Correlation of the Maximal Respiratory Pressures, Respiratory Airflow and Dysphagia in Patients with Acquired Autoimmune Myasthenia Gravis

  

    
Deglutition
    
MIP
    
MEP
    
PCF
    
PEF
  
  

    
% of predicted    (average)
    
% of predicted    (average)
    
Average
    
% of predicted    (average)
  
  

    
Normal
    
83,25
    
98,04
    
495
    
84,56
  
  

    
Mild
    
79,89
    
77,69
    
386
    
73,37
  
  

    
Moderate
    
79,87
    
70
    
363
    
71,93
  
  

    
P
    
0,015*
    
0,011*
    
0,078
    
0,019*
  
  

    
ANOVA: Analysis of Variance for Repeated    Measures.
      
Abbreviations: p: significance; MIP:    Maximal Inspiratory Pressure; MEP: Maximal Expiratory Pressure; PCF: Peak    Cough Flow; PEF: Peak Expiratory Flow; %: percentage, *: p<0,05.
      
 
  

Table 8:  Relationship between dysphagia severity and respiratory results
ofstrength and flow.

Discussion

This study of oropharyngeal dysphagia in individuals with AAMG was able to demonstrate how the stomatognathic system structures influence the swallowing process, and how this function is intrinsically related to respiratory mechanics and breathing function. These findings provide a better comprehension of the changes found in the swallowing process, as well the relationship between swallowing and breathing. The results of this study allow us to reflect about the patient’s AAMG rehabilitation and the integrated care of the multidisciplinary team, focused on the evaluation and treatment of oropharyngeal dysphagia.

The AAMG is a neuromuscular junction disease that may occur in any age group. Some authors show a greater commitment in females between 20 and 40 years old [1,14] and higher prevalence in men around 50 years old [14]. Similar results were found in our study. The average of our population was 38.71 years old for women and 50.17 years old for men. There are conflicting results in literature. However other studies indicate a greater impairment in males, between 70 and 80 years old [15,16].

Various publications, report that women are more often affected than men until the fourth decade, ranging from 2:1 to 4:1 [1,4,14,17- 22]. In accordance to others studies, our data revealed a predominance of women in relation to men, in a ratio of approximately 2:1.

Time for diagnosis could be understood as the interval between the symptoms initiation and the diagnostic conclusion; whereas could be defined as the interval between the symptoms initiation and the day of the clinical evaluation.

Previous investigations [1,23] demonstrated that the time for diagnosis could vary from 1 to 516 months, and 24 hours to 960 months, respectively. These data are in agreement with the elevated variability found in our study, 0 to 228 months related to the time for diagnosis. The diagnosis of AAMG is based on patient’s clinical history added to the physical examination, and tests that assess the neuromuscular function [18], but in some cases is difficult in perform the diagnosis can cause complications for the patient.

Although no data in literature makes this correlation directly, we observed that the higher the time for diagnosis, the higher dysphagia severity. This data reinforces the idea that the proper treatment can be conducted only to the light of the patient’s diagnosis. Treat the patient of a palliative form implies in clinical course with a higher incidence of complications, as dysphagia.

Although no data in literature makes this correlation directly, we observed that the higher the time for diagnosis, the higher dysphagia severity. This data reinforces the idea that the proper treatment can be conducted only to the light of the patient’s diagnosis. Treat the patient of a palliative form implies in clinical course with a higher incidence of complications, as dysphagia.

Even though the literature reports emotional factors, physical trauma and fever [25] as triggering factors of the myasthenic crisis, pulmonary infections could be considered the main factors for their initiation [3]. In addition, the earlier and more severe the impairment of the respiratory muscles, the worst the complications and the myasthenic crisis [26]. Thus, we emphasize the importance of the oropharyngeal dysphagia treatment in order to prevent aspiration pneumonia that can present of relapsing form, when cause was not completely eliminated [27].

The fatigue and fluctuating muscle weakness are the main features of the disease [1,2,28-30], but the symptoms initiation is varied and may be present initially by ocular, bulbar or generalized alterations, alone or in combination. Evidences indicate that the vocal changes can be one of the first and only signs of AAMG [14,31].

In an attempt to make a characterization of the sample relative to the stomatognathic system muscles, we performed a clinical examination of some muscle groups (lips, tongue, buccinator, masticatory and soft palate). We noted that the tongue muscles are the most frequently affected by muscle weakness, followed by masticatory and orbicularis muscles of the lips. Accordingly, the swallowing function is the most altered, in both frequency and gravity, since the aforementioned muscles participate of the swallowing oral phase.

The description of the facial muscles commitment, soft palate, lips, tongue, larynx, pharynx, masticatory muscles, cervical and breathing was performed in a previous study, in order to characterize the bulbar abnormalities found in patients with AAMG [32].The weakness of the buccinators muscles, risorius, and zygomatic major and minor could be associated with the facial muscles paresis, resulting in a tendency of smile verticalization; with consequent impairment of facial expression [33,34].

The orofacial muscle disorders compromised the functions of chewing and swallowing (80%), and muscle weakness was the determining factor of these disorders. The symptoms of bulbar nature (dysarthria, dysphagia, dysphonia and dyspnea) are often described with the clinical findings and have always been attributed to the weakness of the muscle structures involved, with the typical characteristic of fluctuating symptoms [35].

The masticatory muscles, especially the masseter and temporal, are also responsible for mandibular lifting motion [36]. Due to the weaknesses observed in these muscles, associated with the weakness of the orbicularis muscles of the lips, the implications for the AAMG patient may involve the predominance of an oral breathing, distortion of bilabials phonemes, or even anterior food and saliva escape, featuring oral phase swallowing disorders.

Clinical examination data, enabled to define the severity of the swallowing disorders [11]. A careful clinical evaluation needs to consider the performance of swallowing, as well the elements that act as a cause and consequence; pulmonary status, nutritional status and feed pleasure for each individual, making it possible to identify and classify the neurogenic dysphagia [37]. These data were very important to define the therapeutic approaches adopted.

We verify that 80% of patients had swallowing disorders by clinical examination that was considered mild in 50%, and moderate in 30% of the cases. The literature proposes a dysphagia classification in AAMG patients according to the choking presence [38], muscle strength and performance in chewing activities, swallowing, speech and breathing criterion [10,39]. The patients of our study were ambulatory and were not hospitalized. This fact can justify the absence of severe or profound dysphagia degrees in our sample.

Swallowing is described as a function characterized by a dynamic process characterized by a sequence of complex events, interrelated between them [40-42]. The swallowing process is divided didactically in stages: anticipatory phase, divided into oral preparatory and oral phase itself, pharyngeal phase and esophageal phase.

The preparatory oral phase of swallowing is the stage that prepares and qualifies the food bolus. Chewing is the initial stage of preparation, in which food is crushed and humidified. Concomitantly, the body qualifies the food bolus, in relation to its volume, consistency, density and degree of humidification.

Myasthenic patients may experience a significant reduction in maximum bite force, reducing the maximum electromyographic activity of lifters and depressors jaw muscles, and also masticatory inefficiency [43].

The weakness and masticatory fatigue were reported by 80% of patients. And these are symptoms often described in literature [10]. Patients with reduced maximum tongue strength tend to swallow more slowly [43], which can compromise the necessary synchrony for swallowing process.

The swallowing oral phase itself is characterized by the organization and food bolus ejection of the oral cavity to the pharynx. During this phase, the food bolus is placed on the tongue, to start the ejection to the oropharynx, initiating the involuntary phase of swallowing.

The pharyngeal phase is the involuntary phase of swallowing; characterized by a dynamic that directs the pressure flow and muscle adjustments made, such as the lip seal, the elevation of the palate and the pharyngeal constriction, preventing the dissipation of this pressure, helping to protect the airways.

The palate veil alterations in patients with AAMG can be impeditive for the intraoral pressure maintenance, causing hypernasality, articulatory imprecision, and also nasal reflux [44].

In AAMG, the more muscles involved are: tongue, larynx lifts and constrictor of pharynx, according to an electrophysiological evaluation [27]. The larynx changes can generate a dysphonia, with floating character that gets worse with sustained emission; the lossadduction of the vocal folds leads an air leak during phonation, resulting in a breathy voice quality; with the fatigue of tensor of the vocal folds, the pitch can be lowered and become monotonous, the sentences are short and the cough is weak [32].Bulbar disorders such as dysphagia are so frequent in patients with AAMG, but little valued both by doctors and by patients themselves, and this disorder is attributed to involved muscles weakness [4,16].

Neuromuscular disorders may result in respiratory failure and, in some cases, may be the first symptom [22]. The respiratory muscles may also be affected by weakness. This impairment can be expressed by a rapid shallow breathing or for a reduction in maximum voluntary ventilation [3,25]. The degree of respiratory compromise depends of the disease severity, at the beginning can be observed in 1% to 4% of patients with AAMG, in advanced stages of the disease appears between 60% to 80% [2,26]. In our patients, 70% had dyspnea in different levels of effort, and 20% presented associated orthopnea; similar results were found in previous study [45].

To evaluate respiratory muscle strength, we decided to use the air mask and not the nozzle, due a minimally necessity of the facial muscle strength preservation to contain the device well positioned [26]. The frequent involvement of the orbicularis muscle of the lips in patients with AAMG [4,27] would compromise the proper sealing during the examination and may generate false positive results.

Evidences suggest that the use of mask shows no interference in the results of MIP. In the case of the MEP, the authors suggest that the use of the mask may have low values due to possible air leaks, However, the mask use could be associated possibly a lesser air leak in our population if compared to the nozzle [46]. Some authors [46,47] speculate that when the mask is used there is more chance of occurring air leakage during the procedure, especially in the MEP measures; however, when the sealing is adequate, the values are significantly higher than the measurements obtained by the nozzle [46].

In our study the measures were performed with extreme caution in order to prevent air leakage.

Patients with AAMG usually present respiratory muscles weakness, restrictive ventilatory pattern [2], and deterioration of maximal respiratory pressures could occur progressively, with or without concomitant fall FVC [22].

In agreement with other studies [2,22], 35% of the our sample showed a reduction greater than 80% of the predicted value of the MIP and 45% reduction greater than 80% of the predicted value of the MEP.

We observed a good correlation among the maximal respiratory pressures deterioration and the increased severity of dysphagia; favoring the risk of aspiration. Therefore, there is a statistically significant relationship between swallowing and breathing changes.

In our findings, the more serious swallowing disorders, the greater the impairment of respiratory muscle strength. Likewise in relation to the lung flows. This decline in the measures is due to weakness of the respiratory muscles, fatigue and reduced lung compliance [48].

The results of the respiratory evaluation showed a strong correlation of the respiratory muscles strength with the expiratory flow measures and cough. The muscles weakness promotes a decrease of the inspired and exhaled volumes. We also observed a good correlation between MIP and MEP, both of them presented below of the predicted, indicating that the weakness characteristic of the disease affected the inspiration and forced exhalation, so important during cough, and also for airway protection [49].

The MEP values below 40cm H₂O favors the accumulation of secretion in the laryngopharyngeal region. This stasis associated with reduced strength of cough secondary to a difficulty to generate expiratory flow, can increases the risk of occurrence of tracheal aspiration and thus to possible respiratory infections [50].

Effective cough is important for proper protection of the airways in case of laryngeal penetration or tracheal aspiration [51- 56]. The cough starts with inspiratory movement with wide vocal fold abduction, so the greater the inspiratory phase, the greater the effectiveness of cough [49]. In the compression phase, there is a glottal closure associated with activation of the diaphragm and the muscles of the abdominal and thoracic wall, in order to expel air from the airways. In the expiratory phase of cough, the folds abduct of an abrupt form, with air outlet at high speed, with a simultaneous lifting of the soft palate, preventing communication with the nasal cavity, clearing the airway secretions and inhaled particles [57]. The weak cough results in retention of secretions in laryngeal level and in inferior airways [32].

Aspiration pneumonia caused by episodes of tracheal aspiration is one of the most frequent complications of the oropharyngeal dysphagia. Dysphagia, can provide triggering and aggravating factors, such of musculoskeletal nature and breathing; which implies in a multifactorial reasoning to understand this clinical entity, especially in patients with AAMG. Respiratory issue is a factor that deserves special attention, since the respiratory fatigue in high intensity can trigger myasthenic crisis [58,59].

Consider the pathophysiology of the base disease of dysphagia patient, integrating knowledge of related areas, in clinical evaluation and treatment, not only adds value to the diagnosis, but also prevents respiratory and nutritional complications [60] secondary to dysphagia, with a possible worsening of the disease. The better comprehension of this scenario applied to AAMG is very important in the rehabilitation of these patients, since the detection and early dysphagia treatment in patients with AAMG are essential, in terms of prevention, diagnosis and treatment. The development of an interprofessional strategic planning certainly enhances the potential rehabilitation of this patient.

Conclusion

The oropharyngeal dysphagia in patients with AAMG is correlated with the orofacial and respiratory muscular components, translated by measurements of strength and flow. The cough strength and expiratory flow are correlated to the strength of inspiratory and exhalation muscles. The reduction of MIP values, MEP and PEF were indicators of the oropharyngeal dysphagia worsening. In this study both respiratory muscle strength measurements were below of the predicted.

References

1. Cunha FM, Scola RH, Werneck LC. [Myasthenia gravis. Clinical evaluation of 153 patients]. Arq Neuropsiquiatr. 1999; 57: 457-464.
2. Fregonezi GA, Regiane-Resqueti V, Pradas J, Vigil L, Casan P. [The relationship between lung function and health-related quality of life in patients with generalized myasthenia gravis]. Arch Bronconeumol. 2006; 42: 218-224.
3. Rassler B, Hallebach G, Kalischewski P, Baumann I, Schauer J, Spengler CM. The effect of respiratory muscle endurance training in patients with myasthenia gravis. Neuromuscul Disord. 2007; 17: 385-391.
4. Oda AL, Chiappetta AL, Annes M, Marchesan IQ, Oliveira AS. [Clinical, endoscopical and manometric evaluation of swallowing in patients with acquired autoimmune myasthenia gravis]. Arq Neuropsiquiatr. 2002; 60: 986-995.
5. Fambrough DM, Drachman DB, Satyamurti S. Neuromuscular junction in myasthenia gravis: decreased acetylcholine receptors. Science. 1973; 182: 293-295.
6. Khan OA, Campbell WW. Myasthenia gravis presenting as dysphagia: clinical considerations. Am J Gastroenterol. 1994; 89: 1083-1085.
7. Lohi EL, Lindberg C, Andersen O. Physical training effects in myasthenia gravis. Arch Phys Med Rehabil. 1993; 74: 1178-1180.
8. Panda S, Goyal V, Behari M, Singh S, Srivastava T. Myasthenic crisis: a retrospective study. Neurol India. 2004; 52: 453-456.
9. Coronel M. Miastenia Gravis. Revista Médica de Tucumán. 2005; 11: 1-2.
10. Chiappetta ALML, Oda AL. Doenças Neuromusculares. In: Ferreira LP, Befi-Lopes DM, Limongi SCO. Tratado de Fonoaudiologia. São Paulo: Roca. 2004; 330-342.
11. Chiappetta AL, Oda AL, Zanoteli E, Guilherme A, Oliveira AS. [Oropharyngeal dysphagia in the myotonic dystrophy: phonoaudiological evaluation and nasofibrolaryngoscopical analysis].Arq Neuropsiquiatr. 2001; 59: 394-400.
12. American Thoracic Society/European Respiratory Society. ATS/ERS Statement on respiratory muscle testing. Am J Respir Crit Care Med. 2002; 166: 518-624.
13. Neder JA, Andreoni S, Lerario MC, Nery IE. Reference values for lung function tests. II. Maximal respiratory pressures and voluntary ventilation. Brazilian Journal of Medical and Biological Research 1999; 32: 719-727.
14. Montero-Odasso M. Dysphonia as first symptom of late-onset myasthenia gravis. J Gen Intern Med. 2006; 21: C4-6.
15. Diretrizes de MG. Protocolo Clínico e Diretrizes Terapêuticas. Miastenia Gravis. Portaria SAS/MS Number 229 de 10 de maio de 2010 (Retificada em 27/08/2010): 465-488.
16. Carvalho ASR, Silva AV, Ortensi FMF, Fontes SF, Oliveira ASB. Miastenia Grave autoimune: Aspectos clínicos e experimentais. Rev Neurociências. 2005; 13: 138-144.
17. Burch J, Warren-Gash C, Ingham V, Patel M, Bennett D, Chaudhuri KR. Myasthenia gravis--a rare presentation with tongue atrophy and fasciculation. Age Ageing. 2006; 35: 87-88.
18. Noda JL, Sonoda LT, Sangean M, Fávero FM, Fontes SV, Oliveira ASB. O efeito do tratamento muscular respiratório na miastenia grave: revisão da literatura – Trabalho realizado no Setor de doenças Neuromusculares da Universidade Federal de São Paulo (UNIFESP), São Paulo-SP, Brasil.
19. Lavrnić D, Jarebinski M, Rakocević-Stojanović V, Stević Z, Lavrnić S, et al. Epidemiological and clinical characteristics of myasthenia gravis in Belgrade, Yugoslavia (1983-1992). Acta Neurol Scand. 1999; 100: 168-174.
20. Ruiz LRJ; Reibscheid SM; Cataneo AJM; Rezende LAL. Resultado da timectomia ampliada no tratamento de pacientes com Miastenia gravis. J Bras. 2004; 30: 115-120.
21. Goti P, Spinelli A, Marconi G, Duranti R, Gigliotti F, Pizzi A. Comparative effects of plasma exchange and pyridostigmine on respiratory muscle strength and breathing pattern in patients with myasthenia gravis. Thorax. 1995; 50: 1080-1086.
22. García Río F, Prados C, Díez Tejedor E, Díaz Lobato S, Alvarez-Sala R, Villamor J. Breathing pattern and central ventilatory drive in mild and moderate generalised myasthenia gravis. Thorax. 1994; 49: 703-706.
23. Seneviratne J, Mandrekar J, Wijdicks EF, Rabinstein AA. Noninvasive ventilation in myasthenic crisis. Arch Neurol. 2008; 65: 54-58.
24. Agarwal R, Reddy C, Gupta D. Noninvasive ventilation in acute neuromuscular respiratory failure due to myasthenic crisis: case report and review of literature. Emerg Med J 2006; 23:e06.
25. Ferguson IT, Murphy RP, Lascelles RG. Ventilatory failure in myasthenia gravis. J Neurol Neurosurg Psychiatry. 1982; 45: 217-222.
26. Heliopoulos I, Patlakas G, Vadikolias K, Artemis N, Kleopa KA, Maltezos E. Maximal voluntary ventilation in myasthenia gravis. Muscle Nerve. 2003; 27: 715-719.
27. Ertekin C, Aydogdu I. Neurophysiology of swallowing. Clin Neurophysiol. 2003; 114: 2226-2244.
28. Kim JY, Park KD, Richman DP. Treatment of myasthenia gravis based on its immunopathogenesis. J Clin Neurol. 2011; 7: 173-183.
29. Symonette CJ, Watson BV, Koopman WJ, Nicolle MW, Doherty TJ. Muscle strength and fatigue in patients with generalized myasthenia gravis. Muscle Nerve. 2010; 41: 362-369.
30. Drachman DB. Myasthenia gravis. N Engl J Med. 1994; 330: 1797-1810.
31. Cridge PB, Allegra J, Gerhard H. Myasthenic crisis presenting as isolated vocal cord paralysis. Am J Emerg Med. 2000; 18: 232-233.
32. Carpenter RJ 3rd, McDonald TJ, Howard FM Jr. The otolaryngologic presentation of myasthenia gravis. Laryngoscope. 1979; 89: 922-928.
33. Weijnen FG, van der Bilt A, Wokke JH, Kuks JB, van der Glas HW, Bosman F. What's in a smile?: Quantification of the vertical smile of patients with myasthenia gravis. J Neurol Sci. 2000; 173: 124-128.
34. Oda, AL, Chiappetta ALML, Medrano LMM. Miastenia Grave: O resgate do soriso na reabilitação fonouldiológica. O Mundo da Saúde, São Paulo: 2006; 30: 65-72.
35. Buccholz DW, Robbins J. Neurologic diseases affecting oropharyngeal swallowing. In Pearlman AL, Schulze-Delrieu K (eds). Deglutition and its disorders: anatomy, physiology, clinical diagnosis and management. New York: Singular Publishing Group Inc, 1997.
36. Douglas CR. Fisiologia da deglutição. Tratado de Fisiologia Aplicada às Ciências da Saúde 1ª ed. Robe editorial. 895-910 1994.
37. Silva LMC. Disfagia Avaliação e Tratamento. 2ed Rio de Janeiro: Revinter; 2004.cap.2: 18-25.
38. Wolfe GI, Herbelin L, Nations SP, Foster B, Bryan WW, Barohn RJ. Myasthenia gravis activities of daily living profile. Neurology. 1999; 52: 1487-1489.
39. Mertens HG, Balzereit F, Leipert M. The treatment of severe myasthenia gravis with immunosuppressive agents. Eur Neurol. 1969; 2: 321-339.
40. Logemann JA. Evaluation and Treatment of Swallowing Disorders. San Diego. College Hill Press, 1998.
41. Costa MMB; Moscovici M; Pereira AA; Koch HA. Avaliaçäo videofluoroscópica da transiçäo faringoesofágica (esfíncter superior do esôfago). Radiol bras. 1993; 26: 71-80
42. Kahrilas PJ. Anatomy Physiology and Pathophysiology of Dysphagia. Acta Oto-rhino-laryngologica Belg 1994; 48: 97-117.
43. Weijnen FG, van der Bilt A, Wokke JH, Wassenberg MW, Oudenaarde I. Oral functions of patients with myasthenia gravis. Ann N Y Acad Sci. 1998; 841: 773-776.
44. Aronson AE. Early Motor Unit Disease Masquerading as Psychogenic Breathy Dysphonia: A Clinical Case Presentation. Journal of Speech and Hearing Disorders. 1971; 36: 115-124.
45. Lo YL, Leoh TH, Dan YF, Tan YE, Nurjannah S, Ratnagopal P. Repetitive stimulation of the long thoracic nerve in myasthenia gravis: clinical and electrophysiological correlations. J Neurol Neurosurg Psychiatry 2003; 74: 379-381.
46. Fiore Jr JF, Paiani DM, Franceschini J, Chiavegato LD, Faresin SM. Pressões respiratórias máximas e capacidade vital: comparação entre avaliações através do bocal e de máscara facial. J Bras Pneumol 2004; 30: 515-520.
47. Schettino GPP, Tucci MR, Sousa R, Barbas CSV, Amato MBP, Carvalho CRR. Mask mechanics and leak dynamics during noninvasive pressure support ventilation: a brench study. Intensive Care Med 2001; 27: 1887-1891.
48. Tidwell J. Pulmonary management of the ALS patient. J Neurosci Nurs. 1993; 25: 337-342.
49. [No authors listed]. [II Brazilian guidelines for the management of chronic cough]. J Bras Pneumol. 2006; 32 Suppl 6: S403-446.
50. Chiappetta ALML, Oda AL, Hayashi MBC, Stanich P, Oliveira ASB, Gabbai AA - Swallowing evaluation in ALS patients: The importance of MEP to prevent laryngeal penetration and tracheal aspiration. 14th International Symposium on ALS/MND (amyotrophic lateral sclerosis/ motor neuron disorders); 2003; 17-19; Milan, Italy.
51. Koch WM. Swallowing disorders. Diagnosis and therapy. Med Clin North Am. 1993; 77: 571-582.
52. Black LF, Hyatt RE. Maximal static respiratory pressures in generalized neuromuscular disease. Am Rev Respir Dis. 1971; 103: 641-650.
53. Kaplan LM, Hollander D. Respiratory dysfunction in amyotrophic lateral sclerosis. Clin Chest Med. 1994; 15: 675-681.
54. Strand EA, Miller RM, Yorkston KM, Hillel AD. Management of oral-pharyngeal dysphagia symptoms in amyotrophic lateral sclerosis. Dysphagia. 1996; 11: 129-139.
55. Ertekin C, Aydogdu I, Yüceyar N, Kiylioglu N, Tarlaci S, Uludag B . Pathophysiological mechanisms of oropharyngeal dysphagia in amyotrophic lateral sclerosis. Brain. 2000; 123: 125-140.
56. Quadros A, Chiappetta ALM, Silva HCA. Resumo do 14º Simpósio Internacional de Esclerose Lateral Amiotrófica. ABRELA. 2003; 17-19 Milão Itália.
57. Silva LMC, Jacobi JS. Disfagia Orofaríngea e sua importância na pneumologia. In: Jacobi JS, Levy DS, Silva LMC. Disfagia Avaliação e Tratamento. Rio de Janeiro: Revinter; 2004. Cap.12, 163-180.
58. Boitano LJ. Management of airway clearance in neuromuscular disease. Respir Care. 2006; 51: 913-922.
59. Thomas CE, Mayer SA, Gungor Y, Swarup R, Webster EA, Chang I. Myasthenic crisis - Clinical features, mortality, complications, and risk factors for prolonged intubation. Neurology. 1997; 48: 1253-1260
60. Martens L, Cameron T, Simonsen M. Effects of a multidisciplinary management program on neurologically impaired patients with dysphagia. Dysphagia. 1990; 5: 147-151.