12/13/2025

ENoG and EMG: when and why? How to read the results

Author: Designed by Freepik

This article is for information only and is not medical advice. It explains how clinicians use ENoG and EMG in facial nerve palsy based on scientific publications and guidelines. Decisions about testing and interpretation are always made by a qualified healthcare professional.

What is ENoG?

ENoG (electroneurography / electroneuronography) is a test that evaluates how well an electrical impulse travels along the facial nerve and how strong the resulting muscle response is.

In practice, ENoG measures the compound muscle action potential (CMAP) after the facial nerve is electrically stimulated and compares the affected side with the healthy side.

How is ENoG performed?

• A stimulating electrode is placed near the stylomastoid foramen (behind the ear).
• Recording electrodes are placed over selected facial muscles, for example near the nasolabial fold.
• The nerve is stimulated with a supramaximal electrical pulse and the device records the muscle response.
• The result is usually expressed as the percentage of amplitude on the paralyzed side compared with the healthy side (for example: 40%, 10%, etc.).

ENoG is an objective test - the traces can be stored and compared over time.

What is facial EMG?

EMG (needle electromyography) evaluates the electrical activity of facial muscles and, indirectly, the status of their innervation. In facial nerve palsy, typical target muscles include the frontalis, orbicularis oculi and orbicularis oris.

What happens during EMG?

• A very thin needle electrode is inserted into the muscle.
• At rest, the examiner looks for pathological spontaneous activity (for example fibrillation potentials, positive sharp waves) that indicate denervation.
• During voluntary movement (for example raising the eyebrows, closing the eye), the examiner assesses:
  • whether motor unit potentials (MUPs) are present,
  • itheir morphology (for example polyphasic units typical of reinnervation),
  • stopień rekrutacji (ile jednostek angażuje się przy wysiłku).

EMG therefore helps determine whether the muscle is still receiving nerve input, whether there is ongoing denervation, and whether there are signs of regrowth and reinnervation.

Why are ENoG and EMG used in facial nerve palsy?

Across reviews and guidelines, three main purposes are repeatedly mentioned:

1. To estimate the severity of nerve injury
   1. distinguishing milder conduction block (neuropraxia) from more severe axonal damage.
2. To give a prognosis for facial recovery
   1. estimating the likelihood of complete, partial or poor recovery,
   2. identifying patients at higher risk of persistent weakness or synkinesis.
3. To support treatment decisions
   1. selecting candidates for surgical decompression or repair in some cases of severe acute palsy,
   2. planning reconstructive strategies and more targeted rehabilitation.

The AAO-HNSF Bell’s palsy guideline states that electrodiagnostic testing may be offered to patients with complete facial paralysis, but clinicians should not perform electrodiagnostic testing in patients with incomplete paralysis.

When should ENoG be done?

The key factors are time since onset and degree of paralysis.

The 3-21 day window

In facial nerve palsy:

• Around 72 hours after injury, Wallerian degeneration reaches the distal extracranial segment of the nerve.
• Over the next 5-6 days, the ENoG response rapidly decreases.
• Around 21 days after onset, degeneration is essentially complete and the ENoG result usually does not change further.

Therefore:

• Too early (first 72 hours) - ENoG can be misleadingly normal, because degeneration has not yet progressed to the tested segment.
• The most useful prognostic information comes from day 3 to day 21 after symptom onset, especially in acute, complete paralysis.

In clinical practice, many protocols recommend performing the first ENoG around day 3 and then repeating it every 3-5 days to observe the trend.

When does ENoG make sense - and when not?

From guidelines and reviews:

• Main indication:
  • Acute, complete facial paralysis - ENoG helps estimate the proportion of fibers that have degenerated.
• ENoG is usually not performed when:
  • the palsy is incomplete and recovery begins quickly,
  • more than about 3-4 weeks have passed since onset – the result will not significantly change prognosis.

When should EMG be done?

ENoG and EMG are complementary, but their optimal timing differs:

• ENoG - most informative between day 3 and day 21 (acute, complete palsy, prognosis).
• EMG - becomes particularly useful after 10-14 days, when:
  • fibrillation potentials and positive sharp waves can appear as signs of denervation,
  • later, polyphasic reinnervation potentials appear as the nerve regrows.

Unlike ENoG:

• EMG remains useful beyond the first month - it can show ongoing denervation, reinnervation patterns, and synkinesis in chronic phases as well.

How to read an ENoG result - in simple terms

Typically, the report includes:

• the CMAP amplitude on the healthy side,
• the CMAP amplitude on the paralyzed side,
• a percentage ratio (for example 60%, 30%, 5%), indicating how much response remains compared with the healthy side.

You may also see:

• a “degeneration index” - the percentage loss of amplitude (the higher, the more severe the injury),
• prognostic thresholds taken from clinical studies.

Examples from the literature (these are research-based ranges, not direct rules for individual patients):

• Many protocols define “severe degeneration” as ≥90% loss of amplitude (paralyzed side <10% of healthy side) within a certain time window. This threshold has been used to identify patients at higher risk of poor outcome and, in some centers, to select candidates for surgical decompression in severe Bell’s palsy.
• Some newer analyses suggest that around 70-75% degeneration may already be associated with a higher risk of incomplete recovery, especially when combined with other unfavorable factors.

Important:

• ENoG alone does not distinguish a pure conduction block from deeper axonal damage; that is one reason why ENoG should be interpreted together with EMG findings and the clinical picture.

How to read an EMG result - in simple terms

In a facial nerve palsy report you may see, for each muscle:

1. Spontaneous activity at rest
   1. presence of fibrillation potentials and positive sharp waves → indicates active denervation (axonal injury).
2. Activity during voluntary effort
   1. no MUPs (no motor units) → typical of severe axonal loss with no voluntary recruitment,
   2. reduced recruitment but some MUPs present → some units are still functioning or reinnervating, often associated with a better prognosis,
   3. polyphasic, “messy” MUPs → suggest reinnervation (nerve fibers are re-establishing connections).
3. Relationship with ENoG

One study found that:

• Absence of voluntary MUPs on EMG was about 63% sensitive and 92% specific for ≥90% degeneration on ENoG, with a high positive predictive value for severe injury in which some teams consider surgical options.

This is not a simple “yes/no” rule for patients, but it illustrates how EMG:

• helps distinguish heavy axonal loss from milder damage,
• can detect the onset of reinnervation even before it is clearly visible clinically.

Points to remember when interpreting tests

1. Tests do not replace clinical assessment
   Reviews and guidelines emphasize that ENoG and EMG are supporting tools. Results must always be interpreted in the context of:
   1. time since onset,
   2. underlying cause (Bell’s palsy, trauma, tumor, etc.),
   3. neurological and otologic examination.
2. Not every patient needs ENoG/EMG
   The AAO-HNSF guideline states that:
   1. in incomplete palsy or with rapid early improvement, electrodiagnostic tests are generally not required,
   2. offering electrodiagnostic testing is an option mainly in acute, complete paralysis or when the course is atypical.
3. Protocols vary between centers
   Some centers perform serial ENoG/EMG early and systematically; others reserve them for selected cases (trauma, lack of improvement, surgical planning). This reflects differences in resources, experience, and local practice.

Summary: ENoG + EMG as pieces of the puzzle

• ENoG:
  • most useful between day 3 and day 21 after onset in acute, complete palsy,
  • expresses the percentage loss of CMAP amplitude,
  • primarily used for prognosis and, in some centers, for selecting candidates for surgery in very severe cases.
• EMG:
  • becomes informative from around 10-14 days onward and remains useful in later phases,
  • detects denervation, reinnervation and recruitment patterns,
  • helps assess long-term recovery potential and phenomena such as synkinesis.

For patients, the key message is:

• Numbers in an ENoG/EMG report never tell the whole story by themselves.
• The results must be translated into functional meaning by a clinician and the rehabilitation team - only then can they be connected to prognosis, exercise planning, or possible surgical options.

This text is meant to help you understand the terminology found in test reports, not to replace a medical consultation.