Friday, March 5, 2021

TGI MH Friday

Welcome to our new feature – TGI MH Friday! On selected Fridays I’ll post a real case from the MHAUS MH Hotline and you can play MH hotline consultant. We’ll start with one of the most common calls we get on the hotline in one of various forms. A healthy 15-month-old infant underwent uneventful circumcision with sevoflurane via an LMA, and caudal block. HR, BP and ETCO2 were all normal throughout the case. While recovering in the PACU his HR and RR rose and he developed a fever to 38.0 C. He was given oral acetaminophen and is drinking well. One hour later his temperature is 40.1 C and the PACU nurse calls you to assess the child. What will you do? Could this be MH?

Answer:

A common misunderstanding about the presentation of MH is that it may be delayed, even many hours into the postoperative period. This does not seem to be true. In a published analysis of 528 cases of suspected MH reported to the NAMHR, 10 occurred after discontinuation of volatile anesthetic agents. In none of these 10 cases did the first signs of MH appear more than 40 minutes after discontinuation of the volatile anesthetics. Many clinicians worry that acute MH may begin in the postoperative period with hyperthermia as the presenting sign. Based on our experience with calls to the MHAUS hotline, postoperative hyperthermia (i.e., T > 39°C) without additional signs of MH is relatively common, but is not associated with an eventual diagnosis of MH.

Postoperative fever, on the other hand, is very common, and can be alarmingly high, especially following procedures that may be associated with transient bacteremia, such as ENT or urology.

Why has the myth of postoperative MH been perpetuated throughout the years? It probably relates to the more frequent use of succinylcholine in olden days, and the development of postoperative rhabdomyolysis, which can occur in patients with a subclinical myopathy (see last week’s Throwback Thursday case) or when multiple doses of succinylcholine have been administered. These cases were often thought to be somehow linked to MH susceptibility but this association did not last the test of time.

So, what would be my answer to the caller? After confirming the absence of generalized limb rigidity, I would reassure them that this doesn’t represent MH, and they should continue to administer conservative treatments until the fever begins to abate and the child can then be discharged home. Some callers may not be comfortable with this advice because of the rapidity of the temperature rise, in which case I’ll recommend a measurement of ETCO2 by nasal cannula, or if they are still not convinced, they can obtain a venous blood gas (not arterial for Pete’s sake!) to make sure there isn’t a combined respiratory and metabolic acidosis, one of the hallmarks of MH, as well as a serum CK level. Once the temperature returns to a reasonable level the child can be discharged home.

Hope everyone has a relaxing weekend, and for those on call, a safe journey until Monday morning.