Which nerve is most commonly affected by trauma causing a non-comitant deviation?

• A. CN III
• B. CN IV
• C. CN VI
• D. CN II

Answer: (B)

Noncomitant deviations are gaze-dependent misalignments that arise when a single extraocular muscle is weak, so the amount and direction of misalignment change with where you look. Trauma commonly injures the nerve that innervates the superior oblique muscle, the trochlear nerve. When CN IV is affected, the eye cannot depress properly in adduction, producing a hypertropia that worsens in certain gaze positions and with head tilt. This creates vertical diplopia that varies with gaze—a classic noncomitant pattern. The superior oblique palsy from CN IV also often leads to torsional misalignment, reinforcing the gaze-dependent nature of the deviation. In contrast, CN III palsy tends to present with broader signs such as ptosis and a more global eye position change, and CN VI palsy typically yields horizontal diplopia with a different pattern of gaze dependence, making the CN IV pattern the best match for trauma-related noncomitant deviation.