CN III is a pure motor nerve that has both voluntary and involuntary function. Voluntary function includes the intentional movement of the eyes toward a specific direction, as well as opening of the eyelids.
Involuntary CN III function is produced through brainstem reflexes. A brainstem reflex, like a spinal cord reflex, is the initiation of a protective and automatic motor response as a result of a sensory stimulus.
Brainstem and spinal reflexes can be ellicited in conscious or unconscious states as long as the neuronal pathways are intact. The spinal reflex "arc" consists of an input (sensory message) and immediate output (protective motor response). Reflex are independent of brain communication.
An example of a spinal reflex is the automatic withdrawal of your hand in response to a paper cut. The noxious stimulus (sensory input) enters the spinal nerve root, where it travels immediately to the corresponding motor nerve. Stimulation of the motor nerve causes the limb to "jerk away", a protective response that occurs before the painful message reaches the brain for interpretation.
Similar reflex arcs exist between cranial nerves with sensory and motor function. Because most of the cranial nerves are arranged along the brainstem, these reflex arcs are referred to as "brainstem reflexes".
LIGHT REFLEX TESTING
Pupillary light reflex is an example of a brainstem reflex. When light is directed toward one eye, CN II (Optic - sensory nerve) carries the light stimulus to CN III. Light directed toward either eye will immediately stimulate CN III on both sides. Thus, both pupils constrict in response to light directed into either eye (known as direct and consensual light response). This involuntary response occurs continually and allows our pupils to adjust to ambient room light. This reflex is preserved in the unconscious state.
There are 3 separate assessments to make when examining pupils (size, equality and reactivity). Assess size and equality before testing for reactivity with the flashlight. Reactivity is an assessment of the CN II and III brainstem reflex:
- Assess pupillary size: With normal room lighting, open up both eyelids. Allow the pupils to adjust to the room light for a few seconds. Once they have stabilized, determine the size of the pupils in millimeters by comparing them to the pupil size chart.
- Assess pupil equality: While lids are held open, examine the eyes to determine symmetry and shape. The pupils should be the same size and have a round shape.
- Reactivity: Begin with the eyelids closed. The assessment of reactivity is a 4 - step assessment using the "swinging flashlight" technique. Begin by shining light into one eye (we will begin with the right for ease of explanation).
Step One: Open the right eyelid and shine the light into the right eye. Look only into the right eye to assess for a response. A normal response is a brisk constriction of the right pupil. If the pupil gets larger or has a sluggish response, it is considered abnormal. This is an assessment of the direct light response of the right eye.
Step Two: Open the left eyelid as you "swing" the flashlight beam into the left eye. Look only into the left eye and observe the response. This is an assessment of the direct light response of the left eye.
Step Three: Open both eyelids as you swing the flashlight beam back to the right eye. This time, you will only look into the left eye. Assess the left pupil response. This is an assessment of the indirect or consensual light response of the left eye.
Step Four: Open both eyelids as you swing the flashlight beam back to the left eye. This time, you will only look into the right eye. Assess the right pupil response. This is an assessment of the indirect or consensual light response of the right eye.
Acute compression of CN III is a critical event. For expanding lesions that originate in one side of the cerebral cortex, pressure on the brainstem and CN III will generally begin on the side of the lesion. Thus, a right sided brain injury will typically cause the right pupil to become larger and less responsive than the left first. If the lesion increases in size quickly, the left CN III may also become compressed, producing bilateral dilation and loss of reactivity.
- When pupillary dilation or loss of reactivity occurs as a result of CN III compression, both direct and indirect (consensual) light reflex response is impaired in the same eye (e.g., the right pupil is dilated and non-reactive to light shone into either the right or left eye).
- If direct light reflex is lost but indirect light reflex is preserved, true CN III compression is less likely. This suggests an afferent defect is affecting the side with the loss of direct response.
Example: the pupillary reflex is absent in the right eye when light is shone into the right eye, but the right pupil constricts in response to light directed into the left eye.
- In an awake patient whose pupil or pupils do not respond briskly to light, they may describe photophobia (pain in bright lighting). If pupils are unequal, they may also describe blurred or impaired vision.
In conscious patients, eye movement is assessed by having the patient follow an object into the vertical, horizontal and diagonal directions. The object should be 2 or more feet away from the patient (make the movement slow and large to allow time to focus).
Observe for the following:
- The eyes should move in the same direction in tandom. This is called "conjugate gaze". When the eyes separate or move in different directions, the patient has disconjugate gaze. An awake individual with disconjugate gaze will have double vision (called diplopia).
If you observe full horizontal, vertical and diagonal eye movement you have collectively assessed CN I (you have to see the object to follow it), plus CN III (Oculomotor), IV (Trochlear) and VI (Abducens).
All upward and downward gaze is CN III. Horizontal eye movement is CN III and VI. For example, movement of the eyes horizontally toward the left requires the left CN VI and the right CN III (to make the right eye rotate medially and horizontally). Downward gaze toward the nose is CN IV. Impaired horizontal eye movement can occur as a result of problems in CN II (you cannot see an object to follow it), CN VIII (vestibular apparatus stimulated by head rotation), CN VI, contralateral CN III, medial lateral fasciculus or pons.
Voluntary Eye Movement
There are two types of voluntary eye movement - saccades and pursuit. Saccades eye movement occurs when we rapidly switch our gaze from one object to another quickly. Pursuit is the smooth and steady tracking of an object, for example as we look around the room slowly. Saccades can be tested by placing two objects, one on either side of the temporal visual fields. Have the patient look quickly from one side to the other. The gaze should be conjugate. Pursuit movements are tested when you ask a patient to follow an object slowly through a vertical (up and down) and horizontal (right and left) plane.
Saccade movement originates in the frontal eye fields and initiate the CONTRALATERAL CN III (vertical movement) or VI (horizontal eye movement). Pursuit eye movement originates at the top of the brainstem and initiates CN III (vertical) or CN VI (horizontal) on the IPSILATERAL side. Conjugate saccade or pursuit eye gaze is then achieved through involuntary eye movement, by activating the contralateral CN III as described below.
Normal Involuntary Eye Movement (Vestibulo-Ocular Reflex)
Involuntary eye movement is stimulated through the vesibulo-ocular reflex. When we turn our head or neck to the side, or toward an upward or downward vertical position, our eyes will automatically move in the opposite direction to maintain focus on an object. For horizontal gaze, turning our head toward the left for example, will cause the vestibular apparatuses of both the right and left CN VIII to rotate in the opposite direction (toward the right). This in turn will stimulate the right Paramedian Pontine Reticular Formation in the pons. The PPRF simultaneously activates the right (ipsilateral) CN VI (abducens) and the Medial Lateral Fasciculus (MLF). The right CN VI activates the lateral rectus muscle to move the RIGHT eye horizontally toward the right temple. Simultaneous activation of the Medial Lateral Fasciculus carries a stimulus to the LEFT CN III at the top of the brainstem. Stimulation of the left CN III activates the left medial rectus muscle to move the LEFT eye horizontally toward the left temple. Thus, horizontal eye gaze will be conjugate and in the opposite direction to the head rotation.
Involuntary vertical movement causes the eyes to move up when the head is tilted down (and vice vesa). This occurs by simultaneous activation of both CN III in response to a vestibulo-ocular reflex.
Abnormalities in horizontal gaze can occur as a result of vestibular damage, damage to the pons, CN VII or contralateral CN III. It can also be due to damage to the MLF, known as an internuclear ophthalmoplegia (INO). Disconjugate gaze can also occur as a result of supranuclear lesions (above the nucleus of the cranial nerves or in the brain). For example, damage to the frontal eye fields (control contralateral saccades) or temporo-parietal occipital region (ipsilateral pursuit) can cause gaze disorders. A supranuclear lesion can be identified by the presence of impaired eye gaze with intact doll's eyes movement (doll's eyes is a brainstem reflex).
Abnormal eye gaze is common in patients who are awakening from deep sedation and coma and often do not represent pathology to the cranial nerve. In order to follow an object, the patient needs to be awake and cooperative, and able to maintain attention.
When observing eye gaze, you can assess for any shakiness, bouncing or rotational movement as well as conjugate gaze. This is called nystagmus. You can assess for nystagmus during pursuit and saccades movement if patient is able to participate. When nystagmus is observed, notice whether the eyes are "beating" toward the same side repeatedly, or are they beating back and forth in both directions. Also note whether the nystagmus occurs with or without conjugate gaze, and whether it is noted during vertical or horizontal eye movement. Nystagmus may be present during a seizure.
It is often difficult to identify ptsosis if the eyelid weakness is mild. It is easier to look at the whites of the eye and the distance between the upper and lower lids (called the palpebral fissure). Ptsosis causes the palpebral fissure to be narrowed (less white of the eye on the affected side). The eyelid may have a puffy or thickened appearance.
Testing the Unconscious Patient (Brainstem Reflexes)
1. Pupillary Light Reflex should remain intake (this tests CN II and CN III at the top of the brainstem)
2. Dolls Eyes (Oculocephalic Reflex) - in an unconscious patient with intake CN III, CN VI and CN VIII, movement of the head horizontally or vertically should result in contralateral conjugate gaze in the opposite direction. It should not be performed if there is any concern for neck injury.
3. Cold Calorics (Oculovesibular Reflex) - the instillation of cold water into the ear canal of an unconscious patient with head of bed elevated 30 degrees is done to test the pons and CN III, VI and VIII. The eyes of an unconscious patient should deviate slowly toward the irrigated ear and then return quickly to midline when the irrigation stops. As brainstem function deteriorates, movement in either or both direction is lost. Simultaneous irrigation of both ears causes downward deviation of the eyes. If the test is performed in a conscious patient, the eyes will deviate toward the irrigated ear and display horizontal nystagmus that beats toward the opposite eye. A conscious patient may also experience nausea and vertigo. Although rarely used, warm water causes the eyes to deviate in the opposite direction.