A patient presents with inability to close the right eye and drooping of the right angle of mouth. The examiner notes that the right forehead wrinkles are preserved. At which level is the facial nerve lesion?

Explanation

## Localization of Facial Nerve Lesions ### Clinical Presentation Analysis The patient has: - **Inability to close right eye** → orbicularis oculi paralysis - **Drooping of right angle of mouth** → orbicularis oris paralysis - **Preserved right forehead wrinkles** → frontalis muscle is INTACT ### Key Point: Forehead Preservation = Distal Lesion **Key Point:** Preservation of forehead wrinkling (frontalis function) indicates the lesion is DISTAL to the branch that supplies the frontalis muscle. The frontalis is supplied by the **temporal branch** of the facial nerve, which arises from the parotid plexus — AFTER the posterior auricular nerve has already branched off at the stylomastoid foramen. ### Facial Nerve Branching Sequence (Peripheral) ``` Facial nerve exits stylomastoid foramen → Posterior auricular nerve (supplies occipitalis, auricular muscles) → Nerve to posterior belly of digastric & stylohyoid → Enters parotid gland → Parotid plexus → Temporal branch (supplies frontalis & orbicularis oculi upper) → Zygomatic branch (orbicularis oculi lower) → Buccal branch → Marginal mandibular branch (depressors of mouth) → Cervical branch ``` The **posterior auricular nerve** supplies the occipitalis and auricular muscles — it does **NOT** supply the frontalis. The frontalis is supplied by the **temporal branch**, which arises from the parotid plexus distal to the posterior auricular nerve. ### Why Option A is Correct A lesion **distal to the exit of the posterior auricular nerve** (i.e., within or distal to the parotid plexus) can selectively damage the zygomatic and buccal/marginal mandibular branches (causing loss of eye closure and mouth drooping) while sparing the temporal branch to frontalis — explaining preserved forehead wrinkling. ### Differential Localization | Lesion Location | Forehead | Eye Closure | Mouth | Taste | Lacrimation | Hyperacusis | |-----------------|----------|-------------|-------|-------|-------------|-------------| | **Internal acoustic meatus (D)** | Lost | Lost | Lost | Lost | Lost | Lost | | **Proximal to stapedius nerve (C)** | Lost | Lost | Lost | Lost | Preserved | Lost | | **Stylomastoid foramen (B)** | Lost | Lost | Lost | Preserved | Preserved | Preserved | | **Distal to posterior auricular nerve (A)** | **PRESERVED** | Lost | Lost | Preserved | Preserved | Preserved | ### Why Other Options Are Wrong - **Option B (Stylomastoid foramen):** A lesion here affects ALL peripheral branches including the temporal branch → forehead wrinkling would be LOST. - **Option C (Proximal to stapedius nerve):** Intratemporal lesion → forehead lost + hyperacusis present. - **Option D (Internal acoustic meatus):** Most proximal → all functions lost including lacrimation and taste. **Clinical Pearl:** In peripheral (lower motor neuron) facial nerve palsy, forehead sparing localizes the lesion to the parotid plexus or its terminal branches — distal to the posterior auricular nerve. This contrasts with upper motor neuron lesions (e.g., stroke) where forehead is spared due to bilateral cortical representation of the frontalis. [cite: Clinically Oriented Anatomy, Moore et al., 8th ed., Ch. 8; Gray's Anatomy for Students, 3rd ed.]