Right $3^{rd}$ Nerve Palsy Explained

The symptoms described – a right eye deviated downwards and outwards with a dilated pupil – are characteristic of a lesion affecting the right $3^{rd}$ cranial nerve (Oculomotor nerve).

Understanding $3^{rd}$ Nerve Function

• The $3^{rd}$ nerve controls several muscles responsible for eye movement: superior rectus, inferior rectus, medial rectus, and inferior oblique. It also controls the eyelid (levator palpebrae superioris).
• Crucially, it carries parasympathetic fibers that cause pupil constriction.

Symptom Correlation

• Downward and Outward Deviation: When the $3^{rd}$ nerve is damaged, the muscles it controls are weakened or paralyzed. The unopposed action of the $4^{th}$ nerve (superior oblique, causing intorsion and depression) and the $6^{th}$ nerve (lateral rectus, causing abduction) results in the eye turning downwards and outwards.
• Dilated Pupil (Mydriasis): Damage to the $3^{rd}$ nerve disrupts the parasympathetic fibers responsible for pupil constriction. This leads to a dilated and often unresponsive pupil.
• Affected Side: The symptoms specifically mention the "right eye," indicating a palsy of the right $3^{rd}$ nerve.

Why Other Options Are Incorrect

• Left $3^{rd}$ Nerve Palsy: Symptoms would affect the left eye.
• Left $6^{th}$ Nerve Palsy: Would cause inward deviation (esotropia) of the left eye, not downward/outward deviation of the right eye, and wouldn't typically cause pupil dilation.
• Right $4^{th}$ Nerve Palsy: Primarily affects upward gaze and causes torsional issues, leading to upward deviation, not downward/outward. It does not involve pupil control.

Therefore, the clinical presentation directly points to a right $3^{rd}$ nerve palsy.