Nerve monitoring

[Mark]

Starting a new thread to continue the question of nerve monitoring

Here is a link to a pretty extensive academic write up comparing several types of cranial nerve monitoring, so the first issue for a surgery patient is what type did your surgeon use because they don't all work the same 

http://books.google.com/books?id=mFlV1-v_eVwC&pg=PA1944&lpg=PA1944&dq=nerve+monitoring+machines&source=web&ots=WLkI4xQGw-&sig=2NNgEcTygatGXxMasN6pcoZurh4&hl=en&sa=X&oi=book_result&resnum=4&ct=result#PPA1944,M1

Wow, that's a link that stretches the page 

It discusses how they work and what the "weakness or failure risks" are with each

Here's the conclusion on facial nerve monitoring from a Otology seminar I found

Conclusions
   Not a replacement for surgical knowledge or technical skill (US$200/ hour)
   Beware of false positive/ negative & its limitations (not unfailable)
   Modify & advance surgical techniques
   Can’t rely on the instrument to “find the nerveâ€?, visualizing the course of FN by skeletonizing it in its bony canal remains the optimal method of avoiding injury


Future directions
   Computer-based system with simultaneous capacity for EMG & ABR recording
   Rapid data collection with online digital filtering
   Better artifact rejection
   Automated control of stimulation and recording parameters
   User- friendly interfaces and displays of current data as well as trends during the operation
   Relationship between intra-op recordings & ultimate clinical outcome

Here's another which certainly suggests there is variability in machine accuracy and sensitivity settings

Optimal Stimulus Duration for Intraoperative Facial Nerve Monitoring.

Triological Society Papers
Laryngoscope. 109(9):1376-1385, September 1999.
Selesnick, Samuel H. MD, FACS

Abstract:
Objectives/Hypothesis: The charge delivered to the facial nerve during intraoperative facial nerve monitoring (IOFNM) is the product of the stimulation amplitude and the duration for which the pulse of charge is applied. In the literature, no standard for pulse duration exists, precluding meaningful comparison of IOFNM between studies. The optimal stimulus pulse duration can be derived from facial nerve strength duration curve analysis and calculation of chronaxy. Chronaxy is directly related to the time constant, [tau], of the neuronal membrane, and is a function of neuronal membrane resistance and capacitance.

Study Design: A prospective trial of facial nerve stimulation in both an animal and a human model.

Methods: Five rabbits and 17 humans underwent intraoperative stimulation of healthy facial nerves. Pulse durations using pulses of 10, 20, 50, and 100 microseconds were employed, and the corresponding threshold stimulation amplitudes were recorded. From these data sets, strength duration curves were plotted and chronaxy values calculated.

Results: Average chronaxy values of 18 microseconds in the rabbit and 32 microseconds in the human were found. Given IOFNM system accuracy limitations, the optimal pulse duration for facial nerve stimulation is 50 microseconds.

Conclusion: Most commercially available intraoperative monitoring systems employ a pulse duration default setting of 100 microseconds. Doubling of the 50-microsecond optimal pulse duration may result in a loss of sensitivity of predictive facial nerve data. Both the stimulation amplitude and the selected pulse duration should be reported by investigators so that meaningful comparison of the IOFNM data in the literature can be made.

Overall, my impression is that there is enough limits on monitor capabilities and surgical environment variables to make them anything but failsafe in a facial nerve outcome

Mark

[Jim Scott]

Mark:   

I have no idea what type of cranial nerve monitoring my neurosurgeon used (as if knowing that would actually mean anything to me). The monitoring tech was brought in by my neurosurgeon from a hospital in another state and he seemed highly competent.  Actually, he was the last person I spoke to before going under the influence of the anesthesia.  I had no facial or other nerve damage so they must have done something right.  However, I agree with the otolaryngology text you linked to that the surgeon's experience and skill has the most influence on whether cranial nerves are compromised and that nerve monitoring, although a useful tool for the surgeon, is not a substitute for skill. This simply amplifies the oft-made assertion that when discussing procedures relating to AN treatment, be it one of the three surgical approaches or one of the three radiation-based approaches, there are flaws in every procedure and whatever approach one chooses will entail some risk or, as I like to put it: 'there are no guarantees'.

Jim

[Mark]

Jim,

I agree with you a 100% about there being no guarantees any way you go. Since I went the CK route I never had a reason to research the issue of nerve monitoring in surgery in any detail, so it was interesting  to do a little digging into the subject. Having gotten a little education  in terms of the issues and the limitations related to the monitors performance, I did start to wonder how many people go into surgery with a false sense of confidence that it will "guarantee" a good facial nerve outcome.  Clearly, the studies I read put more importance on the skill of the surgeon and his/ her technique than the monitors themselves. Something for folks to consider

Mark

[Kate B]

Jim,

I am like you in that I have no clue as to the type of facial monitoring done during my surgery, other than they did do it at House.  In addition to doctor's skill, another big influential factor is size of the tumor. Most of the data shows that tumors under 2cm have little side effects.

Kate