Evaluation

When evaluating a weak patient always consider the pre-procedural status. A patient having difficulty raising their leg off the bed may be a cause of great concern for a 20-year-old athlete who underwent a cholecystectomy, but may be the baseline strength for an inactive 80-year-old having the same operation. Always consider the operation and anesthetic choice as well. If the same 20-year-old athlete was given a femoral nerve block or epidural for an anterior cruciate ligament (ACL) repair, the level of concern would decrease since the weakness is likely to be due to residual nerve block. Conversely, if he or she underwent a lumbar spinal fusion under general anesthesia, our level of concern would increase as we consider whether this is a surgical complication that needs immediate attention.

Other considerations for weak patients focus on pre-existing disease and home medications. If a patient has cerebrovascular disease with history of transient ischemic event or cerebrovascular accident, the differential diagnosis may consider a central rather than peripheral cause. Similarly, a patient who stopped taking warfarin 7 days prior to surgery but also, accidentally, stopped taking their thyroid hormone replacement may be profoundly hypothyroid with resulting muscle weakness.

When evaluating weak patients a good starting point is the neurological exam focusing on strength, motor function, and sensory. Adequate strength in the operating room (OR) can be assessed by firm handgrip or sustained head lift of at least 5 seconds, and the same can be done in the Post-Anesthesia Care Unit (PACU) if weakness is suspected. Depending on the operation performed, a baseline exam may be present in the patient’s chart from either the surgeon or anesthesiologist. The type of operation performed, anesthetic type and medications given, and operative course is well documented in the anesthesia record available by the patient’s bedside. Any questions not answered by the patient history and anesthesia record can be quickly obtained by a phone call to the OR where both the surgeon and anesthesiologist may still be present. It is generally good policy to notify the surgical and anesthesia services quickly if there are any deficits or neurological changes, since a return trip to the OR may be necessary depending on the underlying cause.

Neuromuscular blocking drugs

Neuromuscular blocking drugs are routinely given during general anesthesia to facilitate airway management, surgical exposure, and safety during operations where undesired movement could be detrimental. That being said, these drugs are also responsible for much of the postoperative morbidity and mortality associated with patient weakness. While new medications have been developed in recent years, our methods of monitoring the level of muscle relaxation are still underutilized and when used are lacking in sensitivity and subject to a wide range of interpretation among different providers. A basic understanding of how neuromuscular blocking drugs work and contribute to postoperative weakness is important when evaluating weak patients with or without neuromuscular disease.

Depolarizing muscle relaxants: Succinylcholine is the most common depolarizer currently used in clinical practice and is composed of two acetylcholine molecules linked through an ester bond.^[3] The ester bond allows rapid hydrolysis by plasma enzymes, rendering succinylcholine breakdown largely independent of liver and kidney function. This makes a succinylcholine block very short-lived and predictable except in cases of inherited or acquired pseudocholinesterase deficiency.

Pseudocholinesterase deficiency: This condition, which is present in 1:3,200 individuals, may not be discovered until the patient is given succinylcholine in the OR and a prolonged recovery is seen. The inherited form is an autosomal dominant condition and affects the quality not the quantity of plasma pseudocholinesterase enzyme. Normal patients will demonstrate 90% recovery from succinylcholine block in 9 to 13 minutes, while heterozygous atypical patients will show prolonged recovery by 50 to 100%, and homozygous atypical patients may have a prolonged block lasting up to 4 to 6 hours.^[3] The acquired form is secondary to drugs such as anticholinesterases, organophosphates, and oral contraceptives. Acquired deficiency can also be due to severe hepatic or renal disease, and even seen in normal physiological states such as pregnancy.^[4] Management of pseudocholinesterase deficiency, whether inherited or acquired, is generally supportive with continued tracheal intubation and ventilation until the block wears off, although transfusion of plasma containing normal pseudocholinesterase may expedite recovery.^[4]

Non-depolarizing muscle relaxants: The non-depolarizing neuromuscular blocking drugs are classified based on duration of action: short, intermediate, and long. They are metabolized and excreted by both hepatic and renal systems (with the exception of cisatracurium), so prolonged duration of action can be seen in patients with respective disease. The effects of non-depolarizers are measured by a train-of-four response using a twitch monitor attached to the overlying surface of various peripheral nerves, most commonly ulnar, posterior tibial, or facial. Four stimuli are given in rapid succession with response measured in number and intensity of twitch height. Misinterpretation of the train-of-four can cause both under- and over-assessment of relaxation and lead to problems with weakness postoperatively.

When non-depolarizers are used during surgery, they are generally reversed prior to extubation by acetylcholinesterase inhibitors, such as neostigmine. This increase in acetylcholine at the neuromuscular junction will compete with the non-depolarizing muscle relaxant at the receptor level and facilitate return of strength.

In weak patients without neuromuscular disease, residual block by non-depolarizing relaxants is often the cause. This can result from failure to administer reversal agent at the end of surgery, inadequate amount of reversal given, or reversal given too early with duration of block exceeding the half-life of reversal and subsequent recurarization. Management is dependent upon both the cause and severity of symptoms. For less severe causes, administration of additional pharmacological reversal agents is sufficient, but patients experiencing difficulties breathing may require intubation and mechanical ventilation.