Topodiagnostic Tests of the Facial Nerve

Principle

1. Greater superficial petrosal nerve (GSPN) — at the geniculate ganglion
2. Nerve to stapedius — at the second genu (vertical/mastoid segment)
3. Chorda tympani — in the vertical mastoid segment
4. Terminal motor branches — after the stylomastoid foramen

The Five Topodiagnostic Tests

1. Schirmer Test (Lacrimal Function)

• Branch tested: Greater superficial petrosal nerve (GSPN) → parasympathetic innervation of the lacrimal gland via the pterygopalatine ganglion
• Technique: A strip of sterile filter paper is folded and placed in the conjunctival fornix of each eye. After 5 minutes, the length of the wetted portion is measured and compared bilaterally.
• Normal: Both sides produce roughly symmetric amounts of tears; the lesser response is generally >54% of the greater.
• Abnormal (positive): Affected side shows less than 50% of the lacrimation on the healthy side; or the total response (both eyes combined) is <25 mm.
• Modified Schirmer test: Uses a cotton thread stained with fluorescein placed in the lateral upper conjunctival sac for only 5–20 seconds — faster but equivalent in principle.
• Localization: An abnormal Schirmer test indicates the lesion is at or proximal to the geniculate ganglion (i.e., in the segment from the CPA to the geniculate ganglion, where motor and GSPN fibers travel together).
• Limitation: May be reduced bilaterally in Bell palsy due to subclinical involvement of other cranial nerves.

2. Stapedial (Acoustic) Reflex Test

• Branch tested: Nerve to the stapedius muscle — branches off just past the second genu in the vertical (mastoid) segment
• Technique: Standard impedance audiometry / tympanometry. In the setting of facial palsy, the test assesses the efferent (motor) limb of the reflex rather than the afferent auditory limb. Can be elicited by ipsilateral or contralateral acoustic, tactile, or electrical stimulation.
• Abnormal: Absent reflex, or amplitude less than 50% of the contralateral side.
• Localization: An absent stapedial reflex with intact lacrimation indicates the lesion is between the geniculate ganglion and the origin of the nerve to stapedius (i.e., in the tympanic segment). A present stapedial reflex places the lesion distal to its origin.
• Prognostic value: Recovery of the stapedial reflex within 2 weeks predicts complete facial recovery within 12 weeks; recovery within 4 weeks predicts recovery within 24 weeks.
• Fisch reported the stapedial reflex is absent in 69% of Bell palsy cases (84% when complete palsy).

3. Taste Testing / Electrogustometry (EGM)

• Branch tested: Chorda tympani — carries special visceral afferent (taste) fibers from the anterior two-thirds of the tongue
• Technique:
  • Psychophysical: Filter paper disks impregnated with NaCl (salty), saccharose (sweet), citric acid (sour), or quinine (bitter) are applied to each side of the tongue separately
  • Electrogustometry (EGM): Bipolar or monopolar electrical stimulation of the tongue at 4 µA–4 mA; threshold to perceive a taste sensation is compared bilaterally. Threshold difference >25% between sides is abnormal. A difference >20 dB across sides or no response is considered clearly abnormal.
• Localization: Abnormal taste with intact stapedial reflex localizes the lesion to the vertical (mastoid) segment between the stapedius branch and the chorda tympani takeoff. Normal taste places the lesion distal to the chorda tympani.
• Note: EGM of the soft palate (testing the palatine branch of the GSPN) may better reflect GSPN function than the Schirmer test.

4. Salivary Flow Test (Submandibular Flow)

• Branch tested: Chorda tympani — parasympathetic secretomotor fibers to the submandibular gland
• Technique (Magielski–Blatt test): Bilateral cannulation of the submandibular (Wharton's) ducts and comparison of stimulated flow rates using a radiolabeled tracer (technetium pertechnetate). Flow is stimulated (e.g., with 6% citric acid) and the percentage of flow on the affected side relative to the normal side is calculated.
• Abnormal: Flow ≤25% of the contralateral side (May & Hawkins criterion for surgical consideration); Ekstrand criterion: <45% after citric acid stimulation correlates with worse prognosis (89% accuracy for complete vs. incomplete recovery).
• Localization: Reduced submandibular flow implies a lesion at or proximal to where the chorda tympani leaves the main trunk (anywhere in the vertical/mastoid segment).
• Advantage over electrical tests: Salivary flow decreases earlier than threshold changes on nerve excitability testing, making it a potentially earlier prognostic indicator.
• Limitation: Time-consuming, uncomfortable for the patient, not widely used in routine practice.

5. Salivary pH

• Technique: Cannulation of the submandibular duct on the affected side; pH of saliva is measured.
• Abnormal: Submandibular salivary pH ≤6.1 predicts incomplete recovery in Bell palsy (accuracy ~91%); normal pH is ≥6.4. Reduced osmotic pressure also correlates with poorer recovery.
• Localization: Same principle as salivary flow — reflects chorda tympani function.
• Limitation: Very limited clinical experience; unclear whether it gives earlier prognosis than other tests.

Localizing the Level of Lesion: Summary Table

Clinical Limitations

"Although rarely employed today, topognostic tests are of historical interest and still are useful in illustrating relevant cranial anatomy." — Cummings Otolaryngology

1. Bell palsy: Typically a diffuse, mixed, partial lesion with varying degrees of demyelination across different fibers. Topodiagnostic tests therefore frequently give paradoxical or misleading results — the "most proximal lesion" principle does not apply when multiple levels are simultaneously affected.
2. Multiple lesions: Tests predict only the most proximal lesion; distal injuries are not identified.
3. Traumatic/focal lesions: Tests are more reliable but usually unnecessary, since imaging and clinical examination suffice.
4. Distal stimulation bias: All electrical tests (ENoG, NET) record distal to the lesion, requiring 3–5 days for Wallerian degeneration to propagate distally before becoming abnormal. Salivary flow testing becomes abnormal earlier.