Approach to Electrodiagnostic Studies in the Intensive Care Unit

Differential Diagnosis of Neurologic Weakness in the ICU

Neurologic causes of profound weakness in an ICU patient include disorders of the central nervous system (CNS) and the peripheral nervous system (PNS) ( Box 40.1 ). Some of these are primary neurologic disorders that result in admission to the ICU, whereas others occur while the patient is hospitalized for unrelated medical conditions ( Box 40.2 ). One of the most common CNS diagnoses leading to weakness in the ICU is encephalopathy. Encephalopathy in the ICU often is multifactorial, secondary to a multitude of causes including electrolyte and metabolic disturbances, sepsis, and medications. Other CNS disorders can manifest as generalized weakness, including stroke, especially of the posterior circulation, seizures, anoxia, subarachnoid hemorrhage, and infectious meningitis. The spinal cord is part of the CNS, and spinal cord disorders can also present as generalized weakness. Infarction, demyelination, or unrecognized trauma in the high cervical cord can present acutely as a flaccid quadriparesis with decreased or absent reflexes and loss of sensation. Remember that an acute CNS disorder often is associated initially with decreased tone and reduced reflexes (i.e., cerebral or spinal shock) and can mimic a PNS problem early on.

In the PNS, profound weakness can occur from a lesion anywhere in the motor unit, from the motor neuron (anterior horn cell) to the motor nerve, neuromuscular junction (NMJ), and muscle. Acute motor neuron disease is very uncommon and occurs only in the setting of paralytic poliomyelitis. As discussed in Chapter 31 , poliomyelitis is a clinical syndrome that occurs from infection by several viruses, with West Nile and other viruses now added to the list. Acute flaccid myelitis (AFM) occurs mostly in children, primarily associated with Enterovirus D68 (EV-D68), as discussed in Chapter 31 . Patients with chronic motor neuron disorders, such as amyotrophic lateral sclerosis (ALS), occasionally present to the ICU when the neurologic condition has not been previously recognized or diagnosed, and the patient comes to medical attention because of a concurrent acute medical problem, usually pneumonia. The typical scenario is that of a patient with bulbar-onset ALS who has undergone an exhaustive medical evaluation looking for a gastrointestinal or otolaryngologic etiology of the speech and swallowing dysfunction. The impaired speech and swallowing eventually lead to aspiration and an accompanying pneumonia, which when superimposed on respiratory muscle weakness from the unrecognized ALS quickly leads to respiratory compromise and the need for intubation. It is only then, in the ICU, as the patient is recovering from the pneumonia but cannot be weaned from the ventilator, that it becomes more apparent that there is more generalized weakness that had not been appreciated earlier.

Moving down the motor unit, the most well-known acute neuropathy that results in marked weakness and respiratory compromise is Guillain-Barré syndrome (GBS). GBS is an acquired motor and sensory polyradiculoneuropathy that usually is demyelinating. Other variants have been described, including axonal forms, one of which is motor and sensory, and the other pure motor. GBS probably has an autoimmune etiology, often triggered by an infection either days or a few weeks earlier. Patients typically present with ascending numbness and weakness over several days, often with simultaneous paresthesias of the fingers and toes. Weakness may affect bulbofacial and respiratory muscles. Some patients present more abruptly, over hours, with associated early respiratory weakness. Other than GBS, it is rare to see an acute neuropathy as the cause for admission to the ICU. Notable exceptions include porphyria and some toxic (e.g., arsenic) neuropathies, which can mimic the presentation of GBS.

The most common severe neuropathy seen in the ICU patient is critical illness polyneuropathy (CIP). CIP usually occurs in patients within 1–3 weeks of ICU admission who have been admitted for a primary medical illness, most often sepsis, systemic inflammatory response syndrome (SIRS), and multiple organ failure. In contrast to GBS, which is usually demyelinating, CIP is an axonal sensorimotor polyneuropathy thought to be due to a complication of SIRS. SIRS is a severe systemic inflammatory response that can be caused by sepsis but is also seen in other settings, including trauma, burns, major organ failure, and/or as a consequence of major procedures. SIRS is thought to be present in most patients hospitalized in the ICU for longer than 1 week. In SIRS, significant cellular and humoral responses are thought to alter the microcirculation in the body, including the microcirculation to nerve and muscle. These responses include changes in endothelial and inflammatory cells, in addition to the expression of numerous cytokines and coagulation factors, among other changes. In prospective studies of ICU patients studied with serial nerve conduction studies, CIP can occur as early as within 3 days after the onset of sepsis. In most patients, CIP is preceded by a septic encephalopathy (aka, toxic metabolic encephalopathy), which is extremely common in ICU patients. CIP usually comes to medical attention only when the patient begins to improve from their primary medical illness but is found to have profound weakness and sensory loss or fails to wean from the ventilator. As CIP results in axonal degeneration, recovery is typically very slow and often incomplete, especially in severe cases. Indeed, in some cases, clinical and electrophysiologic evidence of CIP may remain for years after an ICU admission; rare patients remain profoundly disabled.

CIP is reported to be very common in ICU patients and can occur by itself, or more commonly in association with critical illness myopathy (CIM). Indeed, the two are recognized together so commonly, depending on how closely the patient is examined clinically and electrically, that some have advocated for the term critical illness polyneuromyopathy to describe the neuromuscular syndrome that commonly occurs in the ICU. In one study of ICU patients with SIRS, 50% developed a neuromuscular disorder. Of these, 80% had both CIP and CIM, 10% had CIP alone, and 10% had CIM alone. Even more remarkable, in another study, abnormal EDX findings were present in up to 90% of patients admitted to an ICU for various reasons, especially sepsis and multiorgan failure.

In addition to severe polyneuropathies, mononeuropathies of one or both phrenic nerves can directly result in respiratory compromise. Many of these patients are seen in the ICU. Phrenic neuropathies may be idiopathic, presumably autoimmune and postinfectious, similar in etiology to other mononeuropathies such as Bell’s palsy. In addition, phrenic neuropathy can occur rarely as part of neuralgic amyotrophy, either in isolation or more commonly as part of a more widespread pattern of multiple mononeuropathies. The other situation where unilateral or bilateral phrenic neuropathies occurs is as a complication of thoracic surgery. Some cases of phrenic neuropathy following coronary artery bypass surgery may be due to cold-induced injury occurring secondary to the use of topical cooling with ice slush used during surgery for prevention of myocardial ischemia.

Moving next to the NMJ, several disorders should be considered in the ICU setting. The one disorder of NMJ that presents acutely as rapidly progressive weakness in an adult is botulism. The typical presentation is one of descending paralysis, often associated with gastrointestinal and autonomic symptoms. Of course, a large number of chemical and biologic toxins can poison the NMJ acutely, among them organophosphates, spider venom, and “nerve gas.”

Although myasthenia gravis (MG) typically is diagnosed in an outpatient presenting with ptosis, double vision, slurred speech, and fluctuating weakness, an occasional previously undiagnosed patient may present to the ICU in acute primary respiratory failure. This situation can occur from selective involvement of the diaphragm and other muscles of respiration or, similar to the patient with unrecognized ALS, from bulbar weakness leading to aspiration and pneumonia, quickly followed by respiratory failure.

Patients with Lambert-Eaton myasthenic syndrome (LEMS) are distinctly uncommon in the ICU. First, the disorder is extremely rare. Second, the disorder usually presents subacutely over months, and respiratory muscles are not typically involved. Clinically, LEMS is most often confused with a myopathy. However, rare patients with LEMS present to the ICU as a failure to wean after elective surgery. In these cases, LEMS probably is unmasked when the patient receives a calcium channel blocker or an NMBA at the time of surgery.

Rare patients without any underlying NMJ or muscle disorder fail to extubate as a result of delayed clearance of an NMBA given during anesthesia in preparation for surgery. Most often, these patients have renal insufficiency or frank renal failure and thus fail to clear the NMBA effectively from their system. The most common paralytic agent reported is vecuronium.

The final component of the motor unit is the muscle. By far, the most common muscle disorder seen in the ICU is CIM, also known as acute quadriplegic myopathy , thick myosin filament myopathy , and intensive care myopathy , among many other names. CIM occurs most often in the setting of high-dose intravenous steroids used in conjunction with NMBAs. Rarely, it is seen in association with only one of the two; exceptional cases have been reported in sepsis and multiple organ failure in the absence of steroids and NMBAs. Pathologically, there is dissolution of the thick myosin filaments in most cases. Rarely, there is a necrotizing myopathy on muscle biopsy. One of the most common clinical situations in which CIM occurs is in patients with status asthmaticus, with estimates as high as a third of patients developing some component of CIM. These patients typically are intubated and treated with high-dose intravenous methylprednisolone. Because intubation often is difficult in these patients, pharmacologic paralysis with NMBAs is common. As the asthma improves, it becomes apparent that the patient is flaccid, areflexic, and profoundly weak. Once intubated, the patient may fail to wean for a prolonged period of time. CIM recovers in most patients in 3–6 months. However, in patients with SIRS, CIM often occurs in conjunction with CIP. When both are present, the recovery is much longer and may result in permanent disability because of the CIP component.

Other myopathies seldom cause respiratory arrest or severe generalized weakness in the ICU. Rarely, severe cases of inflammatory myopathy (i.e., polymyositis or dermatomyositis) may result in profound generalized weakness. Likewise, severe toxic myopathies are uncommon in the ICU, although rare cases of rhabdomyolysis associated with alcohol, drugs, or other toxins can present as profound weakness. Periodic paralysis, especially hypokalemic periodic paralysis, presents as severe, rapidly evolving weakness during an attack, but only rarely affects the respiratory muscles. Finally, although extremely rare, the myopathy associated with adult-onset acid maltase deficiency characteristically affects respiratory and abdominal muscles and can present as a primary neuromuscular cause of respiratory insufficiency.

Electrodiagnostic Studies in the ICU: Technical Issues

There are a number of challenging technical issues unique to performing EDX studies in the ICU ( Table 40.1 ). Some are related to patient factors, whereas others involve central and intravenous lines and electrical equipment that interfere with the performance of the study. While good patient rapport and cooperation are indispensable to the efficiency and reliability of the EMG study in the outpatient setting, these goals are much more difficult, if not impossible, to accomplish with the ICU patient. Many ICU patients are encephalopathic and cannot cooperate with the EMG examination. They may become easily agitated, making both the nerve conduction and needle examination difficult to accomplish. On the other hand, patients who are intubated are often sedated with benzodiazepines or narcotics. Some may be placed in a pharmacologic coma with propofol or barbiturates. Although such patients may not be agitated, they are unable to cooperate with routine nerve conduction and EMG studies. Neither the agitated patient nor the sedated patient is able to give the electromyographer proper feedback during the study, for example whether he or she is feeling the stimulus during the nerve conduction studies. Nor can such patients place their limbs in the correct position for the nerve conduction studies or the spontaneous activity assessment portion of the needle examination. Finally, they cannot cooperate with the examiner to activate their muscles when trying to assess motor unit action potentials (MUAPs) during the needle examination.

Because of these and other difficulties (described later), it is always recommended that two individuals perform the study together in the ICU. One person can run the EMG machine while the other performs the nerve conduction studies and needle examination, adjusting the patient’s limbs to the extent possible.

Access to certain anatomic locations in the ICU can be difficult. The presence of arterial lines, especially at the wrist, often interferes with the ability to stimulate the distal median and ulnar nerves. Because the antecubital fossa is a common site for intravenous lines, the proximal median stimulation site may not be accessible. This can be remedied by moving more proximally toward the axilla where the median nerve can often be easily stimulated. The presence of intravenous lines in the antecubital fossa may also make it difficult to flex the elbow during ulnar motor conduction studies. As noted in Chapter 22 , if ulnar motor conduction studies are not performed with the elbow in a flexed position, factitious slowing across the elbow may easily occur.

Patients who are intubated or cannot cooperate due to encephalopathy or sedation will have great difficulty moving to certain positions that are required for some nerve conduction studies and needle EMG. Of the nerve conduction studies, the sural sensory potential is the one most at risk to be compromised because it is optimally performed with the patient rolled onto his or her contralateral side. If the patient cannot be rolled onto the contralateral side or maintain that position, the study can be done with the patient supine and the leg flexed at the knee. This usually will require the assistance of another person to help hold the leg in place, and the waveform may still be suboptimal. Likewise, the tibial H reflex is best performed with the patient prone, which is essentially not possible in any ICU patient. If a central catheter is in place, proximal stimulation (i.e., axilla, Erb’s point, and nerve root) is relatively contraindicated in the ICU patient (see Chapter 43 ). During the needle EMG examination, it often is very difficult or impossible to sample certain muscles because of the patient’s inability to roll on his or her side. Most important among them are the gluteal, hamstring, posterior shoulder girdle, and paraspinal muscles.

In addition to the technical problems posed by the patient in the ICU, several technical problems related to electrical devices in the ICU may compromise the EDX study. First, the typical ICU room is filled with numerous electrical devices that are potential sources of electrical noise. Electrical noise can obscure the nerve conduction potentials (especially sensory potentials, which are orders of magnitude smaller than motor potentials) and needle EMG potentials. Second, ICU patients lie in beds with metal frames and side restraints. These beds usually are electrical devices themselves, with motors, wires, and controls as part of the actual bed. Many of the electrical devices in the room are attached to the patient (e.g., electrocardiograph, blood pressure monitor, etc.). The presence of multiple electrical devices attached to the patient, each with its own ground electrode, increases the potential risk of an electrical injury if the EMG machine is not maintained or if proper protocol is not followed (see Chapter 43 ). Finally, the presence of any line that traverses through the patient’s skin and lies close to the heart (e.g., central catheter, external pacemaker) results in the so-called “electrically sensitive patient.” In this situation, extremely small leakage currents from the EMG machine can pose a risk to the patient, whereas such small currents would be of no consequence to the typical outpatient (see Chapter 43 ).

Important Electrodiagnostic Patterns in the Intensive Care Unit

A limited number of nerve conduction and needle EMG patterns are seen in the ICU, based on the neurologic conditions that may result in respiratory or generalized weakness requiring ICU admission ( Table 40.2 ). Each pattern suggests a specific localization; in some cases, the pattern may suggest additional studies to be performed.