To the Editor:—
The presymptomatic diagnosis of malignant hyperthermia (MH) susceptibility is based on the in vitro contracture test in Europe and the caffeine–halothane contracture test in North America. Both tests are invasive in requiring an open muscle biopsy undertaken in a specialized center. There are 23 MH investigation centers in Europe, but testing facilities are limited to only 6 centers in the United States.1,2
The locus of the ryanodine receptor of skeletal muscle on chromosomes 19q 13.1 has been shown to link to MH, and several mutations have been identified on this gene.3Molecular genetic diagnosis of MH susceptibility in persons from MH families with identified mutations in the ryanodine receptor gene has recently been introduced.1,4
A 29-yr-old woman contacted our MH investigation unit to be tested for MH susceptibility. Being pregnant, she was concerned about a possible cesarean delivery because she had a history of a clinical MH episode. She underwent adenotomy as a 6-yr-old child in 1982. The clinical and laboratory findings on the clinical grading scale for MH episodes ranked the episode in the highest category.5The child was discharged without permanent sequelae on the 11th postoperative day. In 1986, MH testing became available in Switzerland, and an open muscle biopsy and in vitro contracture test were performed on her parents rather than on the child, because she did not consent to being tested. Although the father was MH negative, the mother was MH susceptible. As part of our research program, the frozen muscle biopsy of the mother was subsequently investigated for MH-associated mutations in the ryanodine receptor gene. Mutation G2434R, known to be causative for MH, was identified. Therefore, the patient contacting our center was tested for this mutation and found to be carrier of G2434R. She agreed to have the umbilical blood of her baby taken at labor, to have the newborn assessed for MH susceptibility. Vaginal delivery was uneventful, with the mother receiving lumbar epidural analgesia with 0.12% ropivacaine and 2 μg/ml fentanyl. Umbilical cord blood was collected in an EDTA tube and sent to the Swiss MH investigation unit. DNA was automatically extracted using the MagNA Pure DNA isolation kit I (Roche Diagnostics, Rotkreuz, Switzerland). Exon 45 of the ryanodine receptor gene was amplified by polymerase chain reaction using the following primer set: forward: AGA ACG CCA ATG TGG TGG T; reverse: CTG CAT GAG GCG TTC AAA G. Presence of the mutation was proven by an automated sequencing technique (Applied Biosystems, Rotkreuz, Switzerland) and is shown in figure 1. The results were discussed in detail with the mother, and she was given a warning card about the MH status of her newborn son.
This is the first report of MH susceptibility in a newborn by molecular genetic testing of umbilical cord blood. Although knowledge of MH status may be of lesser importance during daily life, it is valuable to confirm or exclude MH susceptibility before surgery in individuals from families with known MH susceptibility. Although elective procedures can be performed during either regional anesthesia or intravenous anesthesia without triggering agents, patients are likely to be exposed to inhalation agents or succinylcholine during emergency or obstetric interventions, and volatile anesthetics are preferred in pediatric anesthesia, because venous access can be established after induction of anesthesia.
Muscle biopsy and contracture testing must be performed in specialized centers and may not be readily available. Most MH centers do not perform biopsies in infants and children because of the limited availability of skeletal muscle. Therefore, the MH status of this newborn would have remained unknown for at least the first decade of his life.
This report emphasizes some significant points:
Compared with muscle biopsy, sampling for molecular genetic investigations is much easier, and collected tissue can be transported to the center by regular mail. Sampling of umbilical blood is noninvasive. A possible concern might be the potential contamination of umbilical cord blood with maternal nucleated cells. However, the concentration of maternal cells was found to be 10−4to 10−5times lower than neonatal nucleated cells,6and therefore, the identical signal intensity of both alleles in our analyses represent the neonatal MH mutation. For verification, we excluded contamination with maternal DNA by short tandem repeat profiling.
The genetics of MH are complex, because this disease shows substantial locus and allelic heterogeneity.7More than 40 mutations have been identified in the ryanodine receptor, and not all have been proven to be causative of MH. A careful selection of patients eligible for genetic testing of MH susceptibility must be made on the basis of family history and molecular genetics, as well as results of in vitro contracture tests, to prevent unnecessary and costly genetic investigations.
Every pregnant woman with a self or family history of MH and an identified MH-causative mutation should be offered the option of molecular genetic investigations of umbilical cord blood.
A positive test result confirms and hence avoids uncertainty about MH susceptibility in the newborn.
It is important to note that because of the heterogeneity of MH and according to the guidelines,1individuals will still need to have a muscle biopsy to confirm their MH status in case of a negative genetic test.
* University Hospital Basel, Basel, Switzerland. email@example.com