TGI MH Friday, March 19, 2021
Welcome to another issue of TGI MH Friday. Today we’ll discuss how to evaluate a very common problem we all face – the child with a vague history of MH in the extended family. But first, some new PAAD additions. I’m a big fan of Politico’s Playbook, a daily email of current political events. Along with Heather Cox Richardson’s Letters from an American, it’s my morning coffee reading. Playbook has created a nice sense of community by including sections at the end of the newsletter with announcements of birthdays, job transitions, and other “mazel tovs”. Starting Monday next week I’ll try to do the same with fellow PAADers. If you would like a shout-out, please send them to me at LitmanR@chop.edu and I’ll post them for the group.
Today’s case is one that we all encounter from time to time, and it comprises one of the most common calls we get on the MHAUS Hotline. An otherwise healthy 15-month-old boy presents for myringotomy and ear tube insertion for the first time. During the preoperative visit, the parents tell you that the mother’s aunt may have had an episode of MH as a child but was never formally tested for MH susceptibility. No one else in the family has ever had any problems with anesthesia, but those records are not available. Should this child now be considered MH susceptible and receive a non-triggering technique?
To me, there are several important issues to consider in this case: Based on the history, what is the chance that the child is truly MH susceptible? If you decide to go ahead with a non-triggering technique, how would you prepare the anesthesia machine? How would you then anesthetize the child? Finally, how would you counsel the family once the procedure is finished? Let’s take each of these in turn…
Unless we have convincing evidence to the contrary, we need to assume, for safety’s sake, that the aunt’s history of MH is real. How closely related does a “proband” have to be to warrant avoidance of MH triggering agents? Although a classic Mendelian inheritance of MH is an oversimplification, the autosomal dominant mode of inheritance is helpful to estimate the risk of MH susceptibility. Each offspring of an MH susceptible parent has a 50% risk to be MH susceptible. With every generation, the risk of MH susceptibility decreases by 50%. Therefore, the next generation (grandchildren) each have a risk of 25% and so on. If we assume that the child’s mother’s aunt inherited the MH mutation from one of her parents, it means that the child has a 12.5% chance of having inherited it. Using a conservatively high prevalence of MH variants of approximately 1 in 1,500 in the general population, it would take approximately 10 generations to decrease the calculated familial risk to be similar to that of the general population. What that means is that any familial history within 10 generations (essentially everyone) should be cause for suspicion of MH susceptibility and should prompt the use of a non-triggering general anesthetic.
Preparation of the anesthesia machine is relatively straightforward, as long as you have charcoal filters. Once they are attached to the inspiratory and expiratory limbs, clearing residual gas from the machine and anesthesia circuit only takes a few minutes, and there is usually no obstruction to OR throughput. Without the filters, flushing methods vary between anesthesia machines, and take much longer. For extra protection, we also either temporarily remove the vaporizers, or place a big piece of tape over them.
The non-triggering anesthesia technique is also straightforward, but not necessarily easy. There are many variations in practice. I would premedicate the child with oral midazolam, and then place an IV in the OR while the child breathes N2O. Occasionally, we’ll find an expert nurse who will place the IV in the preoperative holding area. Once the IV is placed, propofol and ketorolac can be administered for the procedure.
How, then, to counsel the family? If ever possible, the family member with the highest risk of MH susceptibility should be tested for MH in order to rule out, or confirm, the risk of MH in subsequent generations. That means going back to the maternal aunt and asking that person to get either a genetic test (confirms MH susceptibility), or a contracture biopsy (rules out MH susceptibility). In practice, neither of these get done in most of these cases, and the best one can do is help the parents understand the ramifications of an MH-susceptible label.
In the interest of brevity for this daily PAAD, many details of the above have been omitted, but complete information can be found on the MHAUS website.
As always, I hope everyone has a relaxing weekend, and special thanks to those on call. I hope your days and nights are quiet till Monday morning.