NOCTURNAL OXYGENATION AND PROGNOSIS IN DUCHENNE MUSCULAR DYSTROPHY

	To the Editor :

The nocturnal use of nasal ventilation can correct nocturnal hypoxia and hypercapnia, improve daytime blood gases, and resolve symptoms of chronic alveolar hypoventilation (1, 2). In itself, though, nocturnal-only use of noninvasive ventilation can not be expected to greatly prolong survival because for patients with neuromuscular respiratory muscle weakness, acute respiratory failure and death, or resort to tracheostomy, most often occurs as a result of inability to effectively cough during otherwise benign upper respiratory tract infections (3). Proper intervention with expiratory as well as inspiratory aids is more important to maintain patients free of respiratory failure than is simple use of nocturnal nasal ventilation (4). With time, even when not ill, neuromuscular disease patients can become weaker to the point of requiring continuous noninvasive ventilation, most often via mouthpiece during daytime hours and nasal interface overnight, without ever needing to undergo tracheostomy (3). Many patients who decline invasive mechanical ventilation are willing to use noninvasive ventilation (5) to both prolong survival (2) and improve quality of life (6).

In their recent publication, Philips and colleagues (7) suggested that besides monitoring vital capacity (VC) and Pa_CO2, polysomnography and arterial blood gases and evaluation for REM-related oxygen desaturation should be undertaken. However, we have maintained hundreds of patients who eventually weakened to the point of requiring continuous noninvasive ventilatory support by just considering symptoms, VC, end-tidal CO₂, and nocturnal oximetry. Indeed, considering that many of our DMD patients have had to use support continuously for over 50 yr, it is hard to understand why clinicians would want to prematurely burden essentially asymptomatic patients with nocturnal nasal ventilation largely on the basis of polysomnography findings. Thus, it is not surprising that Phillips and coworkers' 18 patients died around 22 yr of age because, although treated for sleep-disordered breathing, they were left without means for effective assisted coughing or alveolar ventilation during times of greatest need, including during daytime hours. Since many of our DMD patients have had VC measurements of less than 200 ml for 10 yr or more and they are not only alive but also without tracheostomy tubes, "a VC of less than 1 liter" only indicates "death from respiratory failure in 4 yr" when patients are suboptimally managed. When clinicians understand how to use inspiratory and expiratory muscle aids, the VC becomes irrelevant for prognosis.

Further, it is disappointing that echocardiograms were performed for only three patients who were suspected of symptomatic cardiomyopathy and that no treatment was noted even for one patient with global hypokinesis. It has been demonstrated that cardiac neuroendocrines can be normalized and left ventricular ejection fractions improved with treatment (8).

Department of PediatricsNational Yakumo HospitalHokkaido, Japan

John R. Bach

Department of Physical Medicine and RehabilitationUniversity of Medicine and Dentistryof New JerseyNewark, New Jersey

1. Birnkrant, D. J., J. F. Pope, and R. M. Eiben. 1997. Pediatric noninvasive nasal ventilation. J. Child Neurol. 12: 231-236 [Medline].

2. Simonds, A. K., F. Muntoni, S. Heather, and S. Fielding. 1998. Impact of nasal ventilation on survival in hypercapnic Duchenne muscular dystrophy. Thorax 53: 949-952 [Abstract/Free Full Text].

3. Bach, J. R. 1999. Guide to the Evaluation and Management of Neuromuscular Disease. Hanley & Belfus, Inc., Philadelphia.

4. Bach, J. R., Y. Ishikawa, and H. Kim. 1997. Prevention of pulmonary morbidity for patients with Duchenne muscular dystrophy. Chest 112: 1024-1028 [Abstract/Free Full Text].

5. Make, B. J., N. S. Hill, A. I. Goldberg, J. R. Bach, G. J. Criner, P. E. Dunne, M. E. Gilmartin, J. E. Hoffner, R. Kacmarek, T. G. Keens, S. McInturff, W. J. O'Donohue Jr., E. A. Oppenheimer, and D. Robert. 1998. Mechanical ventilation beyond the intensive care unit: report of consensus conference of the American College of Chest Physicians. Chest 113(Suppl.): 289S-344S [Medline].

6. Barthlen, G. M. 1997. Nocturnal respiratory failure as an indication of noninvasive ventilation inpatient with neuromuscular disease. Respiration 64(Suppl. 1):35-38.

7. Phillips, M. F., E. M. Smith, N. Carrol, R. H. T. Edwards, and P. M. A. Calverley. 1999. Nocturnal oxygenation and prognosis in Duchenne muscular dystrophy. Am. J. Respir. Crit. Care Med. 160: 198-202 [Abstract/Free Full Text].

8. Ishikawa, Y., J. R. Bach, and R. Minami. 1999. Cardioprotection for Duchenne's muscular dystrophy. Am. Heart J. 137: 895-902 [Medline].

	From the Authors:

We were interested to read the comments of Drs. Ishikawa and Bach about our recent paper (1) describing a prospective investigation of the role of different monitoring strategies for patients with Duchenne muscular dystrophy. We entirely agree with the points that they raise in their first paragraph, which are very much in keeping with our own approach outlined a number of years ago (2). However, we are concerned that they may have misinterpreted the intent of our paper and perhaps drawn erroneous conclusions. The patients who were included in the study had all undergone polysomnography (PSG) in a defined period in the late 1980s and were then followed up as a population to establish the natural history of their illness, and its relationship to these initial diagnostic findings. In our original submission we made rather stronger statements about the relative merit of different assessment approaches but modified these in the light of the referees' comments. We feel our data do not support the need for routine PSG assessment in patients with Duchenne muscular dystrophy. It was pointed out to us that we had not conclusively established this to be the case in all comers and so we have left the readers of our paper to draw their own conclusions about the relative merits of the different approaches, something which Drs. Ishikawa and Bach have clearly done. In general, we agree with them that patients should not be burdened with ventilatory support on the basis of PSG findings alone and we had thought that we had made this clear in our paper. If we did not we would certainly wish to take this opportunity to do so.

We are a little puzzled at the statement that "many of our patients have had to use support continuously for over 50 years," as this does not accord with our experience of this illness where, regrettably, such prolonged survivals are unknown. At the time when our study cohort was recruited nasal positive pressure ventilation was not widely used. We continue to discuss this form of ventilatory support and have helped to develop guidelines with the Muscular Dystrophy Group of Great Britain and Northern Ireland for the application of this treatment. The difference in the take up of our community may reflect different patients and care or perceptions and this may be an area of further research. We would certainly stand by our observation that a decline in vital capacity to below 1 L is a simple measurement that can be readily applied to indicate the onset of the terminal phase of the illness, at least from the respiratory point of view. We accept that cardiac screening has played a larger role in recent years for the assessment of these patients. We note that all of the references quoted by these correspondents have been published in the last two years and in our defense would say we did not have access to their data at the time when our patients were alive, which sadly was a number of years before publication.

P. M. A. CALVERLY M. F. PHILLIPS

Pulmonary and Rehabilitation Group University of Liverpool Liverpool, United Kingdom

1. Phillips, M. F., P. E. N. Smith, N. Carrol, R. H. T. Edwards, and P. M. A. Calverly. 1999. Nocturnal oxygenation and prognosis in Duchenne muscular dystrophy. Am. J. Respir. Crit. Care Med. 160: 198-202 .

2. Calverly, P. M. A., R. H. T. Edwards, G. A. Evans, and E. J. M. Campbell. 1987. Practical problems in the respiratory care of patients with muscular dystrophy. New Engl. J. Med. 316: 1197-1205 [Medline].