CLINICAL SYNDROMES
Fulminant MH is rare. The usually muted onset is quickly detected
by increased levels of expired carbon dioxide, developing tachycardia, or muscle
rigidity. Onset can be acute and rapid if anesthesia includes a potent inhaled anesthetic
or succinylcholine. It can be delayed and may not be overt until the patient is
in the recovery room. Once initiated, the course of MH can be rapid. When clinical
signs, such as increased expired carbon dioxide, muscle rigidity, tachycardia, and
fever, suggest MH, the association is not strong unless more than one abnormal sign
is observed. A single adverse sign usually does not indicate MH.
Volatile anesthetics and succinylcholine cause affected subjects
to undergo a striking increase in aerobic and anaerobic metabolism, resulting in
intense production of heat, carbon dioxide, and lactate and an associated respiratory
and metabolic acidosis.[11]
[46]
These reactions markedly alter whole-body acid-base balance and temperature because
of skeletal muscle bulk (40%) and are magnified as temperature increases. Whole-body
rigidity occurs in almost all pigs and in most humans. Temperature may exceed 43°C
(109.4°F), PaCO2
may exceed 100 mm
Hg, and pHa may be less than 7.00. Associated with this increased permeability of
muscle are increased serum levels of potassium, ionized calcium, CK (although MH-related
changes do not differ overall from CK changes observed during surgery[78]
),
myoglobin, and serum sodium.[46]
Later, serum potassium
and calcium levels decrease; muscle edema may occur. Sympathetic hyperactivity (e.g.,
tachycardia, sweating, hypertension) occurs early as a sign of increased metabolism.
With metabolic exhaustion, cellular permeability increases with whole-body edema,
including acute cerebral edema. As MH progresses, disseminated intravascular coagulation
and cardiac or renal failure may develop. MH is a disorder of increased metabolism;
it need not involve increased temperature, for example, if heat loss is greater than
production or if cardiac output plummets early. The clinical MH syndrome can occur
as a final common pathway in situations that may not involve susceptibility to MH,
as various disorders may mimic MH ( Table
29-2
).[79]
[80]
[81]
[82]
[83]
[84]
Trismus-Masseter Spasm
Trismus-masseter spasm is defined as jaw muscle rigidity in association
with limb muscle flaccidity after administration of succinylcholine. It is a unique
property of jaw muscle in normal people. Masseter and lateral pterygoid muscles
contain slow tonic fibers that can respond to depolarizers with a contracture.[85]
[86]
This is manifested clinically on exposure
to
succinylcholine as an increase in jaw muscle tone, well defined by van der Spek and
associates.[87]
When this increase in jaw muscle
tone becomes exaggerated, prolonged, and tight ("jaws of steel"),
TABLE 29-2 -- Mimics of malignant hyperthermia
|
Disorder |
Reference |
|
Alcohol therapy for limb arteriovenous malformation |
[81]
|
|
Contrast dye |
[144]
|
|
Cystinosis |
[145]
|
|
Diabetic coma |
[80]
|
|
Drug toxicity or abuse |
[46]
|
|
Environmental heat gain more than loss |
[64]
|
|
Equipment malfunction, increased carbon dioxide |
[114]
|
|
Exercise hyperthermia |
[106]
[107]
[108]
[109]
[110]
[111]
[146]
|
|
Freeman-Sheldon syndrome |
[151]
|
|
Heat stroke |
[109]
[110]
[111]
[112]
[113]
|
|
Hyperthyroidism |
[79]
|
|
Hypokalemic periodic paralysis |
[83]
|
|
Intracranial free blood |
[82]
|
|
Muscular dystrophies (Duchenne, Becker) |
[84]
|
|
Myotonias |
[18]
|
|
Neuroleptic malignant syndrome |
[116]
|
|
Osteogenesis imperfecta |
[147]
|
|
Pheochromocytoma |
[148]
|
|
Prader-Willi syndrome |
[149]
|
|
Rhabdomyolysis |
[46]
[90]
[91]
[92]
|
|
Sepsis |
[46]
|
|
Ventilation problems |
[114]
|
|
Wolf-Hirschhorn syndrome |
[150]
|
the likelihood of MH increases greatly. There is a spectrum of normal responses:
a tight jaw that becomes a rigid jaw and then a very rigid jaw ( Fig.
29-4
). Somewhere in the area of the declining curve is the boundary for
the MH population; the difficulty is in defining it. Trismus may still occur after
pretreatment with a defasciculating dose of a nondepolarizing relaxant. If there
is rigidity of other muscles in addition to trismus, the association with MH is absolute;
anesthesia should be halted as soon as
Figure 29-4
Succinylcholine usually increases jaw muscle tone slightly.
In some patients, this increase is moderate, and in very few, the effect is extreme
(i.e., "jaws of steel"). As much as 50% of this latter group may be susceptible
to malignant hyperthermia (MH). Somewhere in the area of the declining curve is
the boundary for the MH population.
possible and treatment of MH begun. The following discussion considers trismus without
rigidity of other muscles.
After trismus occurs, proper monitoring should include end-expired
carbon dioxide, examination for pigmenturia, and arterial or venous blood sampling
for CK, acid-base status, and electrolyte levels, particularly potassium. The initial
degree of tightness of the jaw and its duration suggest the gravity of the response.
For patients with jaws of steel, the procedure should be halted, especially if the
condition persists for more than several minutes. If the jaw is slightly resistant
to opening, the anesthesiologist may continue anesthesia during proper monitoring.
If the jaw is modestly tight and distinctly a problem, there are two choices: halt
the procedure or continue with nontriggering agents. Any suggestion of MH should
prompt MH therapy, including the use of dantrolene. The patient with a fever may
have an exaggerated response to succinylcholine's effect in increasing jaw muscle
tension. Patients who experience trismus should undergo testing for MH susceptibility.
Trismus occurring after use of nondepolarizing muscle relaxants is not apparently
related to MH or to hyperkalemia.[88]
 |
