Which mechanical issue is most likely to produce a positive forced duction test in an eye movement exam following trauma?

• A. Orbital fracture with muscle entrapment
• B. CN VI palsy
• C. Thyroid eye disease
• D. Myasthenia gravis

Answer: (A)

A positive forced duction test means there is a true mechanical block preventing passive movement of the eye. After trauma, an orbital fracture can trap a rectus muscle within the fracture line, most often the inferior rectus, creating a physical restriction. When you attempt to move the eye passively in the restricted direction, you feel resistance and the eye cannot be moved, yielding a positive test. This mechanical entrapment is the key reason it’s the most likely cause in a traumatic setting.

In contrast, a CN VI palsy is a neural problem where the eye cannot move laterally due to nerve impairment, but passive movement typically remains possible, so the forced duction test would be negative. Thyroid eye disease and myasthenia gravis involve non-mechanical causes of motility limitation (fibrosis of muscles or fluctuating neuromuscular transmission, respectively) and are not the classic post-traumatic mechanical obstruction described here.