Why Intraoperative Neurophysiological Monitoring in MVD surgery, to treat Hemifacial spasm.

Introduction about Monitoring in MVD surgery, to treat Hemifacial spasm.

• Hemifacial spasm (HFS) is a neurological disorder characterized by the irregular and involuntary contractions of the facial muscles innervated by the ipsilateral facial nerve.

Pathophysiology .

• The pathogenesis of Hemi Facial Spasm {HFS} may involve vascular compression of the facial nerve at its origin near the brainstem {pontomedullary} resulting in ephaptic transmission, reason to this abnormal transmission most likely hyperexcitability of the facial nucleus is considered the central mechanism.

Aim of surgery.{MVD}

• Microvascular decompression (MVD) surgery is a treatment option for hemifacial spasm caused by vascular compression of the facial nerve.

• During MVD surgery a neurosurgeon makes a small incision behind the ear and creates an opening in the skull to access the affected nerve.

• The surgeon then identifies the compressing blood vessel and places a cushioning material or such as a Teflon pad, between the vessel and the nerve to relieve the pressure.

• This procedure aims to resolve the compression of the facial nerve, thereby alleviating the symptoms of hemifacial spasm.

Neurophysiological Monitoring.{MVD}

• Intraoperative neurophysiological monitoring represents a valuable tool in performing MVD for HFS . Neuromonitoring Lateral Spread Response {LSR} Modality is a valuable tool make it is possible to ensure the successful decompression of the culprit vessel from the facial nerve root exit zone.

• This is particularly valuable in cases where the culprit vessel is not evident in the surgical field. Second the surgical manipulation of the cranial nerves, perforating vessels ,and brainstem parenchyma may expose patients to a substantial risk of neurological injury, Vestibulocochlear nerve injury resulting in hearing impairment is the main concern in the MVD of the facial nerve for the treatment of HFS.

• From this perspective, Lateral Spread Response (Abnormal Muscle Response).The lateral spread response (LSR) is used to identify the responsible blood vessel and ensure its thorough decompression from the root exit zone of the facial nerve during MVD for HFS.

• In HFS patients the LSR is recorded intraoperatively once the culprit vessel is detached and adequately cushioned to prevent recompression of the facial nerve, the LSR typically disappears within seconds to minutes. The intraoperative disappearance of the LSR has been associated with favorable clinical outcomes.

• IONM still plays a supplementary role, in practice many crucial decisions during surgery rely more on the operator’s experience than on IONM findings.

Lateral Spread Response (Abnormal Muscle Response){MVD}

• The lateral spread response (LSR) can be useful to identify the responsible blood vessel and ensure its thorough decompression from the root exit zone of the facial nerve during MVD for HFS. In maximum of HFS patients the LSR is recorded intraoperatively signal shows Once the culprit vessel is detached and adequately cushioned to prevent re-compression of the facial nerve, the LSR typically reduce or disappears within couple of minute, The findings intraoperative disappearance of the LSR has been associated with favorable clinical outcomes in the postoperative period, although its relationship with long term outcomes .

Image of Lateral spread response (LSR)

To record this signal, image first upper trace recorded from orbicularis oculi muscle and bottom trace recorded from Mentalis muscle , Stimulation electrode kept only Zygomatic arch .For recording and stimulation were used subdermal standard 3 Centimeter long needle electrode.

Bottom the typical delayed electrical activity in the mentalis muscle following stimulation of the temporal branch of the facial nerve, LSR Reduced to disappears during the procedure when the culprit vessel is detached from the facial nerve.

Stimulation and recording technique.

• The LSR usually acquired on the affected side of the face using twisted bipolar needle electrode electrodes for both stimulation and recording purpose a twisted electrode pair can be placed over the orbicularis oculi and mentalis muscles for recording. Another electrode pair for stimulation would be preferred intradermally, approximately 1 cm apart along the zygomatic branch of the facial nerve, which innervates the orbicularis oculi muscle. While there may be some interpersonal variation, the typical location for the stimulation electrode is at the midpoint of an imaginary line between the tragus and the outer corner of the ipsilateral eye. In the standard methodology of LSR monitoring, it is recommended to achieve centripetal transmission of electrical stimulation towards the brainstem by placing the cathode in a proximal position relative to the anode.

How to place the recording electrode and stimulating point.

The recording electrodes for the mentalis muscle are can be inserted at the point shown in the figure.

Yellow lines and black markings indicate facial nerve branches temporal, zygomatic; buccal, mandibular and cervical branch.
Blue markings indicate the facial muscles used in LSR recording temporal , frontalis,
orbicularis oculi or orbicularis oris , mentalis muscle. Red arrows and large markings indicate
the direction of electrical stimulation, stimulating nerve branch, and recording muscle, LSR, lateral
spread response.

stimulation parameter .

• Initially the stimulating parameters are set with an intensity of 5.0–25.0 milliampere and a duration of 0.3 milliseconds. The threshold value for the stimulating intensity is determined by the point at which no further increase in the amplitude {steady state} of the LSR is observed upon increasing the intensity by 1.0–2.0 mA.

Confirmation of a Positive LSR.

The latency of the evoked response is used as a guide to confirm a positive LSR. The onset latency of the compound muscle action potential (CMAP) in each recording electrode of the four muscles should be less than 4 milliseconds. On the other hand, the LSR typically exhibits longer latency, usually exceeding 8 milliseconds. Despite the straightforward definition of a positive LSR, its intraoperative interpretation is not as simple in real-world scenarios. Various patterns of EMG waves emerge during the monitoring process.

Pitfall during recording and stimulation .

• First LSR may be undetectable due to patient specific characteristics or failure in maintaining then anesthetic state.

• Second, an EMG wave occurring within the 4–8 milliseconds interval is not silent, making the characteristics of the LSR less clear. This phenomenon is a consequence of the spread of the CMAP to the muscle where the recording electrodes are inserted, originating from other facial muscles through interconnections between distant facial nerve branches.

• Third inaccurate placement of the recording electrodes may result in the delayed onset latency of the CMAP making it difficult to properly interpret the EMG findings when a mixture of Compound muscle action potential {CMAPs} and LSR is present.

• Fourth there are instances wherein ambiguous findings arise, making interpretations very challenging.

• Finally there is a concern that patients who have received multiple botulinum neurotoxin injections may not exhibit accurate EMG findings.

Confirmation of LSR Disappearance in MVD surgery.

Confirming the disappearance of the LSR is not as easy as confirming its presence below figure will show you Several factors contribute to the complexity of confirming LSR disappearance. In most cases, the LSR becomes apparent when the depth of anesthesia is adequately controlled during the procedure.

The amplitude of the LSR may not completely disappear but rather decrease. In such cases, the presence of additional culprit vessels or incomplete detachment of the culprit vessel from the facial
nerve should be considered.

The baseline EMG activity may not be completely silent, and spikes near the 10 ms interval can sometimes mask the absence of the LSR. Therefore, any haziness in the baseline EMG at the time of IONM installation should be corrected to ensure efficient monitoring.

Lastly similar to the challenge of confirming a positive LSR, the CMAP from adjacent muscles can sometimes mimic the LSR, making it difficult to detect changes in the LSR.

Different pattern of electromyography of various muscle from the top each waveform represents EMG waves of the frontalis (FRONT), orbicularis oculi (OCULI), oris (ORIS), and mentalis (MENT).
Initial (green) and final (black) waves are displayed. The horizontal axis of the grid represents 10ms and the vertical axis represents amplitudes of various values. (A). Disappearance of LSR latency
in both the oris and the mentalis is approximately 10 ms. The amplitudes of both upper waves are similar.

(B) Undetected LSR: No LSR in the oris is recorded throughout the operation, whereas the character-
istic LSR in the mentalis with a latency of 10 ms disappears. (C). Persistent LSR: Latencies in both the

oris and mentalis are approximately 10 ms. The amplitudes of both waves are similar. The LSR in
the oris remains unchanged.

Anesthesia.

Muscle relaxant should be avoided post intubation, to maintain depth of anesthesia TIVA would be preferable

Other modality is F wave .

F-waves have been reported to be useful during MVD for HFS. F-waves represent the
backfiring of the facial motor neurons following antidromic activation. Electrophysiological
studies have shown that F-wave appearance is more persistent in patients with HFS and
tends to decrease after effective decompression. However, unlike the LSR, F-waves are not
specific findings for HFS.

conclusion. MVD.

In MVD surgery for HFS the lateral spread response (LSR) is a valuable indicator in confirming the successful decompression of the facial nerve. Recent modifications to the protocol, such as the inversion of the stimulating electrode position, have shown potential in enhancing the efficacy of LSR monitoring. In summary IONM is a valuable tool in MVD surgery for HFS, facilitating the identification and decompression of the culprit vessel, monitoring critical nerve function, and improving surgical outcomes.

Related to this article. MVD surgery.

Future of Intraoperative Neurophysiological Monitoring IONM in Endoscopic Trans-Nasal Trans-Sphenoid TNTS Approach For Pituitary Adenomas . 

https://neurointraoperative.com/wp-admin/post.php?post=1727&action=edit

https://pubmed.ncbi.nlm.nih.gov/32297629

Question.

• What is Micro Vascular Decompressive Surgery?.

• What is the role of intraoperative neurophysiological monitoring {IONM} in this kind of procedure?

• What exactly does surgical Neurophysiologist in operation rooms while functional and non functional neurosurgical procedure being done?.

• What kind of modality is Lateral Spread Response {LSR} in MVD surgery?.

• Where has to place the stimulation and recording electrode for LSR modality?.