The Simplified Guide to Understanding Eye Movements

Eye muscles and eye movements can be a confusing topic to understand—let’s look at the six ocular muscles and what they do.

There are three cranial nerves that we are interested in when it comes to the movement of the eyeball: the third nerve, the fourth nerve, and the sixth nerve, or, the oculomotor nerve, the trochlear nerve, and the abducens nerve, respectively.

Table 1 shows a breakdown of which nerves supply which muscles. Of note, the superior rectus and inferior oblique comprise the superior branch of the 3rd nerve, while the inferior rectus and medial rectus make up the inferior branch of the 3rd nerve.

Nerve	Muscle
The 3rd nerve (oculomotor)	Superior rectus (SR), inferior oblique (IO), inferior rectus (IR), medial rectus (MR)
The 4th nerve (trochlear)	Superior oblique (SO)
The 6th nerve (abducens)	Lateral rectus (LR)

^{Table 1: Courtesy of Kaajal Nanda.}

The brain-eye connection

The eyes are an extension of the brain. There are many things that can go wrong with eye movements, and if something doesn’t look right, it can give us an indication of what could be going on in the brain.

To figure this out, we need to know which way each muscle moves the eye. And remember, the positioning of the muscle on the eyeball can be different from the movement that the muscle provides. We can also check how well any given muscle is working in specific gaze positions.

The diagram in Figure 1 demonstrates the functions of eye muscles and the primary, secondary, and tertiary actions of each muscle. Don't forget SINRAD—Superiors INtort; Recti ADduct.

^{Figure 1: Courtesy of Kaajal Nanda.}

Table 2 summarizes the eye muscle functions in a grid format.

Muscle	Primary	Secondary	Tertiary
LR	Abduction	None	None
MR	Adduction	None	None
SR	Elevation	Intorsion	Adduction
IR	Depression	Extorsion	Adduction
SO	Intorsion	Depression	Abduction
IO	Extorsion	Elevation	Abduction

^{Table 2: Courtesy of Kaajal Nanda.}

The animations below illustrate the anatomical movements of eye muscles by showing the primary, secondary, and tertiary functions (black arrows) and the resulting movement (gray dotted arrow).

The 4 laws of eye movements

There are several eye movement laws that sum up the way our eyes move: Sherrington’s Law, Hering’s Law, Listing’s Law, and so on.

Sherrington’s law of reciprocal innervation

Sherrington’s law states that increased innervation to a muscle is accompanied by decreased innervation to its antagonist.¹

Hering’s law of motor correspondence

This law states that innervation to yoke muscles is equal.¹ Figure 1 above shows that the right lateral rectus and the left medial rectus are yoke muscles—they work together and are innervated equally to allow both eyes to move into the right gaze.

Not only do we have contralateral antagonists, but we also have ipsilateral antagonists and contralateral synergists. It is important to understand how and when each muscle fires and relaxes, especially in cases of a palsy. This can get a little more complicated with the vertical muscles.

Table 3 simplifies these relationships, and how the muscles respond to a palsied muscle.

Palsied Muscle (Agonist)	O/A Contralateral Synergist	O/A Ipsilateral Antagonist	U/A Contralateral Antagonist
Right lateral rectus (RLR)	Left medial rectus (LMR)	RMR	LLR
Right medial rectus (RMR)	Left lateral rectus (LLR)	RLR	RMR
Right superior rectus (RSR)	Left inferior oblique (LIO)	RIR	LSO
Right inferior rectus (RIR)	Left superior oblique (LSO)	RSR	LIO
Right superior oblique (RSO)	Left inferior rectus (LIR)	RIO	LSR
Right inferior oblique (RIO)	Left superior rectus (LSR)	RSO	LIR

^{Table 3: Courtesy of Kaajal Nanda.}

Donders’ law

Donder’s law states that for each gaze direction, there exists a single corresponding eye orientation irrespective of how the eye was brought to this position.²

Listing’s law

This law specifies what the orientation is in reference to Donders’ law.

Figure 2 illustrates Listing’s law.

^{Figure 2: Courtesy of Kaajal Nanda.}

Eye movements in the optometry clinic

Now, let’s look at some specific types of eye movements that we test for in more neurological cases:

Smooth pursuits

Assessing motility and ensuring the patient can move their eyes in all gaze directions is part of routine testing. When testing smooth pursuits, pay close attention to confirm that those smooth pursuits are “smooth,” and not jerky or delayed in following your target, as neurologic conditions can have unstable, or jerky, smooth pursuits.³ Be sure to also note any muscle restrictions or weaknesses.

Pearl: Use a penlight as a target to assess ocular motility as this aids when looking for any misalignment in various gaze positions.

Saccades

Saccades are rapid eye movements—the fastest movement that the body can generate. The goal of a saccade is to move the eyes to an object of interest and shift it onto the fovea.⁴ Saccadic movement should be tested both horizontally and vertically. We make saccadic movements approximately 100,000 times in one day.⁵

Abnormal saccades can indicate a neurological problem. Saccades can be hypometric (underestimating the distance of the target) or hypermetric (overshooting the target). In the case of hypometric saccades, you may notice a single or several “catch-up” saccades indicating the eyes’ effort to fixate on the target accurately.

A person may have trouble initiating a saccade—this can indicate a problem in the brainstem. If the trouble is the initiation of horizontal saccades, we need to look at the paramedian pontine reticular formation (PPRF) in the pons. However, if the trouble is in the initiation of vertical saccades, we need to look to the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) in the midbrain.^6,7

We might see abnormal saccades in patients with Parkinson’s disease, Gaucher’s disease, Huntington’s disease as well as progressive supranuclear palsy and spinocerebellar ataxia.^8,9,10

Vestibulo-ocular reflex (VOR)

This reflex stabilizes the eyes relative to the external world by compensating for head movement. Disruption of this system can result in symptoms of dizziness, blurred vision, nausea, vertigo, and so on.

When assessing for the VOR, we utilize the “doll’s head maneuver,” i.e., asking the patient to fixate on a target and rotate their head from side to side and up and down. An inability to do this comfortably can be indicative of anything from viral infections or head injuries to multiple sclerosis or brainstem ischemia, amongst other possibilities.³

Pearl: Always think outside of the box when it comes to eye movements and specific symptoms that may otherwise result in a normal eye exam. It is easy to rule out headaches and dizziness caused by poor vision, but if the vision is good, we must consider alternatives. Think about checking these other types of eye movements to ensure they are normal.

Vergence testing

The ability to move our eyes into a convergent or divergent position is crucial in maintaining fusion and stereopsis. Measuring this using a prism bar or synoptophore is useful in understanding a patient’s ability to control their eye alignment. The normal range of horizontal fusion at distance is 16BO-6BI; at near, 25BO-10BI.¹¹

Our convergence range is significantly larger than divergence. Testing vergence movements is a routine assessment in an orthoptic exam and has a lot more significance than most think. The assessment of convergence, specifically, can be indicative of the presence of binocular function (or lack thereof) and be a way to identify potential for fusion.

The examples below indicate possible outcomes of convergence testing in a case of exotropia and esotropia, respectively:

Exotropia: If the deviation increases on convergence, this implies poor control of the deviation, and/or poor potential for fusion post-intervention with prisms, exercises, or surgery.
Esotropia: If the deviation continues to reduce on convergence, the patient may appear binocular at a given distance. This could suggest a potential for fusion post-intervention.

Figures 3 and 4 highlight the potential results of convergence testing in patients with exotropia and esotropia, respectively.

^{Figures 3 and 4: Courtesy of Kaajal Nanda.}

Two takeaways on eye movements

Remembering what each muscle does can be tricky—SINRAD is a helpful tool to retain those primary, secondary, and tertiary functions.
There are many different eye movement laws that explain how the eyes work in synchrony. If you’re a visual learner, use the above diagrams to aid your understanding.
1. Sherrington’s law focuses on the muscle of one eye and the antagonist of the same eye, while Hering’s law focuses on the yoke (or synergist) muscles of both eyes.

In conclusion

Eye movements are complicated—that goes without saying! Use this simplified guide to help diagnose, treat, and monitor more complex cases.