Which single clinical finding best distinguishes a lesion of the facial nerve (CN VII) from a lesion of the trigeminal nerve (CN V)?

Explanation

## Distinguishing CN VII from CN V Lesions ### Motor vs. Sensory Dominance **Key Point:** CN VII is primarily a motor nerve to facial muscles; CN V is primarily sensory to the face with limited motor supply to muscles of mastication. ### Clinical Feature Comparison | Feature | CN VII Lesion | CN V Lesion | |---------|---------------|-------------| | **Eye closure** | **Lost** (motor to orbicularis oculi) | Intact | | **Corneal reflex** | Intact (afferent via CN V) | **Absent** (afferent limb affected) | | **Facial sensation** | Intact | **Lost** | | **Jaw deviation** | Normal | **Toward affected side** (motor to masseter/temporalis) | | **Taste (anterior 2/3 tongue)** | **Lost** | Intact | ### Why Eye Closure Distinguishes Them **High-Yield:** Inability to close the eye on the affected side (lagophthalmos) is pathognomonic for CN VII lesion because: 1. Orbicularis oculi is innervated exclusively by CN VII (temporal and zygomatic branches) 2. CN V has no motor innervation to this muscle 3. This finding is absent in pure CN V lesions **Clinical Pearl:** A CN VII palsy patient cannot voluntarily close the eye, and the eye rolls upward (Bell's phenomenon) when they attempt closure — a protective reflex that is preserved. ### Why Other Options Are Misleading - **Corneal reflex loss** occurs in CN V lesions (afferent limb), not CN VII - **Facial sensation loss** is a CN V finding, not CN VII - **Jaw deviation** reflects CN V motor involvement (muscles of mastication), not CN VII **Mnemonic:** **FACIAL** = **F**ace motor (CN VII), **A**fferent sensation (CN V), **C**orneal reflex needs both, **I**ncisors bite (CN V), **A**ll taste (CN VII), **L**agophthalmos (CN VII only)