Introduction
Acquired neuromuscular diseases or congenital disorders either affect the neuromuscular transmission or the muscle itself. When the neuromuscular transmission is involved, two pathophysiological mechanisms must be distinguished: the abnormality of the nicotinic receptor at the endplate (myasthenia gravis, upregulation of the receptor) and the abnormality of acetylcholine release (Lambert-Eaton myasthenic syndrome, iatrogenic myasthenic syndrome). In both situations pharmacodynamic properties of neuromuscular blocking agents (NMBA) are profoundly affected.

Myasthenia gravis
Myasthenia gravis is an autoimmune disorder of neuromuscular transmission. Antibodies against the acetylcholine receptor at the neuromuscular junction result in a reduction of the number of the functional receptors with a decreased safety margin of neuromuscular transmission. Major symptoms are muscle weakness and fatiguability, relieved by rest. Anticholinesterase drugs provide symptomatic improvement in most patients. Myasthenia gravis is frequently associated with abnormalities of the thymus, so thymectomy is the main surgical procedure offered to these patients [4]. Anticholinesterase drugs should be stopped a few days before surgery if the clinical status of the patient allows it, because they could increase the effect of succinylcholine and inhibit the effect of non depolarising NMBAs. Muscle relaxants are not contraindicated in myasthenia gravis but adequate understanding of the response of the myasthenic patient to NMBAs is necessary for their safe administration. Concerning depolarising NMBAs a resistance (decreased potency) is observed because the number of available receptors at the endplate is reduced [1, 6]. High doses of succinylcholine may be required for rapid sequence tracheal intubation but phase II block is frequent even after a single dose [8]. The sensitivity to, and the duration of action of nondepolarising NMBAs are increased, reducing the intraoperative needs. A reduction of 50 to 75% of the usual doses is common [7, 11]. Nondepolarising NMBAs of intermediate duration of action must be chosen [3]. Administration of reduced doses and the use of neuromuscular monitoring allow early extubation in most cases. All the risk factors which predispose to neuromuscular function impairment must be controlled (i.e. hypothermia, hypokalemia, acidosis). The need for anticholinesterase drugs is decreased in the first 48 postoperative hours. These drugs must be restarted carefully and titrated to avoid the risk of cholinergic crisis.

Myasthenic syndrome
The Lambert-Eaton myasthenic syndrome is characterised by a proximal fatiguability, relieved by exercise. It is mainly associated with malignant tumours (pulmonary). The production of anti-calcium voltage dependent channel antibodies implies a presynaptic neuromuscular blockade with abnormal acetylcholine release [12]. Symptoms disappear with treatment of the underlying disease in 50 to 70 % of cases. Their reappearance often precedes a recurrence of the malignant process. Symptomatic treatment is provided by 3,4 diaminopyridine which promotes release of acetylcholine at the endplate. The response to succinylcholine seems to be normal but the sensitivity to nondepolarising NMBAs is increased [17]. If muscle relaxants are required, titration beginning with the tenth of the usual dose and careful monitoring of neuromuscular blockade are necessary. Pyridostigmine is not effective and calcium antagonists are contraindicated.

Muscle disorders
Duchenne muscular dystrophy or pseudohypertrophic dystrophy
Duchenne muscular dystrophy (DMD) is the most common of childhood muscular dystrophy (3 per 10 000 births). The disease is caused by an X-linked recessive gene and is often undiagnosed until the age of 3 to 5 years. The initial symptoms involve the proximal muscle groups of the pelvis. Kyphoscoliosis may develop and skeletal muscle atrophy predisposes to long bone fractures. Elevated plasma creatinine kinase concentrations results from muscle fibre necrosis. Pulmonary complications and congestive heart failure are the main causes of death which occurs between the ages of 15 and 25 years [9]. Anaesthesia is required for scoliosis correction, tendon releases for contractures or exploratory laparotomy for ileus. These procedures improve the quality of life and must be carried out early in order to limit the operative risk. Anaesthesiologists must consider impaired cardiac function and the risk of life-threatening cardiac dysrythmia and avoid depression of cardiac contractility [5]. Pulmonary and cardiac function have to be evaluated preoperatively (respiratory function tests, echocardiography, Holter) because impaired muscle function limits symptomatology [10, 14]. Succinylcholine is formally contraindicated because of a the risk of lethal hyperkalemia than can simulate malignant hyperthermia. Therefore, all malignant hyperthermia triggering agents must be avoided. Sensitivity to nondepolarising NMBAs is increased. A recent study demonstrated a significant increase in sensitivity to vecuronium for both maximal effect and duration of action [15]. Most postoperative complications are respiratory.

Myotonic dystrophy
Myotonic dystrophy designates a group of hereditary degenerative diseases of skeletal muscles characterised by persistent contracture of skeletal muscles after their stimulation, resulting from abnormal calcium metabolism. Steinert's disease is the most common and serious form of this group (3 to 5 per 100 000 population). It seen in adults. Cardiac dysrythmia and conduction defect with pulmonary aspiration are the main causes of death [2, 13]. Like in DMD succinylcholine is formally contraindicated because of the risk of lethal hyperkalemia. A myotonic crisis can be triggered by succinylcholine, hypothermia, surgical manipulations, electrocautery or drugs (clofibrate, propanolol, neostigmine, potassium) [16]. Nondepolarising NMBAs are not effective in these crisies. The use of nondepolarising NMBAs, if they are needed, requires monitoring the neuromuscular blockade. Reversal agents can precipitate skeletal muscle contraction by facilitating depolarisation of the endplate.

Up-regulation of skeletal muscle acetylcholine receptor
Clinical conditions in which the neuromuscular responses simulate an increased receptor number include denervation, disuse muscle atrophy, thermal and direct muscle trauma, infection, and chronic treatment with antagonists of neuromuscular transmission [8]. In these conditions the new acetylcholine receptor (or immature) differs from the normal (or mature) with the substitution of the e subunit for the g subunit. The pharmacological effects of NMBAs are profoundly modified. The increased sensitivity to agonists such as succinylcholine may induce a lethal hyperkalemic response. Therefore, succinylcholine is formally contraindicated from 48 hours after injury and until symptoms disappear. The sensitivity to non depolarising NMBAs is decreased because the number of receptors in the extra-junctional area is increased [8].

Conclusion
Neuromuscular diseases represent an entity including very various pathologies with different patterns. The perioperative complications (cardiac and respiratory) are serious and potentially lifethreatening. Preoperatively, an evaluation of the pulmonary and cardiac systems, which are often involved in these diseases must be performed. Some anaesthetic agents may lead to specific complications. If muscle relaxants are needed careful monitoring of neuromuscular blockade is necessary. Succinylcholine is formally contradicated in muscle disorders and in conditions with up-regulation acetylcholine receptor.

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