Neuromuscular disorders associated with failure to wean from the ventilator

[Critical illness polyneuropathy: case report]

The critical illness polyneuropathy has an acute onset and an axonal predominantly motor nature. It occurs in sepsis or in multiple organ failure usually requiring mechanical ventilation in critical care units. Electroneuromyography corroborates the diagnosis. Usually it courses satisfactorily. We report on a 35-year-old female patient who, after a permanence in a critical care unit due to sepsis and removal of a dead phetus, developed tetraparesis. She had an important improvement in four months. Electromyography showed reduction of amplitude of motor and sensory action potentials, positive waves and fibrillations. The sural nerve biopsy showed axonmyelinic neuropathy. These findings are consistent with those in literature and we believe they support the diagnosis of critical illness neuropathy.

Weaning from mechanical ventilation

For most mechanically ventilated patients, weaning can be accomplished quickly and easily. However, there is a smaller group of ventilated patients who fail to wean and remain ventilator-dependent. These patients account for a significant amount of health care costs and pose a great challenge for clinicians. Detailed knowledge of the etiology and pathophysiology of weaning failure is very important for the "treatment" of difficult to wean patients, and is thoroughly presented in this article. Based on this physiological background, strategies and techniques are proposed that are useful for the gradual transition to spontaneous ventilation.

Weaning from mechanical ventilation

For the ventilator-dependent patient, weaning should be accomplished by withdrawing support safely, efficaciously, and efficiently. Success depends largely on physiologic determinants of respiratory system function, avoidance of ventilator-associated complications, and attention to patient readiness. Recent clinical trials, predictors of weaning, current techniques of weaning, the concept of reloading the respiratory pump, and determinants of ventilator dependency are all discussed.

Clinical and electrophysiological findings in critical illness polyneuropathy

Sixty two patients with critical illness polyneuropathy (CIP) were studied prospectively to determine the clinical and electrophysiological profile, to assess the prognostic value of respiratory electrophysiology in determining the duration of ventilation and to analyze the role of neuromuscular blocking agents (NMBA) and steroids. Limb motor and sensory nerve conductions, bilateral phrenic nerve onset latencies, bilateral diaphragmatic compound muscle action potentials (CMAP), unilateral diaphragmatic needle electromyography (EMG), limb muscle EMG, time on the ventilator, time in the intensive care unit (ICU), dosage of NMBA and steroids were analyzed in 62 patients. The diagnosis of CIP was made by clinical criteria, electrophysiological criteria and exclusion of any other condition suspicious of an axonal neuropathy. The results of phrenic nerve conduction studies and diaphragmatic EMG were compared to normal mean values in 25 healthy subjects. The most common finding in our study were reduced CMAPs and abnormal spontaneous activity in muscle, occuring in 100%. Forty per cent had reduced CMAPs but normal sensory nerve action potentials (SNAP). These patients had normal CK-levels and normal findings, unspecific changes, type 2 fibre atrophy or denervation atrophy on muscle biopsy. Seventy seven per cent of patients had abnormal diaphragmatic CMAPs and spontaneous activity in the diaphragm indicating denervation of the diaphragm is common in CIP. There was no statistically significant relationship to the dosage of NMBA and steroids, and the respiratory electrophysiological studies, duration of ventilation and stay in the ICU.

Electromyography of the diaphragm in neuromuscular disease

We compared the diaphragmatic electromyographic (EMG) recordings from 32 patients with known neuromuscular disease and respiratory symptoms (23 neuropathies, 9 myopathies) to recordings from 23 normal subjects. Turns analysis of 219-ms sections, or epochs, of the EMG demonstrated a significant overlap between diagnostic groups, although some epochs from neuromuscular patients were significantly different from normal. Empirical rules were derived to infer neuropathic and myopathic involvement of the diaphragmatic EMG.

Quantitation in EMG

The neuromuscular system may be affected by disorders of the central nervous system as well as other disorders affecting motoneurons, axons, neuromuscular transmission, the sarcolemmal membrane, the contractile elements and other components of the muscle fibers themselves. One or a combination of these possibilities can present in patients in the critical care unit. This paper reviews various qualitative and quantitative methods for assessing the various components of the peripheral contributions to the electrical and force output as well as the central motor drive to motoneurons. These methods all have their own strengths and weaknesses but many are complementary and together, can provide important diagnostic and prognostic information to guide management.

Electrophysiological respiratory studies in the critical care unit

Respiratory electrophysiological studies are useful in the investigation and monitoring of respiratory failure. Phrenic nerve conduction studies and needle electromyography of the diaphragm are invaluable in establishing the diagnosis, determining the severity, and following the progression of peripheral respiratory muscle dysfunction. In addition to these established methods, repetitive phrenic nerve stimulation is of diagnostic value in patients with neuromuscular transmission defects and dyspnea. The diagnosis of impaired central respiratory drive can often be accomplished by the newly-developed techniques of transcortical magnetic stimulation of the motor cortex with recording of the diaphragm and phrenic nerve somatosensory evoked potentials. These studies are of particular value in critically ill patients where both the central and peripheral lesions may impair respiration.

Lambert-Eaton myasthenic syndrome presenting with severe respiratory failure

Two cases of Lambert-Eaton myasthenic syndrome (LEMS) who presented with primary respiratory failure are reported. In each case, although not initially suspected clinically, the electrophysiological findings, which included reduced compound muscle action potential amplitudes, decrement to 3-Hz stimulation, and potentiation after 40-Hz stimulation, led to the diagnosis in the critical care unit. Electrophysiological studies of the respiratory system, including repetitive nerve stimulation of the phrenic nerve, were extremely valuable in management. As shown by these cases, the severe respiratory failure in LEMS is reversible with treatment. Thus, LEMS should be considered in cases of unexplained respiratory failure, other clinical features of the disorder sought, and the electrophysiological hallmarks looked for including studies of the respiratory system.

Critical illness polyneuropathy in multiple organ dysfunction syndrome and weaning from the ventilator

BACKGROUND

Acute axonal polyneuropathy has been found in patients with multiple organ dysfunction syndrome. This 'critical illness polyneuropathy' (CIP) has been associated with difficult weaning from the ventilator in retrospective studies.

OBJECTIVE

To test the hypothesis that CIP is related to the degree and number of organ dysfunctions, and to weaning problems.

DESIGN

Prospective study of 18 months.

SETTING

A multidisciplinary intensive care unit in a general hospital

SUBJECTS

Thirty-eight patients under 75 years of age who had been mechanically ventilated for more than 7 days, without previous signs of or risk factors for polyneuropathy.

MEASURES

Organ dysfunctions were quantified using a dynamic scoring system (0-12 points). Electromyography studies were performed during mechanical ventilation to identify patients with and without CIP.

RESULTS

CIP was present in 18 out of 38 patients and associated with an increased organ dysfunction score (5.3 +/- 1.8 vs. 3.6 +/- 1.5; p = 0.003) and number of organs involved [median (range): 4 (3-5) vs. 2 (1-4); p = 0.009], in particular cardiovascular (p = 0.003), renal (p = 0.04), and hematopoietic failure (p = 0.04). Patients with polyneuropathy were ventilated longer, but this was not clearly due to more difficult weaning [median: 16.5 (1-48) vs. 9.5 (1-38) days; p = 0.26]. Polyneuropathy was present in 2 of 4 patients with normal weaning.

CONCLUSIONS

Axonal polyneuropathy is related to the severity of multiple-organ-dysfunction syndrome. Its presence does not necessarily implicate difficult weaning from artificial ventilation.

Sepsis and the systemic inflammatory response syndrome: neuromuscular manifestations

OBJECTIVE

To describe the various conditions of peripheral nerve, neuromuscular junction, and muscle associated with the systemic inflammatory response syndrome (SIRS).

DATA SOURCES

Publications in the scientific literature and personal observations during the last 15 yrs.

DATA EXTRACTION

Computer search of the literature and review of patient records relating to polyneuropathy, neuromuscular transmission defects, and myopathies associated with sepsis, the septic syndrome, and SIRS.

SYNTHESIS

SIRS is a new concept in which infection and trauma induce a systemic inflammatory response affecting the microcirculation to organs throughout the body. The nervous system is commonly affected in the forms of septic encephalopathy and critical illness polyneuropathy. Neuromuscular blocking agents and corticosteroids may have additional toxic effects on the neuromuscular system that are manifest as transient neuromuscular blockade, an axonal motor neuropathy, or a thick filament myopathy. Clinical examination in the critical care unit is often unreliable and electrophysiologic studies, at times accompanied by magnetic resonance imaging of the spinal cord, measurement of the circulating creatine phosphokinase concentration, and muscle biopsy, are often necessary to establish the diagnosis. Variants of critical illness polyneuropathy may occur outside the critical care unit. The precise mechanism of these neuromuscular conditions is not known, and further basic research is needed.

CONCLUSIONS

A variety of neuromuscular conditions complicates SIRS. The identification of these conditions is important in patient management and in rendering a prognosis.