Abnormal palatopharyngeal muscle morphology in sleep-disordered breathing

Abstract

The aim of the present study was to investigate whether histopathological changes can be detected in two soft palate muscles, the palatopharyngeus and the uvula, in 11 patients with long duration of sleep-disordered breathing (SDB). Muscle samples were collected from patients undergoing uvulo-palatopharyngoplasty (UPPP). Reference samples from the corresponding areas were obtained at autopsy from five previously healthy subjects. Muscle morphology, fibre type and myosin heavy chain (MyHC) compositions were analysed with enzyme-histochemical, immunohistochemical and biochemical techniques. The muscle samples from the patients, and especially those from the palatopharyngeus, showed several morphological abnormalities. The most striking findings were (i) increased amount of connective tissue, (ii) abnormal variability in fibre size, (iii) increased proportion of small-sized fibres, (iv) alterations in fibre type and MyHC compositions, (v) increased frequency of fibres containing developmental MyHC isoforms. Our findings point towards a pathological process of denervation and degeneration in the patient samples. Conclusively, the morphological abnormalities suggest a neuromuscular disorder of the soft palate in SDB patients.

Introduction

Sleep-disordered breathing (SDB) is characterised by loud snoring, repetitive episodes of upper airway obstruction occurring during sleep and daytime sleepiness [1]. In its severe form, SDB has received increased attention because of the increased risk of diurnal hypertension, nocturnal dysrhythmia, pulmonary hypertension, ventricular failure, myocardial infarction and stroke [1]. SDB is recognised as a partial or complete upper airway collapse, with or without hypopneas or apneas. The obstruction of the pharynx may occur at one or more sites in the upper airway system [2], [3], [4]. Videoradiographic recordings and high speed MRI have demonstrated that the obstruction is often located in the velopharyngeal region [4], [5]. As the functional activity of the muscles of the upper airways is of vital importance in order to maintain the stability and patency of the upper airway during inspiration, a muscle dysfunction might be one cause of SDB. Information on the muscular role in the pathophysiology and pathogenesis behind the airway instability and development of SDB is so far limited.

Morphological analysis of the muscle structure, fibre type composition and expression of myosin heavy chain (MyHC) isoforms gives reliable information on muscle function and dysfunction. Human muscle fibres can generally be classified into types I and II fibres on the basis of the differences in enzyme-histochemical reaction for myofibrillar ATPase (mATPase). The type II fibres can be further subdivided into types IIA, IIAB and IIB and IIC depending on their staining density following preincubation in acid pH [6], [7]. The usefulness of fibre typing comes with the close correlation between the mATPase activity and the physiological properties of the muscle fibres [8], [9], [10]. The mATPase activity is located in the myosin heavy chain (MyHC) molecule, which is the major contractile protein in the muscles. MyHCs exist in multiple isoforms and the MyHC composition of the fibres, thereby dictates their contractile velocity and contraction force [11], [12]. The major MyHC isoforms expressed in adult human limb muscles are slow MyHC, fast A MyHC and fast X MyHC [13], [14], [15], [16]. Slow MyHC is present in type I fibres and is co-expressed with fast MyHC in type IIC/IM fibres. Fast A MyHC is mainly present in type IIA fibres and fast X in type IIB fibres. A gene homologue to the rodent fast B MyHC has also been identified in humans [17], but no myosin related to this gene has so far been identified in human skeletal muscles. Additional MyHC isoforms such as fetal, embryonic and α-cardiac are not normally expressed in adult limb muscle fibres but are present in normal human adult masticatory [18], [19], [20], [21], [22] and soft palate muscles [23] as well as intrafusal fibres in muscle spindles [24]. However, during pathological conditions, and mainly those including regeneration, expression of fetal and embryonic MyHCs can be induced in limb muscles [25], [26], [27]. In the present study, we have investigated whether morphological signs of a localised myogenic or neurogenic myopathy can be detected in two of the soft palate muscles, the palatopharyngeus and uvula muscles, in individuals with a long duration of SDB. The muscle samples were thoroughly analysed with morphological, enzyme-histochemical, immunohistochemical and biochemical methods to evaluate muscle and muscle fibre morphology, fibre type composition and expression of MyHC isoforms. The data from the patient muscles were compared with the data from corresponding muscles from the reference subjects.