Localising a Facial Nerve Palsy — Topognostic Testing

The facial nerve gives off its branches at fixed points along its course through the temporal bone (see the intratemporal course and the branches). That fixed anatomy is what makes a facial palsy localisable: a lesion knocks out every branch below it and spares every branch above it, so the pattern of what is lost and what is preserved points to the level of injury. Testing those functions — lacrimation, the stapedial reflex, and taste — is topognostic (or topodiagnostic) testing. It matters most in traumatic and iatrogenic palsy, where the level guides whether and where to explore the nerve.

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The three testable branches, from proximal to distal

Working down the intratemporal course, three branches are given off in order, and each drives a bedside or audiological test:

• Greater petrosal nerve (GSPN) — leaves at the geniculate ganglion and carries secretomotor fibres to the lacrimal gland. Tested by lacrimation (Schirmer test).
• Nerve to stapedius — leaves in the tympanic/mastoid segment. Tested by the acoustic (stapedial) reflex on tympanometry.
• Chorda tympani — leaves in the mastoid segment, above the stylomastoid foramen, and carries taste from the anterior two-thirds of the tongue plus secretomotor fibres to the submandibular and sublingual glands. Tested by taste (gustometry) and, historically, salivary flow.

Below the chorda tympani the nerve is purely motor to the face, so a lesion there affects movement only.

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The tests

Schirmer test (lacrimation). A strip of filter paper is placed in the lower conjunctival fornix of each eye and the wetted length compared after five minutes. The test is taken as positive — implying a lesion at or proximal to the geniculate ganglion — when the affected side produces less than half the lacrimation of the normal side, or when the combined wetted length of both eyes is under about 25 mm.

Stapedial (acoustic) reflex. Measured on the tympanometer. Loss of the reflex on the affected side localises the lesion at or proximal to the nerve to stapedius; an intact reflex places it distal to that branch. Its absence also correlates with a more proximal, often more severe injury.

Taste (gustometry). Taste on the anterior two-thirds of the tongue is tested with the four modalities (or electrically, electrogustometry). Impaired taste implicates the chorda tympani and places the lesion at or proximal to it.

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The level-by-level pattern

Reading the three tests together brackets the lesion between two branches:

Level of lesion	Lacrimation (Schirmer)	Stapedial reflex	Taste (chorda tympani)	Facial movement
At / proximal to geniculate ganglion (IAC, geniculate)	Reduced	Lost	Lost	Lost
Between geniculate and stapedius (tympanic segment)	Normal	Lost	Lost	Lost
Between stapedius and chorda tympani (mastoid segment)	Normal	Normal	Lost	Lost
Distal to chorda tympani (at/below stylomastoid foramen)	Normal	Normal	Normal	Lost

The logic is one-directional: a lesion abolishes every function whose branch leaves below it and spares every function whose branch leaves above it. So the highest-preserved function marks the lower boundary of the lesion, and the lowest-lost function marks its upper boundary.

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Where topognostic testing helps — and its limits

Topognostic testing is most useful in traumatic and iatrogenic palsy, where knowing the level informs surgical decision-making, and in teaching the anatomy. It is less decisive in Bell’s palsy, where the lesion is typically at or around the geniculate/labyrinthine segment and the tests often localise there regardless, adding little to management.

Two caveats matter. First, the tests assess the level, not the severity — for prognosis and the decision to decompress, electrophysiological tests (electroneuronography and electromyography) are what quantify the degree of degeneration. Second, high-resolution imaging (CT of the temporal bone for trauma or bony pathology, MRI with contrast for tumour or Bell’s/Ramsay Hunt) has largely superseded topognostic testing for anatomical localisation — but the underlying scheme remains the clearest way to understand why a facial palsy presents the way it does, and it is a staple of otology examinations.

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Frequently Asked Questions

What does topognostic (topodiagnostic) testing mean? It is the use of functional tests — lacrimation, the stapedial reflex, and taste — to localise the level of a facial nerve lesion along its intratemporal course, based on which branches are affected and which are spared.

How does the Schirmer test localise the lesion? The greater petrosal nerve, which supplies the lacrimal gland, leaves the facial nerve at the geniculate ganglion. Reduced lacrimation on the affected side therefore implies a lesion at or proximal to the geniculate ganglion. It is regarded as positive when the affected side wets less than half the normal side, or the two eyes together wet under about 25 mm.

If taste and the stapedial reflex are normal but the face is paralysed, where is the lesion? Below the chorda tympani — at or beyond the stylomastoid foramen — where the nerve is purely motor. All the branches tested leave above that point, so their functions are preserved while facial movement is lost.

Does topognostic testing tell you the prognosis? No. It localises the level of the lesion, not its severity. Prognosis and the decision to decompress rest on electrophysiological testing (electroneuronography/EMG), which measures how much of the nerve has degenerated.

Is topognostic testing still used now that we have MRI and CT? Imaging has largely taken over anatomical localisation, especially for tumours and trauma. But the topognostic scheme remains the clearest framework for understanding the clinical picture of a facial palsy, and the individual tests (Schirmer, stapedial reflex) still contribute in selected cases — which is why it endures in otology training and examinations.