597
activation in acute lung injury after bums and smoke inhalation. Lancet 1988; ii: 872-74. 16. Boschetto P, Mapp CE, Picotti G, Fabbri LM. Neutrophils and asthma. Eur Respir J 1989; 2 (suppl 6): 456-59S. 17. Clark CJ, Kinsella J, Reid WH, Campbell D. The role of fibreoptic bronchoscopy in early diagnosis of smoke inhalation injury. Thorax
pulmonary alveolar macrophage
1988; 43: 828. 18. Boulaye L. Increases in airway responsiveness following acute exposure to respiratory irritants. Reactive airway dysfunction syndrome or occupational asthma? Chest 1988; 94: 476-81. 19. Stenton SC, Kelly CA, Walters EH, Hendrick DJ. Induction of bronchial hyperresponsiveness following smoke inhalation injury. Br J Dis Chest 1988; 82: 436-38.
20. Dehaut P, Rachiele A, Martin RR, Malo JL. Histamine dose-response curves in asthma: reproducibility and sensitivity of different indices to assess response. Thorax 1983; 38: 516-22. 21. Vincent NJ, Knudson R, Leith DE. Factors influencing pulmonary resistance. J Appl Physiol 1970; 29: 336-43. 22. Orehek J, Massari JP, Gayrard P, Grimaud C, Chorpin J. The effect of short term, low level nitrogen dioxide exposure on bronchial sensitivity of asthmatic patients. Clin Invest 1976; 57: 301-07. 23. Islam MS, Vastag E, Ulmer WT. Sulphur dioxide induced bronchial hyperreactivity against acetal choline. Int Arch Arveits Med 1972; 29: 221-32. 24. Sumik T. Combustion products of polymeric materials containing nitrogen in their chemical structure. J Fire Flammability 1973; 4:15-22.
Muscle fibre type and habitual
Although anatomical abnormalities of the upper airway have been recorded in some patients with obstructive sleep apnoea (OSA), a muscle tone dysregulation also seems to have an important role in this disorder. Since habitual snoring is the initial stage of OSA, the structural characteristics of upper
airway
muscles
(medium pharyngeal
constrictor
muscle [MPCM]) from 13 men (9 non-snorers and 4 habitual snorers) were studied. MPCM fibre structure in non-snorers was broadly similar to that in normal limb muscles, with the exception that fibre diameters were smaller for all fibre types. Compared with limb muscles, MPCM had a smaller proportion of type lib fibres and a higher proportion of types I and Ila fibres. MPCM in habitual snorers had an abnormal distribution of fibre types (low percentage of type I and type IIb fibres and high percentage of type IIa fibres) compared with non-snorers (p < 0·001) and the type IIa fibres were hypertrophic. No myopathic or neurogenic
changes were seen. Two possible hypotheses explain the abnormal distribution of fibre types in snorers. First, a constitutionally determined reduction of slow alphamotor neurons induces an adaptive transformation of type IIb to type IIa fibres and a hypertrophy of type IIa fibres; or, second, motor neurons change their patterns of discharge and, hence, of activation, and modify fibre-type distribution of MPCM as an adaptation to the anatomical characteristic of upper airway and habitual snoring. Introduction Tonic electromyographic activity is absent during quiet, relaxed wakefulness both in man and in animals, even (though transiently) when the upper motor neuron is damaged and there is clinical spasticity.Furthermore, there is no tonic electromyographic activity during any of the stages of sleep in healthy men and in most patients with spastic hemiparesis.2 Not all skeletal muscle has this characteristic. For example, hyoid muscles in the pharynx
snoring
continuously discharge during wakefulness and non-rapid eye movement (REM) sleep and lose their tonic electromyographic activity only during REM sleep.3 Tonic activity of pharyngeal constrictors disappears during sleep apnoea in man as does phasic inspiratory discharges of some upper airway muscles, such as genioglossus and stylopharyngeus.4,5 Some patients with obstructive sleep apnoea (OSA) have obvious anatomical abnormalities, such as micrognathia, enlarged tonsils or adenoids, macroglossia, and soft palate hypertrophy, which have a role in airway obstruction. In overweight patients it has been suggested that deposition of fatty tissue around the upper airway is a causal factor of OSA. In some patients, computed tomography and magnetic resonance imaging studies show an abnormal narrow airway behind the soft palate.6,7 However, in many obese or non-obese patients with OSA, there is no anatomical abnormality or there is only a slight narrowing of the oropharynx. In patients without anatomical abnormalities, OSA is thought to be due to an excessive loss of muscle tone in the upper airway or an abnormal reflex regulation of upper airway function during sleep.8 So far, little attention has been given to the possibility that structural changes in the pharyngeal muscles contribute to airway obstruction. Knowledge of the morphological characteristics of the normal middle pharyngeal constrictor muscle (MPCM) is needed to assess the possible primary muscle alterations of the pharynx in OSA patients. Since habitual snoring is the initial stage of OSA,9 we decided to defme histological and histochemical characteristics of MPCM specimens from snorers and non-snorers.
Patients and methods Between 1987 and 1990, 13 men who had laryngeal carcinoma in situ and who were going to have total laryngectomy were selected according to the following criteria: body mass index (BMI) less than 28; no clinically evident obstruction of the upper airway; no ADDRESSES. Sleep Disorders Centre (Prof S. Smirne, MD, S. lannaccone, MD, L Ferini-Strambi MD) and Department of Neurology (M Comola, MD, R. Nemni, MD) State University and Scientific Institute H S.Raffaele, Milan; and Department of Otolaryngology, Hospital Niguarda Ca’ Granda, Milan, Italy (E. Colombo, MD) Correspondence to Prof S. Smirne, Department of Neurology, University of Milan, H S Raffaele, Via Prinetti 29, 20127
Milan, Italy.
598
previous radiotherapy, chemotherapy, or corticosteroid treatment; and no other organic diseases, excluded by physical and laboratory examinations. All selected patients were smokers and some had had tonsillectomy and/or adenoidectomy. 9 patients were non-snorers (mean BMI [SD] 26-4 [1-9]; range 25-8-27-9) aged 38-61 years (mean [SD] 53-5 [7-6]), and 4 were habitual snorers (mean BMI [SD] 26-7 [0-3]; range 26-4-27-1), aged 58-68 years (mean [SD] 63-3 [4-1]). Habitual snorers were defined as people who snored every night and all night, as reported by the household in a simple questionnaire. A cylinder of muscle tissue (about 8 mm long and 4 mm wide) was taken from the MPCM during surgery. The same site from each patient was sampled-ie,1 cm from the tip of the greater process of the hyoid bone. Muscle samples were rapidly frozen by submersion in isopentane, cooled in liquid nitrogen (— 160°C), and successive transverse sections (10 lUll) were cut with the cryostat. The following histological stains and histochemical reactions were applied to all muscle specimens: haematoxylin/eosin, modified Gomori trichrome, NADH tetrazolium reductase, Sudan black B, periodic acid-Schiff, and ATPase at pH 9-4, 46, and 4-2. The histological pattern of muscle specimens was evaluated by the criteria of Dubowitz.10 Additionally, in each biopsy sample, three random fields of sections stained with ATPase pH 4-6 were photographed (x 100) and 200 muscle fibres were analysed for fibre type distribution and diameter. The fibre distribution was calculated for type I, type IIa, and type IIb according to Dubowitz.1O The "lesser fibre diameter", defined as the maximum diameter across the lesser aspect of the muscle fibre," was measured. The person who examined the muscle specimens did not know to which group the patients had been assigned. The two groups were compared by means of simple analysis of variance.
Results Mean diameter and proportion of fibre types in MPCM shown in the table. In the analysis of muscle fibre diameters we considered the mean of mean values in the two groups because the within-subject variability was negligible compared with the between-subject variability (one-way are
Anova). MUSCLE FIBRE TYPES IN MEDIUM PHARYNGEAL CONSTRICTOR MUSCLE OF NON-SNORERS AND HABITUAL SNORERS I
NS
=
not
significant
I
Non-snorers
Histological patterns of the MPCM were broadly similar those of normal skeletal muscles, apart from fibre diameters. MPCM fibres had the usual polygonal aspect and eccentric nuclei, and were grouped in secondary to
fascicles of normal architecture. There was no difference in endomysium or perimysium connective tissue, and there no were pathological deposits of lipidic or mucopolysaccharidic material. Oxidative enzymic activity was extended throughout the muscle fibre in a regular ordered network. As judged by the ATPase reaction, at various pHs, the MPCM had a normal fibre mosaic, including type I, IIa, and lIb muscle fibres. The distribution curve of muscle fibre diameters was bell-shaped for type I and type IIa fibres, as in limb muscles most commonly studied in healthy subjects.10 The distribution curve of type IIb diameters was not bellshaped : more than 85 % of the muscle fibres had a diameter in the range 10-30 um. Habitual snorers
patients, type IIa fibre diameter was greater than that of non-snorers. Habitual snorers had an abnormal distribution of fibre types compared with non-snorers: type I and type IIb fibres were reduced and type IIa fibres were increased (table). In these 4
significantly
Discussion Studies of normal limb muscles have shown that there is a large variation in fibre diameter among individuals. However, if biopsy samples are taken from the same site and the same depth in the muscle from each patient, as we have done, variability is negligible.ll In muscles most commonly sampled in biopsy studies, such as the vastus lateralis or biceps brachii, type I, IIa, and IIb fibres are present approximately in equal proportion,’ and the distribution curve of fibre diameters is bell-shaped with diameters in the range of 35 pm to 75 J.llTI.1O In our non-snorers, the MPCM fibre sructure was similar to that of normal skeletal muscles, but fibre diameter was uniformly reduced compared with muscles most commonly sampled.13 Only about 10% fast-twitch fatigue sensitive fibres (type IIb), and nearly 90% fatigue resistant fibres (including slow-twitch oxidative fibres [type I] and fast-twitch oxidative-glycolytic fibres [type IIa] were seen in non-snorers). Since type I muscle fibres are well endowed with mitochondria (they work oxidatively using fatty acids as a source of acetyl-CoA) and type IIa fibres can increase their oxidative capacity, it follows that most muscle fibres in human MPCM are metabolically fit for fatigue resistance. The high percentage of fatigue-resistant fibres points to a continuing tonicity of the pharyngeal wall that should help to keep the lumen open during respiration by counteracting the intrathoracic negative pressure. By contrast, Leese and Hopwood13 found a higher proportion of type II fibres in 50 MPCM samples taken at necropsy (67% vs 51 % in our study). Unfortunately, these investigators could not differentiate type IIa from type IIb fibres with their methods (ATPase at pH 4-2 on samples obtained within 24 hours of death). In our experience, fibre typing of post-mortem MPCM is imprecise even 3 hours after death. MPCM seems to play a fundamental part in the pathogenesis of OSA. Obstruction is due to negative inspiratory pressure, which causes collapse of the narrow
599
Guilleminault and colleagues4 demonstrated a temporary absence of electromyographic MPCM activity and collapse of pharyngeal walls on inspiration during sleep apnoea. Why should our habitual snorers have an abnormal distribution of fibre type compared with non-snorers? We believe that the difference in the mean age between the two groups is not important since there are changes in muscle fibre size and in distribution of fibre type with advancing age only in people over 70 years old.15,16 Skeletal muscle fibres change in response to different functional demands. For example, exercise is a well-known stimulus for the growth of skeletal muscles. Changes include hypertrophy of the fibres, increased proportion of type IIa fibres, decreased proportion of type IIb fibres, and increased resistance to fatigueY-19 Fast-twitch and slow-twitch fibres are innervated by fast conducting and slowly conducting alpha-motor neurons, respectively, which determine the enzymic activity of muscle fibres and hence their type.2o Cross-innervation ’21 immobilisation, denervation, and electrical stimulation22 studies show that skeletal muscle fibres can switch from one type to another. We therefore suggest two possible hypotheses to explain the abnormal distribution of fibre types in our snorers. First, the reduction of slow alpha-motor neurons and, hence, of type I fibres is constitutionally determined; this structural modification induces a compensatory hypertrophy of type IIa fibres and a shift of type I Ib fibres to IIa fibres. Second, motor neurons change their patterns of discharge and, therefore, of MPCM activation, as an adaptation to the anatomical characteristic of upper airway and habitual snoring. In animals, immobilisation and/or continuous low-frequency electrical stimulation change type II to type I fibres,22 whereas denervation and intermittent bursts of high-frequency stimulation produce the reverse change.23 In man, however, endurance exercise increases respiratory enzyme activity but does not induce the interconversion of type I and type II fibres.18 Wade et al24 showed a strong relation between fatness and muscle fibre type with a proportion of type I fibres inversely related to fatness. Obesity is one of the most frequent features ofOSA,25 and it has been suggested that its effect is related to a diffuse fatty infiltration and narrowing of the pharynx.7 Could a reduction of type I fibres be a causal factor not only for obesity but also for habitual snoring? Since OSA is determined by many factors, the relative importance of muscle fibre distribution in MPCM has to be evaluated in a larger number of snorers.
and/or hypotonic airway. 14
We thank Dr S. Palazzi, and Dr G. Calori, Istituto Scientifico H S. Raffaele, for help with the statistical analysis.
REFERENCES 1. Basmajian
JW, Szatmari A. Effect of largactil (chlorpromazine) on human spasticity and electromyogram. Arch Neurol 1955; 73: 224-31.
2. Tauber ES, Coeman
RM, Elliot D, Weitzman ED. Absence of tonic electromyographic activity during sleep in normal and spastic nonmimetic skeletal muscles in man. Ann Neurol 1977; 2: 66-68. 3. Hishikawa Y, Sumitsuji N, Matsumoto K, Kaneko Z. H-reflex and EMG of the mental and hyoid muscles during sleep, with special reference to narcolepsy. Electroencephalogr Clin Neurophysiol 1965; 18: 487-92. 4. Guilleminault
C, Hills MW, Simmons FB, Dement WC. Obstructive sleep apnea: electromyographic and fiberoptic studies. Exp Neurol 1978; 62: 48-67. 5. Remmers JE, deGroot J, Saverland K, Anch AM, Pathogenesis of upper airway occlusion during sleep. J Appl Physiol 1978; 44: 931-38. 6. Suratt PM, Dee P, Atkinson RL, Armstrong P, Wilhoit SC.
Fluoroscopic and computed tomographic features of the pharyngeal airway in obstructive sleep apnea. Am Rev Respir Dis 1983; 127: 487-92. 7. Horner RL, Mohiaddin RM, Lowell DG, et al. Sites and sizes of fat deposits around the pharynx in obese patients with obstructive sleep apnea and weight matched controls. Eur Resp J 1989; 2: 613-22. 8. Sullivan CE, Issa FG, Ellis E, et al. Treatment of cardiorespiratory disturbances during sleep. In: Emser W, Kurtz D, Webb WB, eds. Sleep, aging and related disorders. Basel: Karger, 1987: 47-67. 9. Lugaresi E, Mondini S, Zucconi M, Montagna P, Cirignotta F. Staging of heavy snorers disease. A proposal. Bull Eur Physiopathol Resp 1983; 19: 590-94. 10. Dubowitz V, ed. Muscle biopsy: a practical approach. 2nd ed. London: Baillière Tindall, 1985: 82-128. 11. Lexel J, Henriksson-Larsen K, Sjostrom M. Distribution of different fibre types in human skeletal muscles. 2. A study of cross-sections of whole m. vastus lateralis. Acta Physiol Scand 1983; 117: 115-22. 12. Åstrom KE, Adams RD. Pathological reactions of skeletal muscle fibre in man. In: Walton JN, ed. Disorders of voluntary muscle. London: Churchill Livingstone, 1981: 151-208. 13. Leese G, Hopwood D. Muscle fibre typing in the human pharyngeal constrictors and oesophagus: the effect of ageing. Acta Anat 1986; 127: 77-80. 14. Rojewski TE, Schuller DE, Clark RW, Schmidt HS, Potts RE. Videoendoscopic determination of the mechanism of obstruction in obstructive sleep apnea. Otlaryngol Head Neck Surg 1984; 92: 127-31. 15. Grimby G, Danneskiold-Samsoe B, Hvid K, Saltin B. Morphology and enzymatic capacity in arm and leg muscles in 78-82 year old men and women. Acta Physiol Scand 1982; 115: 124-34. 16. Grimby G, Saltin B. The ageing muscle. Clin Physiol 1983; 3: 209-18. 17. Thorstensson A. Muscle strength, fibre types and enzyme activities in man. Acta Physiol Scand 1976; 443 (suppl): 7-45. 18. Barnard RJ, Edgerton VR, Peter JB. Effect of exercise on skeletal muscle. I. Biochemical and histochemical properties. J Appl Physiol 1970; 28: 762-66. 19. Salmons S, Henriksson J. The adaptive response of skeletal muscle to increased use. Muscle Nerve 1981; 4: 94-105. 20. Buller AJ. The physiology of the motor unit. In: Walton JN, ed. Disorders of voluntary muscle. London: Churchill Livingstone, 1974: 20-30. 21. Buller AJ, Eccles JC, Eccles RM. Interaction between motoneurons and muscles in respect of the characteristic speed of their response. J Physiol (Lond) 1960; 150: 417-39. 22. Cotter M, Phillips P. Rapid fast to slow fibre transformation in response to chronic stimulation of immobilized muscles of the rabbit. Exp Neurol 1986; 93: 531-45. 23. Al-Amood WS, Lewis DM. The role of frequency in the effects of long-term intermittent stimulation of denervated slow-twitch muscle in the rat. J Physiol 1987; 392: 377-95. 24. Wade AJ, Marbut MM, Round JM. Muscle fibre type and aetiology of obesity. Lancet 1990; 335: 805-08. 25. Guilleminault C, van de Hoed J, Milter MM. Clinical overview of the sleep apnea syndromes. In: Guilleminault C, Dement WC, eds. Sleep apnea syndromes. New York: Alan Liss, 1978: 1-12.
From The Lancet Deaths under chloroform In recording more deaths from this anaesthetic we must repeat what we have before urged: that more systematic accounts of such occurrences should be given. The lay press, to which we owe most of our information, cannot, for obvious reasons, be expected to give scientific statements as to such fatalities. Coroners are not always, unfortunately, possessed of the special medicolegal knowledge requisite for putting the necessary questions.... It seems to us most important that in every instance cases of death occurring under anaesthetics should be reported in the medical journals, and that there should be stated: the standing and special experience of the chloroformist, the amount of the anaesthetic given, the method adopted, the time required to effect anaesthesia, the amount of struggling; whether heart or respiration failed first; some particulars as to the patient’s previous history, whether, for example, he was addicted to habits of intemperance; whether he suffered from any general disease--eg, such as causes lardaceous change, syphilis, tuberculosis; whether evidence of cardiac or arterial degeneration existed.
activation in acute lung injury after bums and smoke inhalation. Lancet 1988; ii: 872-74. 16. Boschetto P, Mapp CE, Picotti G, Fabbri LM. Neutrophils and asthma. Eur Respir J 1989; 2 (suppl 6): 456-59S. 17. Clark CJ, Kinsella J, Reid WH, Campbell D. The role of fibreoptic bronchoscopy in early diagnosis of smoke inhalation injury. Thorax
pulmonary alveolar macrophage
1988; 43: 828. 18. Boulaye L. Increases in airway responsiveness following acute exposure to respiratory irritants. Reactive airway dysfunction syndrome or occupational asthma? Chest 1988; 94: 476-81. 19. Stenton SC, Kelly CA, Walters EH, Hendrick DJ. Induction of bronchial hyperresponsiveness following smoke inhalation injury. Br J Dis Chest 1988; 82: 436-38.
20. Dehaut P, Rachiele A, Martin RR, Malo JL. Histamine dose-response curves in asthma: reproducibility and sensitivity of different indices to assess response. Thorax 1983; 38: 516-22. 21. Vincent NJ, Knudson R, Leith DE. Factors influencing pulmonary resistance. J Appl Physiol 1970; 29: 336-43. 22. Orehek J, Massari JP, Gayrard P, Grimaud C, Chorpin J. The effect of short term, low level nitrogen dioxide exposure on bronchial sensitivity of asthmatic patients. Clin Invest 1976; 57: 301-07. 23. Islam MS, Vastag E, Ulmer WT. Sulphur dioxide induced bronchial hyperreactivity against acetal choline. Int Arch Arveits Med 1972; 29: 221-32. 24. Sumik T. Combustion products of polymeric materials containing nitrogen in their chemical structure. J Fire Flammability 1973; 4:15-22.
Muscle fibre type and habitual
Although anatomical abnormalities of the upper airway have been recorded in some patients with obstructive sleep apnoea (OSA), a muscle tone dysregulation also seems to have an important role in this disorder. Since habitual snoring is the initial stage of OSA, the structural characteristics of upper
airway
muscles
(medium pharyngeal
constrictor
muscle [MPCM]) from 13 men (9 non-snorers and 4 habitual snorers) were studied. MPCM fibre structure in non-snorers was broadly similar to that in normal limb muscles, with the exception that fibre diameters were smaller for all fibre types. Compared with limb muscles, MPCM had a smaller proportion of type lib fibres and a higher proportion of types I and Ila fibres. MPCM in habitual snorers had an abnormal distribution of fibre types (low percentage of type I and type IIb fibres and high percentage of type IIa fibres) compared with non-snorers (p < 0·001) and the type IIa fibres were hypertrophic. No myopathic or neurogenic
changes were seen. Two possible hypotheses explain the abnormal distribution of fibre types in snorers. First, a constitutionally determined reduction of slow alphamotor neurons induces an adaptive transformation of type IIb to type IIa fibres and a hypertrophy of type IIa fibres; or, second, motor neurons change their patterns of discharge and, hence, of activation, and modify fibre-type distribution of MPCM as an adaptation to the anatomical characteristic of upper airway and habitual snoring. Introduction Tonic electromyographic activity is absent during quiet, relaxed wakefulness both in man and in animals, even (though transiently) when the upper motor neuron is damaged and there is clinical spasticity.Furthermore, there is no tonic electromyographic activity during any of the stages of sleep in healthy men and in most patients with spastic hemiparesis.2 Not all skeletal muscle has this characteristic. For example, hyoid muscles in the pharynx
snoring
continuously discharge during wakefulness and non-rapid eye movement (REM) sleep and lose their tonic electromyographic activity only during REM sleep.3 Tonic activity of pharyngeal constrictors disappears during sleep apnoea in man as does phasic inspiratory discharges of some upper airway muscles, such as genioglossus and stylopharyngeus.4,5 Some patients with obstructive sleep apnoea (OSA) have obvious anatomical abnormalities, such as micrognathia, enlarged tonsils or adenoids, macroglossia, and soft palate hypertrophy, which have a role in airway obstruction. In overweight patients it has been suggested that deposition of fatty tissue around the upper airway is a causal factor of OSA. In some patients, computed tomography and magnetic resonance imaging studies show an abnormal narrow airway behind the soft palate.6,7 However, in many obese or non-obese patients with OSA, there is no anatomical abnormality or there is only a slight narrowing of the oropharynx. In patients without anatomical abnormalities, OSA is thought to be due to an excessive loss of muscle tone in the upper airway or an abnormal reflex regulation of upper airway function during sleep.8 So far, little attention has been given to the possibility that structural changes in the pharyngeal muscles contribute to airway obstruction. Knowledge of the morphological characteristics of the normal middle pharyngeal constrictor muscle (MPCM) is needed to assess the possible primary muscle alterations of the pharynx in OSA patients. Since habitual snoring is the initial stage of OSA,9 we decided to defme histological and histochemical characteristics of MPCM specimens from snorers and non-snorers.
Patients and methods Between 1987 and 1990, 13 men who had laryngeal carcinoma in situ and who were going to have total laryngectomy were selected according to the following criteria: body mass index (BMI) less than 28; no clinically evident obstruction of the upper airway; no ADDRESSES. Sleep Disorders Centre (Prof S. Smirne, MD, S. lannaccone, MD, L Ferini-Strambi MD) and Department of Neurology (M Comola, MD, R. Nemni, MD) State University and Scientific Institute H S.Raffaele, Milan; and Department of Otolaryngology, Hospital Niguarda Ca’ Granda, Milan, Italy (E. Colombo, MD) Correspondence to Prof S. Smirne, Department of Neurology, University of Milan, H S Raffaele, Via Prinetti 29, 20127
Milan, Italy.
598
previous radiotherapy, chemotherapy, or corticosteroid treatment; and no other organic diseases, excluded by physical and laboratory examinations. All selected patients were smokers and some had had tonsillectomy and/or adenoidectomy. 9 patients were non-snorers (mean BMI [SD] 26-4 [1-9]; range 25-8-27-9) aged 38-61 years (mean [SD] 53-5 [7-6]), and 4 were habitual snorers (mean BMI [SD] 26-7 [0-3]; range 26-4-27-1), aged 58-68 years (mean [SD] 63-3 [4-1]). Habitual snorers were defined as people who snored every night and all night, as reported by the household in a simple questionnaire. A cylinder of muscle tissue (about 8 mm long and 4 mm wide) was taken from the MPCM during surgery. The same site from each patient was sampled-ie,1 cm from the tip of the greater process of the hyoid bone. Muscle samples were rapidly frozen by submersion in isopentane, cooled in liquid nitrogen (— 160°C), and successive transverse sections (10 lUll) were cut with the cryostat. The following histological stains and histochemical reactions were applied to all muscle specimens: haematoxylin/eosin, modified Gomori trichrome, NADH tetrazolium reductase, Sudan black B, periodic acid-Schiff, and ATPase at pH 9-4, 46, and 4-2. The histological pattern of muscle specimens was evaluated by the criteria of Dubowitz.10 Additionally, in each biopsy sample, three random fields of sections stained with ATPase pH 4-6 were photographed (x 100) and 200 muscle fibres were analysed for fibre type distribution and diameter. The fibre distribution was calculated for type I, type IIa, and type IIb according to Dubowitz.1O The "lesser fibre diameter", defined as the maximum diameter across the lesser aspect of the muscle fibre," was measured. The person who examined the muscle specimens did not know to which group the patients had been assigned. The two groups were compared by means of simple analysis of variance.
Results Mean diameter and proportion of fibre types in MPCM shown in the table. In the analysis of muscle fibre diameters we considered the mean of mean values in the two groups because the within-subject variability was negligible compared with the between-subject variability (one-way are
Anova). MUSCLE FIBRE TYPES IN MEDIUM PHARYNGEAL CONSTRICTOR MUSCLE OF NON-SNORERS AND HABITUAL SNORERS I
NS
=
not
significant
I
Non-snorers
Histological patterns of the MPCM were broadly similar those of normal skeletal muscles, apart from fibre diameters. MPCM fibres had the usual polygonal aspect and eccentric nuclei, and were grouped in secondary to
fascicles of normal architecture. There was no difference in endomysium or perimysium connective tissue, and there no were pathological deposits of lipidic or mucopolysaccharidic material. Oxidative enzymic activity was extended throughout the muscle fibre in a regular ordered network. As judged by the ATPase reaction, at various pHs, the MPCM had a normal fibre mosaic, including type I, IIa, and lIb muscle fibres. The distribution curve of muscle fibre diameters was bell-shaped for type I and type IIa fibres, as in limb muscles most commonly studied in healthy subjects.10 The distribution curve of type IIb diameters was not bellshaped : more than 85 % of the muscle fibres had a diameter in the range 10-30 um. Habitual snorers
patients, type IIa fibre diameter was greater than that of non-snorers. Habitual snorers had an abnormal distribution of fibre types compared with non-snorers: type I and type IIb fibres were reduced and type IIa fibres were increased (table). In these 4
significantly
Discussion Studies of normal limb muscles have shown that there is a large variation in fibre diameter among individuals. However, if biopsy samples are taken from the same site and the same depth in the muscle from each patient, as we have done, variability is negligible.ll In muscles most commonly sampled in biopsy studies, such as the vastus lateralis or biceps brachii, type I, IIa, and IIb fibres are present approximately in equal proportion,’ and the distribution curve of fibre diameters is bell-shaped with diameters in the range of 35 pm to 75 J.llTI.1O In our non-snorers, the MPCM fibre sructure was similar to that of normal skeletal muscles, but fibre diameter was uniformly reduced compared with muscles most commonly sampled.13 Only about 10% fast-twitch fatigue sensitive fibres (type IIb), and nearly 90% fatigue resistant fibres (including slow-twitch oxidative fibres [type I] and fast-twitch oxidative-glycolytic fibres [type IIa] were seen in non-snorers). Since type I muscle fibres are well endowed with mitochondria (they work oxidatively using fatty acids as a source of acetyl-CoA) and type IIa fibres can increase their oxidative capacity, it follows that most muscle fibres in human MPCM are metabolically fit for fatigue resistance. The high percentage of fatigue-resistant fibres points to a continuing tonicity of the pharyngeal wall that should help to keep the lumen open during respiration by counteracting the intrathoracic negative pressure. By contrast, Leese and Hopwood13 found a higher proportion of type II fibres in 50 MPCM samples taken at necropsy (67% vs 51 % in our study). Unfortunately, these investigators could not differentiate type IIa from type IIb fibres with their methods (ATPase at pH 4-2 on samples obtained within 24 hours of death). In our experience, fibre typing of post-mortem MPCM is imprecise even 3 hours after death. MPCM seems to play a fundamental part in the pathogenesis of OSA. Obstruction is due to negative inspiratory pressure, which causes collapse of the narrow
599
Guilleminault and colleagues4 demonstrated a temporary absence of electromyographic MPCM activity and collapse of pharyngeal walls on inspiration during sleep apnoea. Why should our habitual snorers have an abnormal distribution of fibre type compared with non-snorers? We believe that the difference in the mean age between the two groups is not important since there are changes in muscle fibre size and in distribution of fibre type with advancing age only in people over 70 years old.15,16 Skeletal muscle fibres change in response to different functional demands. For example, exercise is a well-known stimulus for the growth of skeletal muscles. Changes include hypertrophy of the fibres, increased proportion of type IIa fibres, decreased proportion of type IIb fibres, and increased resistance to fatigueY-19 Fast-twitch and slow-twitch fibres are innervated by fast conducting and slowly conducting alpha-motor neurons, respectively, which determine the enzymic activity of muscle fibres and hence their type.2o Cross-innervation ’21 immobilisation, denervation, and electrical stimulation22 studies show that skeletal muscle fibres can switch from one type to another. We therefore suggest two possible hypotheses to explain the abnormal distribution of fibre types in our snorers. First, the reduction of slow alpha-motor neurons and, hence, of type I fibres is constitutionally determined; this structural modification induces a compensatory hypertrophy of type IIa fibres and a shift of type I Ib fibres to IIa fibres. Second, motor neurons change their patterns of discharge and, therefore, of MPCM activation, as an adaptation to the anatomical characteristic of upper airway and habitual snoring. In animals, immobilisation and/or continuous low-frequency electrical stimulation change type II to type I fibres,22 whereas denervation and intermittent bursts of high-frequency stimulation produce the reverse change.23 In man, however, endurance exercise increases respiratory enzyme activity but does not induce the interconversion of type I and type II fibres.18 Wade et al24 showed a strong relation between fatness and muscle fibre type with a proportion of type I fibres inversely related to fatness. Obesity is one of the most frequent features ofOSA,25 and it has been suggested that its effect is related to a diffuse fatty infiltration and narrowing of the pharynx.7 Could a reduction of type I fibres be a causal factor not only for obesity but also for habitual snoring? Since OSA is determined by many factors, the relative importance of muscle fibre distribution in MPCM has to be evaluated in a larger number of snorers.
and/or hypotonic airway. 14
We thank Dr S. Palazzi, and Dr G. Calori, Istituto Scientifico H S. Raffaele, for help with the statistical analysis.
REFERENCES 1. Basmajian
JW, Szatmari A. Effect of largactil (chlorpromazine) on human spasticity and electromyogram. Arch Neurol 1955; 73: 224-31.
2. Tauber ES, Coeman
RM, Elliot D, Weitzman ED. Absence of tonic electromyographic activity during sleep in normal and spastic nonmimetic skeletal muscles in man. Ann Neurol 1977; 2: 66-68. 3. Hishikawa Y, Sumitsuji N, Matsumoto K, Kaneko Z. H-reflex and EMG of the mental and hyoid muscles during sleep, with special reference to narcolepsy. Electroencephalogr Clin Neurophysiol 1965; 18: 487-92. 4. Guilleminault
C, Hills MW, Simmons FB, Dement WC. Obstructive sleep apnea: electromyographic and fiberoptic studies. Exp Neurol 1978; 62: 48-67. 5. Remmers JE, deGroot J, Saverland K, Anch AM, Pathogenesis of upper airway occlusion during sleep. J Appl Physiol 1978; 44: 931-38. 6. Suratt PM, Dee P, Atkinson RL, Armstrong P, Wilhoit SC.
Fluoroscopic and computed tomographic features of the pharyngeal airway in obstructive sleep apnea. Am Rev Respir Dis 1983; 127: 487-92. 7. Horner RL, Mohiaddin RM, Lowell DG, et al. Sites and sizes of fat deposits around the pharynx in obese patients with obstructive sleep apnea and weight matched controls. Eur Resp J 1989; 2: 613-22. 8. Sullivan CE, Issa FG, Ellis E, et al. Treatment of cardiorespiratory disturbances during sleep. In: Emser W, Kurtz D, Webb WB, eds. Sleep, aging and related disorders. Basel: Karger, 1987: 47-67. 9. Lugaresi E, Mondini S, Zucconi M, Montagna P, Cirignotta F. Staging of heavy snorers disease. A proposal. Bull Eur Physiopathol Resp 1983; 19: 590-94. 10. Dubowitz V, ed. Muscle biopsy: a practical approach. 2nd ed. London: Baillière Tindall, 1985: 82-128. 11. Lexel J, Henriksson-Larsen K, Sjostrom M. Distribution of different fibre types in human skeletal muscles. 2. A study of cross-sections of whole m. vastus lateralis. Acta Physiol Scand 1983; 117: 115-22. 12. Åstrom KE, Adams RD. Pathological reactions of skeletal muscle fibre in man. In: Walton JN, ed. Disorders of voluntary muscle. London: Churchill Livingstone, 1981: 151-208. 13. Leese G, Hopwood D. Muscle fibre typing in the human pharyngeal constrictors and oesophagus: the effect of ageing. Acta Anat 1986; 127: 77-80. 14. Rojewski TE, Schuller DE, Clark RW, Schmidt HS, Potts RE. Videoendoscopic determination of the mechanism of obstruction in obstructive sleep apnea. Otlaryngol Head Neck Surg 1984; 92: 127-31. 15. Grimby G, Danneskiold-Samsoe B, Hvid K, Saltin B. Morphology and enzymatic capacity in arm and leg muscles in 78-82 year old men and women. Acta Physiol Scand 1982; 115: 124-34. 16. Grimby G, Saltin B. The ageing muscle. Clin Physiol 1983; 3: 209-18. 17. Thorstensson A. Muscle strength, fibre types and enzyme activities in man. Acta Physiol Scand 1976; 443 (suppl): 7-45. 18. Barnard RJ, Edgerton VR, Peter JB. Effect of exercise on skeletal muscle. I. Biochemical and histochemical properties. J Appl Physiol 1970; 28: 762-66. 19. Salmons S, Henriksson J. The adaptive response of skeletal muscle to increased use. Muscle Nerve 1981; 4: 94-105. 20. Buller AJ. The physiology of the motor unit. In: Walton JN, ed. Disorders of voluntary muscle. London: Churchill Livingstone, 1974: 20-30. 21. Buller AJ, Eccles JC, Eccles RM. Interaction between motoneurons and muscles in respect of the characteristic speed of their response. J Physiol (Lond) 1960; 150: 417-39. 22. Cotter M, Phillips P. Rapid fast to slow fibre transformation in response to chronic stimulation of immobilized muscles of the rabbit. Exp Neurol 1986; 93: 531-45. 23. Al-Amood WS, Lewis DM. The role of frequency in the effects of long-term intermittent stimulation of denervated slow-twitch muscle in the rat. J Physiol 1987; 392: 377-95. 24. Wade AJ, Marbut MM, Round JM. Muscle fibre type and aetiology of obesity. Lancet 1990; 335: 805-08. 25. Guilleminault C, van de Hoed J, Milter MM. Clinical overview of the sleep apnea syndromes. In: Guilleminault C, Dement WC, eds. Sleep apnea syndromes. New York: Alan Liss, 1978: 1-12.
From The Lancet Deaths under chloroform In recording more deaths from this anaesthetic we must repeat what we have before urged: that more systematic accounts of such occurrences should be given. The lay press, to which we owe most of our information, cannot, for obvious reasons, be expected to give scientific statements as to such fatalities. Coroners are not always, unfortunately, possessed of the special medicolegal knowledge requisite for putting the necessary questions.... It seems to us most important that in every instance cases of death occurring under anaesthetics should be reported in the medical journals, and that there should be stated: the standing and special experience of the chloroformist, the amount of the anaesthetic given, the method adopted, the time required to effect anaesthesia, the amount of struggling; whether heart or respiration failed first; some particulars as to the patient’s previous history, whether, for example, he was addicted to habits of intemperance; whether he suffered from any general disease--eg, such as causes lardaceous change, syphilis, tuberculosis; whether evidence of cardiac or arterial degeneration existed.
