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.īrainstem and spinal reflexes can be ilicited in conscious or unconcious states as long as the neuronal pathways are intact. Involuntary CN III function is produced through brainstem reflexes. Voluntary function includes the intentional movement of the eyes toward a specific direction and opening of the eyelids. 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).ĬN III is a pure motor nerve that has both voluntary and involuntary function. CN III dysfunction causes the eyelid on the affected side to become "droopy". 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).Pupil dilation and loss of reactivity to light (inability of the pupil to constrict to light).Loss of CN III function causes the following abnormalities ON THE SAME SIDE (ipsilateral): What does loss of 3rd Cranial Nerve function cause? Correlation of pupil abnormalities with clinical findings, history and trends is important. Direct eye injuries can produce CN III abnormalities.
Pupillary abnormalities can also be chronic. This can lead to problems with eye movement or eyelid opening where pupil function is spared. The deep blood supply of CN III can become impaired by vascular diseases, hypertension or diabetes. 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).
By contrast, the CN III control of eye movement and eyelid opening runs deep in the centre of the nerve. 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. A new and sudden finding of pupillary dilation and loss of reactivity suggests supratentorial herniation or expanding volume at the top of the brainstem. What is the significance of monitoring for CN III function?Īcute loss of CN III function is an important sign of a raised intracranial pressure with expanding mass lesion. You can remember this function because the Oculomotor nerve starts with the letter "O" for eye "O"pening. Eyelid Elevation Cranial Nerve III also controls the ability to open the eyelid.
It activated the medial rectus, superior rectus, inferior rectus and inferior oblique muscles to cause orbital rotation. The ability to move the eye in all other directions is controlled by the 3rd cranial nerve. Cranial Nerve IV (Trochlear) controls downward eye movement toward the nose, and Cranial Nerve VI (Abducens) controls horizontal eye movement toward the temple. Eye Movement The 3rd cranial nerve also controls eye muscle movement. The parasympathetic response of the pupil (or "return to normal") is constriction. Pupil Constriction Each one of the two 3rd cranial nerves controls the parasympathetic response of the pupil on the same side (ipsilateral). They control eye muscles on the same side of the body (ipsilateral). They are Lower Motor Neurons (LMN) (second order neurons). The 3rd cranial nerves are pure motor nerves. The two 3rd cranial nerves (oculomotor nerves) are located at the top of the brainstem - one to the right and one to the left.
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