Where are the 3rd Cranial Nerves located?

The two 3rd cranial nerves (oculomotor nerves) are located at the top of the brainstem - one to the right and one to the left.  They sit at the level of the tentorium.
The 3rd cranial nerves are pure motor nerves.  They are Lower Motor Neurons (LMN) (second order neurons). They control eye muscles on the same side of the body (ipsilateral). See Diagram.

What is the function CN III?

Pupil Constriction
Each one of the two 3rd cranial nerves controls the parasympathetic response of the pupil on the same side (ipsilateral).  The parasympathetic response of the pupil (or "return to normal")  is constriction. 
Eye Movement
The 3rd cranial nerve also controls eye muscle movement.  Cranial Nerve IV (Trochlear) controls downward eye movement toward the nose, and Cranial Nerve VI (Abducens) controls horizontal eye movement toward the temple.  The ability to move the eye in all other directions is controlled by the 3rd cranial nerve.   It activated the medial rectus, superior rectus, inferior rectus and inferior oblique muscles to cause orbital rotation. See Diagram.
Eyelid Elevation
Cranial Nerve III also controls the ability to open the eyelid. You can remember this function because the Oculomotor nerve starts with the letter "O" for eye "O"pening.
What is the significance of monitoring for CN III function?
Acute loss of CN III function is an important sign of a raised intracranial pressure with expanding mass lession. A new and sudden finding of pupillary dilation and loss of reactivity suggests supratentorial herniation.
Loss of pupillary reactivity is the most important urgent CN III finding. The pupillary control provided by CN III is located along the periphery of the nerve.  By contrast, the CN III control of eye movement and eyelid opening runs deep in the centre of the nerve.  Consequently, a mass that presses on the outside of CN III will usually impact the pupillary function first (for example as a result of an aneurysm or raised ICP). 
The deep blood supply of CN III can become impaired by vascular diseases, hypertension or diabetes. This can lead to problems with eye movement  or eyelid opening where pupil function is spared. 
Pupillary abnormalities can also be chronic.  Direct eye injuries can produce CN III abnormalities. Correlation of pupil abnormalities with clinical findings, history and trends is important.

What does loss of 3rd Cranial Nerve function cause?

Loss of CN III function causes the following abnormalities ON THE SAME SIDE (ipsilateral):
  • Pupil dilation and loss of reactivity to light (inability of the pupil to constrict to light)
  • Inability to follow and object in direction of CN III (the quickest test is to observe upward gaze which is all CN III; the eye on the affected side does not look upward)
  • Inability to open the eyelid.  CN III dysfunction causes the eyelid on the affected side to become "droopy". This is called ptsosis. 

Complete CN III Palsy

In complete CN III palsy, the affected eye is "down and out" (the eyelid is down, and the eye is rotated downward and outward due to loss of opposing upward movement).

How do you test CN III function?

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 and 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 ilicited in conscious or unconcious 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".


Pupillary light reflex is an example of a brainstem reflex. When light is directed toward eye, CN II (Optic - sensory nerve) will carry the input to CN III.  Light directed toward either eye will immediately stimulate CN III in both eyes.  Thus, both pupils constrict in reponse to light directed into either eye. This involuntary response occurs continually and allows our pupils to adjust to ambient room light.  This reflex is preserved in the unconsious state.


There are 3 dseparate assessments to make when examining pupils (size, equality and reactivity). Reactivity is an assessment of the CN II and III brainstem reflex:

  1. 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 milimeters by comparing them to the pupil size chart (available on graphic record)
  2. Assess pupil equality:  While lids are held open, examine the eyes to determine symmetry and shape.  The pupils should be the same size and the should have a round shape.
  3. 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 the most serious problem and pupillary dilation or loss of reactivity is the most important change. When CN III compression occurs, the pupil on the side of the lesion will become dilated and less reactive (loses its ability to constrict to light). If the mass increases in size, the pupil on the other side may also become dilated and less reactive/non-reactive. This is a catastrophic finding and indicates herniation of the brain.

  1. When pupillary dilation or loss of reactivity occurs as a result of CN III compression, both direct and indirect 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).
  2. If direct light reflex is lost but indirect light reflex is preserved, true CN III compression is less likley.  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. This suggests a problem with the sensory input to the right eye (called an afferent defect).  Diabetes, vascular conditions or eye diseases may cause afferent defects.


In consious 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:

  1. 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 right requires the right CN VI and the left CN III (to make the left eye move horizontally toward the nose). Downward gaze toward the nose is CN IV.

    The communication between CN III and VI to facilitate horizontal eye movement require communication between CN VIII (vestibular apparatus provides sensory input or head respositioning).  The sensory input enters the pons area of the brain where the right CN VI sends a message to the left CN III to work together for right horizontal eye gaze.  Impaired horizontal eye movement can occur as a result of problems in CN VIII, VI, III or the pons.


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 palbebral fissure).  Ptsosis causes the palbebral fissure to be narrowed (less white of the eye on the affected side).  The eyelid may have a puffy or thickened appearance.


Last Update: January 19, 2019
Brenda Morgan, CNS, CCTC