F-Waves, H-Reflex, Blink Reflex

F-waves, H-reflexes, and the blink reflex extend nerve conduction studies to test proximal nerve segments, polysynaptic spinal reflexes, and cranial nerve circuits — regions inaccessible to standard motor and sensory NCS. These late responses provide complementary information particularly valuable for proximal demyelinating neuropathy, S1 radiculopathy, brainstem function, and facial nerve disorders. This page covers F-wave technique and interpretation, H-reflex methodology, the blink reflex, and the clinical settings where each adds diagnostic value.

F-Wave

Physiology

  • Motor axons are stimulated antidromically (impulse travels proximally to the cell body).
  • A small percentage of motor neurons backfire orthodromically, sending a second impulse down the same axon.
  • This produces a small late response after the CMAP.
  • F-wave tests motor nerve conduction over the entire length of the nerve, including proximal segments.

Recording

  • Standard motor NCS setup.
  • Same supramaximal stimulation as motor NCS.
  • Sweep speed adjusted for late response: 10–20 ms/div.
  • 10–20 consecutive stimuli to record multiple F-waves.

Measurements

  • F-wave latency: time from stimulus to onset of F-wave.
  • Shortest F-wave latency: most consistent measure; typical normal.
  • F-wave persistence: percentage of stimuli that produce a measurable F-wave.
  • F-wave chronodispersion: difference between shortest and longest F-wave latency.
  • F-M ratio: F-wave latency / CMAP latency; corrects for limb length.

Normal Values (Approximate, Adult)

Nerve Shortest F-wave latency (ms)
Median (record APB) ≤30
Ulnar (record ADM) ≤30
Peroneal (record EDB) ≤55
Tibial (record AH) ≤55

(Adjust for height and lab norms.)

Clinical Applications

  • Demyelinating neuropathy: prolonged F-wave latency, sometimes the first abnormality.
  • GBS: prolonged or absent F-waves often before distal latency or CV changes.
  • CIDP: prolonged F-waves; chronodispersion increased.
  • Radiculopathy: F-wave may be prolonged if severe radiculopathy with proximal demyelination; usually requires multiple roots.
  • Proximal nerve injury: F-wave changes when distal NCS is normal.

Interpretation Caveats

  • F-waves are variable — small differences are normal.
  • Reduced persistence is significant only if marked (e.g., F-waves present in <50% of stimuli when normally present in ≥80%).
  • Side-to-side asymmetry >3 ms is usually pathologic.
  • Height affects F-wave latency — taller patients have longer F-waves.
  • Aging affects F-waves modestly.

H-Reflex

Physiology

  • Electrical analog of the muscle stretch (ankle) reflex.
  • Submaximal stimulation activates Ia afferent fibers (low-threshold sensory).
  • Ia fibers monosynaptically excite α-motoneurons in the spinal cord.
  • Result: muscle response (H-reflex) follows the latency of the Ia afferent + synaptic delay + motor efferent + neuromuscular junction.
  • Tests primarily the S1 reflex arc when recorded from soleus or gastrocnemius.

Recording

  • Patient prone (or seated with leg flexed).
  • Stim at popliteal fossa over tibial nerve.
  • Active electrode over soleus muscle (gastrocnemius for some protocols).
  • Reference over Achilles tendon.
  • Start with LOW stimulation intensity, increase gradually.
  • H-reflex emerges first at low intensity; CMAP increases at higher intensity.
  • H-reflex disappears at supramaximal stimulation (antidromic motor impulses collide with reflex efferent).

Measurements

  • H-reflex latency: time from stimulus to onset of H-reflex.
  • Normal latency: 28–35 ms in adults (soleus); varies with limb length.
  • Side-to-side comparison: latency difference >1.5 ms usually pathologic.
  • Amplitude: less reliable than latency due to variability.

Clinical Applications

  • S1 radiculopathy: prolonged or absent H-reflex with normal distal NCS.
  • Peripheral neuropathy: H-reflex may be prolonged or absent due to peripheral or proximal involvement.
  • Spinal cord lesion: H-reflex may be hyperreflexic with reduced presynaptic inhibition.
  • Bilateral absence of H-reflexes: peripheral neuropathy, severe S1 radiculopathies, or alcoholic neuropathy.

Blink Reflex

Physiology

  • Stim of supraorbital branch of trigeminal nerve (V1) on one side.
  • Two responses recorded from orbicularis oculi:
    • R1: ipsilateral early response (8–13 ms); pontine reflex via short trigeminal-facial chain.
    • R2: ipsilateral and contralateral late response (28–40 ms); medullary reflex via lateral medullary tract.
  • Tests trigeminal (V1) afferent, brainstem, and facial nerve efferent pathways.

Recording

  • Stim supraorbital nerve.
  • Record bilaterally from orbicularis oculi.
  • Filters: 20 Hz low-frequency, 10 kHz high-frequency.
  • Sweep: 10–20 ms/div.
  • 10–20 repetitions; analyze averaged or representative trace.

Measurements

  • R1 latency (ipsilateral).
  • R2 latency (ipsilateral and contralateral).
  • Side-to-side comparison.

Normal Values (Approximate)

  • R1: 10–13 ms.
  • R2 ipsilateral: 30–35 ms.
  • R2 contralateral: 30–37 ms.
  • Side-to-side difference: <1 ms for R1, <3 ms for R2.

Clinical Applications

  • Bell’s palsy and other facial nerve disorders: prolonged or absent R1 and R2 on affected side; helps localize injury and assess severity.
  • Trigeminal nerve lesions: prolonged or absent R1 and R2 when stimulating the affected V1 branch.
  • Brainstem lesions: variable patterns:
    • Pontine lesion: prolonged R1 alone.
    • Medullary lesion: prolonged R2 alone or asymmetrically.
    • Lateral medullary stroke (Wallenberg): characteristic R2 abnormalities.
  • Multiple sclerosis: blink reflex abnormalities common (asymmetric R1 or R2).
  • Acoustic neuroma: V1 may be affected by cerebellopontine angle mass.

Trigeminal SSEP (Brief)

  • Sometimes performed for V2 or V3 evaluation.
  • Stimulate the trigeminal nerve branch and record over the contralateral scalp.
  • Useful in select cases of suspected trigeminal pathway disease.

Tibial Reflex Studies in Specific Situations

Long Loop Reflexes

  • Stim peripheral nerve, record voluntary muscle response after a delay.
  • Tests cortico-spinal loop.
  • Used in research and some clinical settings (e.g., bulbar dysfunction in ALS, distinguishing functional from organic).

Practical Use of Late Responses

Suspected GBS

  • F-wave abnormalities (prolonged latency, absent F-waves) often the first NCS finding.
  • Even with normal distal latencies and CVs, abnormal F-waves can confirm proximal demyelination.

Suspected CIDP

  • F-wave prolongation, increased chronodispersion.
  • Combined with distal NCS findings for EFNS/PNS criteria.

Suspected S1 Radiculopathy

  • H-reflex prolonged or absent.
  • Distal motor/sensory NCS typically normal.
  • Combine with EMG findings.

Bell’s Palsy Prognostication

  • Blink reflex (R1 latency, R1 amplitude) tracks recovery.
  • If R1 returns by 7 days: good prognosis.
  • If R1 absent >14 days: poorer prognosis.
  • Combine with motor NCS of facial nerve.

Brainstem Stroke Localization

  • Blink reflex helps localize within pons (R1) vs medulla (R2).
  • Combined with imaging.

🔍 Did You Know?

In acute Guillain-Barré syndrome (GBS), F-wave abnormalities are often the EARLIEST NCS finding — sometimes the only finding in the first few days of illness before distal latencies, conduction velocities, or amplitudes change measurably. This is because GBS pathology starts proximally (at the nerve roots and proximal nerve segments) before affecting more distal segments. The F-wave, which tests the entire length of the nerve including proximal portions, captures this early proximal demyelination. The clinical implication is profound: in a patient with rapid ascending weakness, a normal distal NCS does not exclude GBS — measurement of F-wave latencies (or persistence) is essential. A patient with progressive weakness, areflexia, and prolonged F-waves with normal distal latencies has GBS until proven otherwise, and treatment with IVIG or plasmapheresis should not be delayed waiting for distal NCS changes. For electrodiagnosticians, the lesson is that routine NCS without F-waves can miss early GBS; for practicing neurologists, the take-home is that “the NCS was normal” should never end the workup if the clinical picture suggests GBS — ask specifically whether F-waves were measured and what they showed. The same principle applies to CIDP and other proximal demyelinating neuropathies. The F-wave is a humble test technically but profoundly important diagnostically.

Pitfalls and Pearls

  • F-wave physiology: antidromic motor stimulation → backfire from motor neurons → small late response.
  • F-wave tests entire nerve length: including proximal segments.
  • Shortest F-wave latency: most consistent measure.
  • F-wave persistence: % of stimuli producing F-wave; <50% suggestive of pathology.
  • F-wave in early GBS: often the first NCS abnormality.
  • F-wave in CIDP: prolonged latency, increased chronodispersion.
  • F-wave height-adjusted: F-M ratio corrects for limb length.
  • H-reflex: electrical analog of ankle reflex; tests S1 reflex arc.
  • H-reflex recorded from soleus: with submaximal stim at popliteal fossa.
  • H-reflex side-to-side >1.5 ms: usually pathologic.
  • S1 radiculopathy: prolonged/absent H-reflex with normal distal NCS.
  • Blink reflex R1: 10–13 ms ipsilateral; pontine.
  • Blink reflex R2: 30–35 ms ipsilateral; 30–37 ms contralateral; medullary.
  • Bell’s palsy: blink reflex prognosticates recovery.
  • Brainstem stroke localization: R1 (pons) vs R2 (medulla).
  • Always combine late responses with standard motor and sensory NCS.
  • Side-to-side comparison: more useful than absolute values for many findings.

References

  1. Kimura J. Electrodiagnosis in Diseases of Nerve and Muscle: Principles and Practice. 4th ed. Oxford University Press; 2013.
  2. Preston DC, Shapiro BE. Electromyography and Neuromuscular Disorders. 4th ed. Elsevier; 2021.
  3. Fisher MA. F-waves—physiology and clinical uses. ScientificWorldJournal. 2007;7:144-160.
  4. Burke D, Adams R, Skuse N. The effects of voluntary contraction on the H reflex of human limb muscles. Brain. 1989;112(Pt 2):417-433.
  5. Aramideh M, Ongerboer de Visser BW. Brainstem reflexes: electrodiagnostic techniques, physiology, normative data, and clinical applications. Muscle Nerve. 2002;26(1):14-30.