclerosis (a demyuelinative plaque extending between midline tracts)

Findings: no complete adduction with conjugate eye movements. But adduction possible with convergence eye movements because vergence commands pass directly to the medial rectus motorneurons without entering the MLF (carries III, IV, VI)

Pontine Gaze Pulsy

Etiology: metastses in the left pons. Abducens nucleus on the involved side was intact. Saccadic and pursuit inputs to it are impaired. Convergence inputs pass to the third nerve nucleus directly

Findings: able to make saccades and smooth pursuit movements horizontally to the right and vertically. Able to drive eyes conjugately over into the left field with head rotation (vestibulo-ocular reflex). Possible to drive right eye into the left field during a convergence eye movement

unable to move eyes across the midline to the left with a saccade or smooth pursuit

Slow saccades in olivopontocerebellar atrophy

Etiology: disease of the pons

Findings: slowing of horizontal sacades. Vertical saccades normal

Opsoclonus ("saccadomania") – rapid irregular nonrhythmic movements of the eye in the horizontal and vertical axes. Saccadic oscillations with horizontal, vertical, and torsional components.

Etiology: neuroblastoma in children, gynecological tumors or lung cancer in adults. Following viral encephalitis

Pathogenesis: burst neurons (project the saccadic commands to ocular motor neurons) are given free rein and cause saccadic oscillations. Cancer tumors destroy regulartory cells of burst cell neurons

Saccadic dysmetria – lack of executing any definitive movement to a point in a harmonious manner

Etiology: midline cerebellar cyst

Findings: overshoots the target of desired vision and has to make a correction Þ localization to the dorsal vermis of the cerebellum

Latent Nystagmus – jerky nystagmus brought about by covering one eye

requires no neurological exam; reflects impaired development of pathways that encode binocular cues on visual motion

Esotropia (convergent strabismus (lack of parallelism of visual axes)

Congenital Nystagmus – horizontal

Findings: mixed jerk (slow and quick phases) and pendular waveforms, changes amplitude, and frequency with gaze change. ALWAYS horizontal. Needs no neurological examination

Acquired Nystagmus

Findings: Oscillopsia – the illusion of movement of the environment and impaired visual acuity

Circular nystagmus of Pelzaeus-merzbacher disease – rare hereditary disorder of myelin

Findings: horizontal and vertical nystagmus