Electroneuronography in determining the prognosis of Bell’s Palsy

Bell’s palsy or idiopathic facial nerve palsy is the commonest cause of facial nerve paralysis. It is sudden in onset and unilateral. The cause of Bell’s palsy remains controversial. Inflammation of the facial nerve and subsequent compression of the facial nerve within the facial canal has been suggested as a cause. Infection by Herpes Simplex type 1 virus has also been implicated as a causal factor. Bell’s palsy is a lower motor neuron type of facial palsy.

Left sided Bell's Palsy. From eMedicine

In cases of Bell’s palsy associated with neuropraxia of the facial nerve there is full recovery of the facial nerve function. The management consists of medical and surgical interventions. Surgical management involves decompression of the facial nerve to remove the pressure on the nerve. Electroneuronography has been suggested as a prognostic test in case of facial nerve palsy and to determine if surgical intervention is needed. The prognostic value of electroneuronography has been investigated by Danielides V et al. in a study.

What is electroneuronography? It is an objective test to determine the number of functioning nerve fibers of the affected side in comparison to the unaffected side. The facial nerve is stimulated supramaximally on both the affected and the normal side. The amplitudes of the elicited muscle summation potentials are then measured and compared. The peak to peak amplitude is directly proportional to the number of intact nerve fibers and hence gives a measure of the neuronal degeneration. In the study, the usefulness of electroneuronography was determined using House-Brackmann facial nerve grading system as the criteria. The House- Brackmann facial nerve grading system is as follows –

• Grade I. Normal – Normal facial appearance and function in all areas.
• Grade II Mild dysfunction – slight dysfunction noticeable only on close inspection, normal symmetry and tone at rest. Forehead – moderate to good function. Eye – incomplete closure. Mouth – slight asymmetry.
• Grade III. Moderate dysfunction – obvious but not disfiguring asymmetry, normal symmetry and tone at rest. Forehead – slight to moderate movement. Eye – complete closure with effort. Mouth – slightly weak motion with maximum effort.
• Grade IV. Moderately severe dysfunction – obvious weakness with possible dysfunction. Forehead – no motion. Eye – incomplete closure. Mouth – asymmetric with maximum effort.
• Grade V. Severe dysfunction – only minimally perceptible motion, asymmetry at rest. Forehead – no motion. Eye – incomplete closure. Mouth – slight movement.
• Grade VI. Total paralysis – no movement at all at any level, obvious asymmetry at rest.

The study found electroneuronography to be a good prognostic indicator of facial nerve palsy. 97% subjects whose muscle action potential were within 51%-95% of normal made grade I or full recovery of the facial nerve, with degree of recovery decreasing with decreasing muscle action potential as determined by electroneuronography. The accuracy of electroneuronography in predicting the prognosis was 97.6%. Hence electroneuronography is an useful prognostic test to determine whether early surgical intervention is needed or medical intervention will suffice. Surgical decompression is recommended when the intact facial nerve fibers falls below 10% of the normal as determined by electroneuronography, measured within 14 days of onset of symptoms.

Reference:

Danielides, V., Skevas, A., & Cauwenberge, P. (1996). A comparison of electroneuronography with facial nerve latency testing for prognostic accuracy in patients with Bell's palsy European Archives of Oto-Rhino-Laryngology, 253 (1-2) DOI: 10.1007/BF00176700