The R/D ratio isolates which compartment carries the facial nerve conduction delay: it divides the R1 latency of the Blink Reflex (full trigemino–facial reflex arc, intracranial) by the direct CMAP latency (stimulation at the stylomastoid foramen, distal extracranial conduction).
R/D = R1 latency (ms) / Direct CMAP latency (ms)
Mandatory technical rule
Both latencies (R1 and direct CMAP) must be recorded exclusively from the orbicularis oculi. Using the nasalis muscle (common in Bell's palsy prognostic protocols) invalidates the ratio, because it breaks the anatomical-distance proportionality used in the 3.6 ± 0.5 reference.
Right (R)
Mandatory recording site: orbicularis oculi
Left (L)
Mandatory recording site: orbicularis oculi
Topographic interpretation
About this index and references
The R/D ratio (Reflex/Direct) is a neurophysiological tool that quantifies how much of the facial nerve conduction time occurs inside the skull (intracranial/intratemporal) versus outside the skull (distal extracranial). Dividing R by D isolates the compartment where conduction is most delayed.
1. Physiological rationale
R — Reflex (long afferent + efferent loop)
The R1 latency represents the full trigemino–facial reflex arc: stimulus at the supraorbital nerve (V), entry into the brainstem, synapse at the facial nucleus, and descent along the entire facial nerve down to the recording muscle.
D — Direct (short efferent loop)
The direct CMAP latency is obtained by stimulating the facial nerve right after it exits the skull (stylomastoid foramen or parotid region), reflecting exclusively distal peripheral conduction.
2. Mandatory technical rule (recording electrode)
For the ratio to preserve the anatomical-distance proportionality, both latencies (R1 and direct CMAP) must be recorded from the orbicularis oculi. In Bell's palsy prognostic protocols the nasalis is frequently used for amplitude/axonal viability — but the literature is categorical: the nasalis CMAP latency must not be used in the R/D ratio. Mixing R1 (orbicularis oculi) with CMAP from the nose invalidates the 3.6 ± 0.5 reference.
3. Formula and parameters
R/D = R1 latency (ms) / Direct CMAP latency (ms)
- Expected normal value: 3.6 ± 0.5
- Normal range (± 2 SD): 2.6 to 4.6
4. Result interpretation
R/D > 4.6 — Increased (proximal / intracranial lesion)
R1 increased out of proportion to the CMAP. Slowing occurs in the segment before the stylomastoid foramen. Clinical considerations: Bell's palsy (intratemporal edema), Miller-Fisher syndrome, brainstem lesions.
R/D < 2.6 — Decreased (distal / extracranial lesion)
The direct CMAP is disproportionately prolonged relative to R1. Slowing occurs after the stylomastoid foramen. Suggests systemic demyelinating polyneuropathy (e.g., Charcot-Marie-Tooth — CMT), generally not in isolated facial neuropathy.
R/D 2.6 – 4.6 — Normal
Proportion preserved. If R1 and CMAP are both prolonged but the ratio stays within range, the lesion involves the facial nerve uniformly along its entire length.
5. Clinical application
The R/D ratio is a topographic localization tool — always interpret alongside clinical assessment and the rest of the neurophysiological study (distal latencies, amplitudes, ipsi- and contralateral R1/R2, full motor conduction, facial EMG). It does not replace the complete examination.
References:
· Kimura J. Electrodiagnosis in Diseases of Nerve and Muscle: Principles and Practice. 4th ed. Oxford University Press; 2013.
· Aramideh M, Ongerboer de Visser BW. Brainstem reflexes: electrodiagnostic techniques, physiology, normative data, and clinical applications. Muscle Nerve. 2002;26(1):14–30.
· Valls-Solé J. Neurophysiological assessment of trigeminal nerve reflexes in disorders of central and peripheral nervous system. Clin Neurophysiol. 2005;116(10):2255–2265.
· Preston DC, Shapiro BE. Electromyography and Neuromuscular Disorders: Clinical–Electrophysiologic Correlations. 4th ed. Elsevier; 2021. (Chapters on Blink Reflex and facial nerve studies.)