A 58-year-old man presents with loss of taste on the anterior 2/3 of the tongue and hyperacusis (sensitivity to loud sounds). Which cranial nerve lesion best explains both findings?

Explanation

## CN VII Lesion: Taste + Hyperacusis ### Anatomy of CN VII (Facial Nerve) **Key Point:** CN VII carries three functional components: motor (facial muscles), parasympathetic (lacrimal and salivary glands), and special sensory (taste to anterior 2/3 tongue via chorda tympani). ### Branches and Functions of CN VII | Branch/Component | Function | Lesion Effect | |------------------|----------|---------------| | **Motor** | Facial muscles, stapedius | Facial droop, loss of eye closure | | **Chorda tympani** | Taste (anterior 2/3 tongue), parasympathetic to submandibular/sublingual glands | Loss of taste, dry mouth | | **Stapedial nerve** | Innervates stapedius muscle | Hyperacusis (loss of dampening) | | **Greater petrosal nerve** | Parasympathetic to lacrimal gland | Dry eye | ### Why CN VII Explains Both Findings **Loss of taste on anterior 2/3 tongue:** - CN VII carries the chorda tympani branch - Chorda tympani is the **only** nerve supplying taste to the anterior 2/3 of the tongue - CN IX (glossopharyngeal) supplies taste to the posterior 1/3 - A CN VII lesion proximal to the chorda tympani branch abolishes taste anteriorly **Hyperacusis (sensitivity to loud sounds):** - CN VII innervates the stapedius muscle via the stapedial nerve - Stapedius dampens vibrations of the stapes in response to loud sounds (acoustic reflex) - Lesion of CN VII → paralysis of stapedius → loss of dampening → **hyperacusis** - This is a classic and highly specific finding for CN VII lesion **High-Yield:** The combination of **loss of taste (anterior 2/3 tongue) + hyperacusis** is pathognomonic for CN VII lesion. Hyperacusis is rarely tested but is a high-yield discriminator. **Mnemonic:** **TASTE-STAPED** — CN VII carries **TASTE** (chorda tympani) and innervates **STAPEDIUS** (hyperacusis when paralyzed). **Clinical Pearl:** In Bell's palsy (idiopathic CN VII palsy), patients often report that normal sounds seem unbearably loud on the affected side — this is hyperacusis and confirms CN VII involvement. ### Why Other Nerves Do Not Fit | Nerve | Taste Function | Stapedius | | |-------|---|---|---| | **CN IX** | Posterior 1/3 tongue only | No | | | **CN X** | Epiglottis/pharynx only | No | | | **CN XII** | No taste function | No | | [cite:Clinically Oriented Anatomy 8e Ch 9]