Which cranial nerve deficit results in the eye going down and out?

The eye going down and out is typically indicative of a deficit in cranial nerve III, also known as the oculomotor nerve. This nerve is responsible for innervating most of the extraocular muscles that control eye movement, including the superior rectus, inferior rectus, medial rectus, and inferior oblique muscles. Damage or dysfunction of this nerve leads to a loss of control over these muscles, resulting in the eye being unable to move properly, thus assuming a position of "down and out" due to the unopposed action of the lateral rectus (innervated by CN VI) and the superior oblique (innervated by CN IV).

In patients with CN III palsy, other associated features may also include ptosis (drooping eyelid) and pupillary dilation, as the parasympathetic fibers that constrict the pupil also travel with the oculomotor nerve and can be affected. Understanding the role of cranial nerve III in eye movement is crucial for diagnosing conditions related to ocular motility and can assist clinicians in localizing neurological deficits during examination.