As survival rates of the critically ill increase with the advancement of modern medicine, the incidence of neuromuscular disturbances in these patients becomes a profound complication that needs further attention. “Intensive care unit acquired weakness” (ICUAW) is a spectrum of underdiagnosed conditions that includes both critical illness neuropathy (CIN) (also referred to as critical illness polyneuropathy (CIP)) and critical illness myopathy (CIM). While studies show that pure neuropathy is rare, patients more often develop a combination of both CIN and CIM, or they can develop a pure CIM. Therefore, it is imperative to understand the key features and electrodiagnostic findings of each condition to ensure prompt recognition, prevent further disability, and ensure better patient outcomes. Prognostically, while patients with critical illness neuropathy often suffer from a long-term impairment, those with a pure critical illness myopathy have been shown to improve relatively rapidly with minimal residual disturbance.
Some of the risk factors for developing CIM include high-dose glucocorticoid use, neuromuscular blocking agents, COPD or asthma exacerbation, and hyperglycemia.
The pathophysiology of CIM is theoretically a combination of microvascular, metabolic, and electrical alteration, which leads to an inflammatory cascade. The atrophy is proposed to be due to increased muscle proteolysis coupled with a decrease in muscle protein synthesis. These processes result in myosin loss, as well as thick fiber necrosis and apparent muscle atrophy, which may be evident in a muscle biopsy. Creatine kinase may be normal or transiently elevated in a patient with CIM.
It bears mention that steroid myopathy may mimic the symptoms of CIM by similarly affecting proximal muscles with profound weakness. However, steroid myopathies affect type 2B muscle fibers, and therefore EMG testing in steroid myopathy will be grossly normal because the test evaluates only the faster, type 1 motor units.
Both critical illness neuropathy and critical illness myopathy may clinically present with difficulty weaning off a ventilator or, if extubated, the patient may complain of profound weakness, flaccid limbs, and may have a reduction in deep tendon reflexes. In pure CIM, patients will usually complain of more proximal weakness. Manual muscle strength testing may reveal the weakness of bilateral shoulders and hips. While deep tendon reflexes may be reduced, sensory fibers are not affected, and therefore, the sensation will be intact. The weakness can affect the extraocular and facial muscles. Most concerning, the diaphragm muscle may be affected as well. Of note, CIM typically spares the bulbar muscles and lacks autonomic instability as clinical features that can help narrow down the differential and rule out neuromuscular junction disorders and myopathies.
Weaning complications are due to the involvement of the diaphragm. Other accessory respiratory muscles may also be affected. The clinician must always keep in mind that Wallerian degeneration may take at least 14 days after the nerve injury to become apparent on an electrodiagnostic test. Therefore, testing should be ordered judiciously only in the appropriate patients, and usually about two weeks after the onset of symptoms.
Performing electrodiagnostic studies in critically ill patients have few absolute contraindications. Needle EMG is contraindicated in those with severe bleeding disorders. Needles should also never be inserted into areas of active soft tissue infection. Nerve conduction studies are contraindicated in patients with implanted cardiac defibrillators or if connected to external defibrillators. Patients should have screening for pacemakers, and electrical stimulation should not be performed directly on or near the device itself.
Before performing any diagnostic study, a comprehensive review of the patient's history and clinical course, as well as a complete physical exam, must be performed. A thorough and compassionate diagnostician will inform the patient and family at the bedside of the indications and overview of the studies which will be performed. To accurately diagnose CIM with electrodiagnostic testing, one should ideally examine at least three extremities, performing both sensory and motor nerve conduction studies, as well as needle EMG testing in both proximal and distal muscles for comparison.
As with all nerve conduction studies, the temperature of the patient's limbs should be kept warm. Colder temperatures can surreptitiously increase amplitudes, prolong latencies, and slow conduction velocities. To minimize the electrical interference which exists in the ICU, a notch filter should be used and, if possible, all unnecessary machines turned off, including unplugging the hospital bed.
As with all electrodiagnostic studies in any setting and for any indication, the risk of complications is low. There is always a small risk of bleeding or introducing infection with needle studies.
Unlike in critical illness neuropathy (which is a primarily axonal sensorimotor peripheral neuropathy), critical illness myopathy (if not superimposed on CIN) would demonstrate normal sensory and motor nerve conduction studies. The needle EMG would be the aspect of the exam to reveal abnormal spontaneous potentials, similar to what would appear in other primary myopathies. It is imperative to understand, however, that often CIN and CIM occur simultaneously.
Sensory Nerve Conduction Studies
In a pure case of CIM, sensory fibers are not affected, and therefore, sensory nerve conduction studies will be normal. If sensory nerve action potentials (SNAP) amplitudes are consistently affected, the diagnostician must consider a coexisting CIN.
Motor Nerve Conduction Studies
As with sensory nerve conduction studies, motor nerve conduction studies (especially when testing distal muscles) should be normal in critical illness myopathy. Unlike critical illness neuropathy, where NCS affects the distal muscles more profoundly, in CIM, the proximal muscles are more affected. Therefore in a severe case of CIM where there is significant atrophy, compound muscle action potentials (CMAP) amplitudes may be mildly decreased. As this is an intrinsic muscle disorder, myelin is not affected. Therefore conduction velocities and distal latencies will generally be normal.
Needle EMG will frequently reveal abnormal spontaneous potentials in proximal muscles at rest, including positive sharp waves (PSWs) and fibrillation potentials (fibs). Individual myopathic motor units may show early recruitment with minimal contraction, will be polyphasic with shorter durations and smaller amplitudes. These findings will be more prominent in proximal muscles than distal muscles.
Needle EMG of the diaphragm muscle may reveal abnormal spontaneous potentials as well.
Often patients with a prolonged ICU admission will develop profound weakness or failure to wean off a ventilator. Intensivists, physiatrists, and neurologists must recognize that further evaluation with electrodiagnostic testing is warranted in these circumstances. As healthcare professionals, appropriate disease recognition and diagnosis of critical illness myopathy will help expedite patient recovery and can improve long term quality of life.
While there is a vast differential in the patient with weakness which develops in the ICU, NCS and EMG studies may be able to identify the etiology and further guide the management. After arriving at the diagnosis of CIM, the interprofessional team must include a team of physicians, physical therapists, occupational therapists, speech therapists, nutritionists, social workers, and case managers. These professionals can work together to coordinate early mobilization and aggressive multifaceted rehabilitation.
Typically, there is a long and challenging recovery ahead for patients with CIM upon leaving the critical care setting. Only with a coordinated effort between the various medical disciplines and departments will the best outcomes for patients be achieved.
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