• Open Access

Pulmonary Dysfunction and Skeletal Muscle Changes in Horses with RAO


Corresponding author: Prof Dr Heidrun Gehlen, Equine Clinic, Department of Veterinary Medicine, Ludwig-Maximilians University Munich, Veterinärstr 13, D-80539 Munich, Germany; e-mail: gehlen@pferd.vetmed.uni-muenchen.de.


Background: Chronic pulmonary diseases (recurrent airway obstruction [RAO]) have been reported to alter skeletal muscle cells in humans. The purpose of this study was to evaluate a potential relationship between pulmonary and muscle variables in horses with a clinical diagnosis of RAO. Muscle biopsies from healthy horses and from horses with RAO were investigated and the relationship between the severity of lung disease and the degree of muscular changes was determined.

Hypothesis: We hypothesized that chronic pulmonary disease can lead to changes of the skeletal muscle in horses.

Animals: Fifteen healthy horses (control) and 50 horses with RAO were examined.

Methods: In a prospective clinical trial, a complete lung examination was performed in all horses. In all horses, muscle enzyme activity at rest and after exercise and muscle biopsies from the M. gluteus medius were examined.

Results: None of the horses had clinical or histologic signs of primary or neurogenic myopathies. According to the clinical, endoscopic, and radiographic findings and with a scoring system, the horses with RAO were grouped according to the severity of pulmonary findings (15 horses mild, 24 horses moderate, 11 horses severe RAO). Pathologic changes of the skeletal muscle (fiber atrophy or fiber hypertrophy, myofibrillar degeneration, hyperplasia of mitochondria, and ragged-red-like fibers) were identified in most horses with RAO but in only a few individual control horses. In addition, a marked depletion of muscle glycogen storage was evident in the RAO horses but not in the control group. Other pathologic changes of skeletal muscle such as centralized nuclei and regenerating fibers were rare, but were more frequent in horses with lung diseases than in the control group. The degree of muscle cell changes was also graded with a scoring system and correlated with the severity of pulmonary disease (r= 0.55).

Conclusion: Chronic pulmonary disease in horses is associated with structural changes in skeletal muscle.

Clinical Importance: Because chronic pulmonary disease may affect muscles, early and effective therapy may prevent these changes. This finding could be of clinical importance but requires further studies.

In recent years, chronic obstructive pulmonary disease (recurrent airway obstruction [RAO]) in humans has become increasingly thought of as a systemic disease affecting many tissues and organs in addition to the lungs. The skeletal muscles in particular have been the target of much research focusing on whether the universally observed exercise limitation reflects a systemic effect of RAO on muscle or simply is the consequence of extreme, long-term inactivity.1,2 Histopathologic examinations of muscle biopsies in humans affected with RAO have identified changes in the cross-sectional area of fibers, fiber atrophy and hypertrophy, and variations in the mitochondrial enzyme profiles. In addition, clinical findings such as muscle catabolism and decreased muscle strength have been described.1,3,4 A cause and effect relationship, however, remains to be established for the link of RAO to peripheral muscle dysfunction because the histologic findings used to propose a myopathic state (eg, fiber atrophy, excessive fiber size variation, partial intramuscular glycogen depletion, centralized nuclei) may also occur in normal subjects who are extremely inactive or in patients with other primary or neurogenic myopathies.5,6

In horses, RAO is a common disease that often leads to poor performance and exercise intolerance. Similar systemic effects on muscles of RAO as described in humans may be possible in horses and additionally may explain lack of performance.

The purpose of this study was to evaluate a potential relationship between pulmonary and muscle variables in horses with a clinical diagnosis of RAO. In addition, any correlation between the severity of pulmonary disease and the degree of muscle changes was examined. The horses examined were without signs of primary or neurogenic myopathies, were without loss of muscle mass, and were not receiving systemic medications or drugs that could alter skeletal muscles.

Material and Methods


Fifty horses with chronic pulmonary disease (RAO) were included in this study (31 geldings, 17 mares, 2 stallions; mean age, 10 ± 6 years; mean body weight, 560 ± 77 kg; mean height, 165 ± 8 cm). These horses had a history of chronic and recurrent clinical signs of pulmonary disease (eg, cough, dyspnea, tachypnoe), but no history of musculoskeletal disease. Most of the horses were warmbloods (n = 44). The others were trotters (n = 4) and Thoroughbreds (n = 2). Twenty-six of these horses were not exercised for weeks to months before examination (untrained) and 24 horses were exercised regularly (riding or treadmill, 45–60 minutes every day = trained). Most of the horses had not received any medical treatment for 6 weeks before examination. Only 5 horses with RAO had been treated with a combination of β-2-mimetic agents, secretory expectorants from the ambroxol- or dembroxol group, and drugs with effects on disulfide bridges (eg acetylcysteine) until 4 weeks before examination.

In addition, 15 healthy warmblood horses (10 geldings, 5 mares; mean age, 8 ± 2 years; mean body weight, 575 ± 40 kg; mean height, 164 ± 6 cm) served as a control group (7 horses untrained without any exercise for more than 1 month; 8 horses trained with the same exercise level as the trained RAO group). None of the horses in this group had received any medical treatment for 6 weeks before examination. All horses (control and RAO) were stabled in the same environment and with the same management of exercise and diet for a time period of 1 week.

All experimental procedures were approved by the Institute of Animal Care of the Veterinary University, Hannover and were in accordance with guidelines established by the National Institutes of Health. For the RAO horses admitted to the equine clinic, informed consent of the owner was obtained.

Examination of the Lung

The lung examination included history, clinical lung auscultation, arterial blood gas (ABG) analysis, endoscopic examination, cytological evaluation of tracheobronchial secretions (TBS) or bronchoalveolar lavage (BAL) fluid (in cases without visible TBS), and radiographic examination.

The diagnosis of RAO was based on more than 1 typical clinical sign (eg, dyspnea at rest, tachypnea, chronic or recurrent cough, prolonged inspiration or expiration), artrio-alveolar-oxygen-difference (AaDO2) >7 mmHg on ABG analysis, increased tracheal secretion and thickened tracheal bifurcation, predominance of neutrophils in cell counts of TBS or BAL analysis, and Curshmann's spirals in TBS and peribronchial infiltrations visible on radiographs. The diagnosis of additional exercise-induced pulmonary hemorrhage (EIPH) was based on radiographic alveolar or mixed alveolar-interstitial opacities in the caudodorsal lung fields and identification of red blood cells or macrophages containing hemosiderin in TBS.

The pulmonary findings were analyzed by a scoring system (ie, increased respiratory rate: 1 point; audible crackles and wheezes: 2 points, Table 1a). According to the total score (sum of points), the horses were divided into 4 groups (Table 1b): healthy (control, 0–1 point), mild (2–4 points), moderate (5–7 points), and severe RAO (>7 points).7

Table 1.   Grading of pulmonary findings during clinical lung examination with a scoring system.
(1a) Pulmonary FindingHorses (n)Score
Spontaneous coughing91
Increased respiratory rate171
Mucous nasal discharge21
Audible crackles and wheezes192
AaDO2 >6 mmHg391
AaDO2 >14 mmHg112
Thickened tracheal bifurcation441
Increased tracheal secretion441
>25% neutrophils in TBS or BAL501
Curshmann's spirals in TBS221
(1b) Lung ScoreLung StatusHorses (n)
  1. TBS, tracheobronchial secretion; BAL, bronchoalveolar lavage; RAO, recurrent airway obstruction; AaDO2, artrio-alveolar oxygen-difference.

2–4Mild RAO15
5–7Moderate RAO24
>7Severe RAO11

Muscle Examination and Muscle Biopsy

Skeletal muscles were inspected and palpated, and muscle-specific enzymes creatine kinase, aspartate aminotransferase, lactate dehydrogenase, and serum lactate concentrations were determined before and after (0, 4, and 24 hours postexercise) a standardized lounging exercise test (10 minutes walk, 10 minutes trot, 5 minutes canter).

Horses with abnormal findings in any of the above variables were excluded from this study because of the potential presence of a primary muscular disease. One to 2 days after the exercise tolerance test (3 days after the exercise test in 2 horses) muscle biopsies were taken from the right M. gluteus medius with a Bergström4 muscle biopsy punch under sedation with detomidinea (0.01–0.1 mg/kg IV). Samples were taken at a depth of 50–80 mm from the skin surface and placed in a sterile tube, stored at 4 °C, and transported to the laboratory within 24 hours for histopathologic evaluation.b No complications were noted after the muscle biopsies.

Muscle biopsies were freed of fat and fascia, embedded in tragantt,c and frozen in isopentane (−135 °C) precooled in liquid nitrogen. Sections (10 μm) were air-dried, acetone-fixed at −20 °C for 10 minutes, and then stained with hematoxylin-eosin and by Gomori-trichrome staining, used for visualization of myofibrillar and mitochondrial changes.

Forty-one muscle samples (7 from control horses and 34 from RAO horses) were reacted with nicotinamide-tetrazolium-reductase combined with ATPase after incubation at a pH between 9.4 and 9.68 in order to determine muscle fiber typing (type 1 and 2) as well as fiber type-specific alterations and fiber type groupings.

Oil red staining was used to evaluate any endomysial and interstitial pathological fat storage. The acid phosphatase stain was used to monitor the presence of pathologic enzyme activity indicating cell necrosis. Periodic acid Schiff (PAS) staining for identification of glycogen in the muscle was performed in 47 horses (12 control; 35 RAO). Muscle sections for PAS staining were cut in pairs, and 1 section was pretreated with diastase provide to negative controls for the estimation of glycogen content. Muscle biopsies were examined microscopically at various magnifications from 2.5 up to 40. Investigation was blinded and done by 2 independent investigators.

A semiquantitative analysis was performed by scoring the frequency of the histologic muscle findings (0 = not visible, 1 = isolated, 2 = multiple, 3 = frequently visible). Additionally, the occurrence of the histologic findings was given as a percentage for each group.

Muscle cell glycogen content was also scored as 0 = variable regular glycogen content or 1 = no glycogen visible.


Data were examined for normal distribution by an analysis of the model residuals using Q-Q plots and the Kolmogorov-Smirnov test. Normally distributed data were included in a descriptive analysis. For these variables, the arithmetic means (x̄) and standard deviation (s) were calculated. Data that were not Gaussian in distribution were evaluated by nonparametric methods. Differences between the groups were compared by one-way analysis of variance (ANOVA) with Ryan-Einot-Gabriel-Welsch Multiple Range posthoc test. Associations between lung and muscle examinations were evaluated by calculating Spearman' correlation coefficients (r spear). Comparison of the histologic variables were performed with the chi-square test or the Fischer's exact test. Nonparametric one-way ANOVA (Kruskal-Wallis test) and Wilcoxon 2-sample test were used to compare glycogen storage between groups.

Estimates of the muscle-specific enzymes between 4 RAO groups at rest and after exercise were tested by calculating a 2 factorial ANOVA with repeated measurements on the factor time. Analyses were carried out with the statistical software (SAS, version 9.1).d

The procedure MIXED was used for the analysis of the linear model. P-values below .05 were considered to be significant in all tests.


Results of the Lung Examinations

Horses from the control group had no clinical signs of lung disease, normal ABG analysis, no tracheal mucus accumulation in endoscopy, and normal thoracic radiographs.

All 50 horses with pulmonary disease had history of recurrent respiratory problems of >4 months duration. Clinical examination revealed the typical pulmonary clinical signs listed in Table 1a. The mean AaDO2 in RAO horses was higher (10.3 ± 3 mmHg) than in the control group (4 ± 1 mmHg, P≤ .01). The results of the endoscopic examination are also presented in Table 1a. TBS analysis identified macrophages containing hemosiderin in 10 horses. In 6 horses, no TBS was visible. In these horses, BAL fluid contained 28–39% neutrophils. Radiography revealed broncho-interstitial patterns in 45 horses and bronchiectasis in 11 horses. All 50 horses were diagnosed with chronic RAO, and 10 of these horses also showed EIPH. Results of the clinical and endoscopic examinations were used for grading the severity of RAO with a scoring system (Tables 1a and 1b). According to the results 15 horses were affected with mild RAO. Twenty-four horses had moderate RAO, and 11 horses showed severe RAO (Table 1b).

Results of Muscle Examinations

Inspection and palpation of skeletal muscles revealed no signs of hardening, swelling, or increased warmth before or after exercise. Stiffness or other clinical signs of myopathy were also not observed, neither at rest nor after exercise. All horses had normal muscle enzyme activities at rest, and nearly all horses had normal muscle enzyme activity after exercise without differences between resting and exercise. Two horses (1 horse with moderate and 1 horse with severe RAO) showed a significant increase of CK activity only 4 hours after exercise (1,500 and 2,460 U/L). The CK activity of these horses was normal 24 hours after exercise and the activity of other muscle enzymes was in the reference range after exercise. In total, there were no statistical differences in muscular enzyme activity or lactate concentrations at rest or after exercise between the groups (Table 2).

Table 2.   Results of muscle enzymes and lactate concentration in control and RAO horses at rest and after exercises (mean ± SD).
Muscle Profile
(n = 15)
(n = 15)
(n = 24)
(n = 11)
  1. RAO, recurrent airway obstruction.

CK rest74 ± 2368 ± 1576 ± 6195 ± 68
CK 0 postexcercise98 ± 8277 ± 1575 ± 2972 ±19
CK 4 (hours) postexcercise82 ± 2468 ± 18203 ± 59246 ± 50
CK 24 (hours) postexcercise79 ± 2067 ± 22139 ± 1764 ± 16
Lac rest0.9 ± 0.20.9 ± 0.20.9 ± 0.31.2 ± 0.4
Lac 0 postexcercise1.9 ± 1.21.3 ± 0.62.6 ± 2.13.1 ± 2.8
Lac 4 (hours) postexcercise0.9 ± 0.20.9 ± 0.20.9 ± 0.20.9 ± 0.2
Lac 24 (hours) postexcercise1 ± 0.20.9 ± 0.21 ± 0.41.2 ± 0.4
AST rest129 ± 61119 ± 37122 ± 30105 ± 17
AST 0 postexcercise139 ± 54128 ± 24130 ± 34109 ± 45
AST 4 (hours) postexcercise127 ± 62114 ± 15131 ± 57113 ± 21
AST 24 (hours) postexcercise128 ± 55104 ± 23129 ± 6796 ± 8.6
LDH rest279 ± 57275 ± 67287 ± 121242 ± 69
LDH 0 postexcercise303 ± 45266 ± 67280 ± 119254 ± 69
LDH 4 (hours) postexcercise290 ± 71239 ± 63344 ± 314301 ± 160
LDH 24 (hours) postexcercise295 ± 43269 ± 68335 ± 171250 ± 67

Untrained horses showed significantly higher lactate concentrations immediately after exercise (2.9 ± 0.7 mmol/L) compared with the trained horses (1.6 ± 0.4 mmol/L, P < .05). Differences between control (53% trained, 47% untrained) and RAO horses (48% trained, 52% untrained) were not observed.

Control Group

In the control group, only 2 horses showed minor histopathologic alterations in skeletal muscle. In 1 horse the fiber caliber spectrum was slightly variable, and in another horse isolated mitochondrial activations, visible as subsarcolemmal red rims in the Gmori trichrome stained sections, were found in individual muscle fibers. Pathologic changes of intramuscular fat storage (oil red stain) occurred in only 1 horse. No other histopathologic findings were evident in the control group. No dramatic muscular glycogen deficiency comparable to that found in some of the RAO horses showing muscle completely free of PAS-positive intrasarcoplasmatic material could be found in the control group.

RAO Group

The occurrence and extent of histologic changes in muscle cells in RAO horses are listed in Table 3. In horses with RAO, a pathologically expanded bimodal fiber caliber spectrum (Fig 1), attributable to fiber atrophy and fiber hypertrophy of varying degrees, was seen (86.7% of those with mild, 91.6% with moderate, and 81% of those with severe RAO). NADA/TR + ATPase staining revealed pathologic fiber caliber variations in type 1 and type 2 fibers. Atrophy of individual muscle fiber types was visible in 14 horses with RAO (6 horses with mild, 7 horses with moderate, and 1 horse with severe RAO, Figs 1 and 2). Atrophy of both muscle fiber types was found in 11 horses with RAO (5 horses with mild, 5 horses with moderate, and 1 horse with severe RAO), and selective type 2 fiber atrophy in 2 horses (1 horse with mild, 1 horse with moderate RAO) and 1 horse (with moderate RAO) showed only atrophy of the type 1 muscle fibers. Fiber type grouping did not occur in any of the examined horses. Mitochondrial activation (hyperplasia) of muscle cells was found significantly (P≤ .001) more frequently in horses with RAO (75–91%, Figs 3 and 4) than in control horses (7%). Myofibrillar alterations such as myofibrillar stunting or thickening, central nuclei, and muscle fiber divisions were observed only in horses with RAO.

Table 3.   Results of histologic examination of muscle biopsies.
GroupsHealthy (n = 15)Mild RAO (n = 15)Moderate RAO (n = 24)Severe RAO (n = 11)
Muscle score0123012301230123
  1. All findings were scored according to the extent and severity of the histologic changes (0 = without changes, 1 = mild changes, 2 = moderate changes, 3 = severe changes).

  2. RAO, recurrent airway obstruction.

Fiber atrophy122102661081422351
Fiber hypertrophy131104551081422351
Angular fibers15000150002310010100
Mitochondrial activation132002760187831622
Altered of myofibers150007530135603710
Central nuklei1500013110184207310
Fiber splitting1500012030182227040
Cell necrosis15000150002301011000
Muscle score 0–5  2–5  2–19  5–16 
Figure 1.

 Muscle biopsies of a control horse without pathologic changes (above) and of a horse with severe recurrent airway obstruction showing alterations in the fiber caliber spectrum and beginning endomysial fibrosis (below). HE stain, × 75.

Figure 2.

 Muscle biopsies of a normal healthy horse without signs of lung disease (above) and of a horse with severe recurrent airway obstruction (below: 1 = mitochondrial activation [red rim], 2 = thickening of myofibers, 3 = variation of the fiber caliber spectrum). Gomori-Trichrome stain, × 75.

Figure 3.

 Muscle biopsies of a control horse without histopathologic changes (above) and of a horse with severe recurrent airway obstruction with moderate mitochondrial activation in several fibers in the form of red rim fibers. Gomori-Trichrome stain, × 75.

Figure 4.

 Muscle biopsy of a horse with moderate recurrent airway obstruction and exercise-induced pulmonary hemorrhage and severe mitochondrial activations on several fibers in the form of red rim fibers (above). Degenerating muscle fiber within the same biopsy with a tendency to ragged red fiber (below). Gomori-Trichrome stain, × 160.

Muscular glycogen storage (PAS stain) seemed to be decreased in 31 horses affected with RAO, whereas 13 horses (5 horses with mild, 4 horses with moderate, and 4 horses with severe RAO) showed some weakly PAS-positive intrasarcoplasmatic material, and 2 horses (1 horse with mild, 1 horse with severe RAO) were free of PAS-positive material throughout the muscle biopsy section (Fig 5). Two horses with moderate RAO were identified with an additional massive accumulation of PAS-positive material in intrasarcoplasmatic and subsarcolemmal locations. A positive correlation was identified between the severity of RAO and the frequency of histopathologic and histochemical changes in the muscle biopsies (r= 0.55, Figs 6 and 7).

Figure 5.

 Skeletal muscle of a horse without lung disease and with physiologic glycogen storage (above) and of a horse with severe recurrent airway obstruction and a glycogen depletion (below). PAS stain, × 100.

Figure 6.

 Muscle cell changes in horses with RAO of different degree. FKV, changes in cross-sectional area; MA, mitochondrial activation; MV, altered myofibers; ZK, central nuclei; FT, fiber splitting; AF, angular fibers; N, cell necrosis.

Figure 7.

 Correlation between severity of pulmonary findings (lung score) and extent of histologic muscle cell changes (muscle score, r= Spearman' correlation coefficient).

Histopathologic and histochemical differences in muscle biopsies between trained and untrained horses were not observed in either the control or the RAO group.


In humans, deconditioning related to a progressive decrease in daily activity often is quoted as being one of the main reasons why patients with RAO have peripheral myopathy.9 The primary cause of muscle tissue destruction in patients with RAO may be a result of systemic inflammation and muscle oxidative stress, which causes noticeable myocyte damage.10 Human patients with RAO showed decreased muscle fibers, fiber atrophy, and a muscle fiber shift from oxidative to glycolytic fiber types.4 Recent studies in humans, however, have suggested that other factors such as exposure to systemic corticosteroids,11 malnutrition,12 hypoxia,13 and apoptosis14 may also contribute to the alterations in peripheral muscle function observed in patients with RAO.15,16

In the present study, histologic myopathic alterations were observed in horses with RAO but not in healthy horses. Some of the histologic findings in the muscle biopsies of the RAO horses (eg, fiber atrophy, fiber size variation, glycogen depletion, centralized nuclei) may also occur in horses with extreme inactivity and loss of muscle mass (muscle dysfunction) or in horses with primary or neurogenic myopathies. In the present study, none of the horses was completely inactive. In both groups (RAO and control) we examined trained and untrained (but not completely inactive) horses and we found no differences in histology or histochemistry of the muscle biopsies between them. Additionally, none of the horses showed a loss of muscle mass. Thus, we excluded muscle dysfunction induced by inactivity as a factor in the RAO horses. In human patients with RAO, significantly higher blood lactate concentrations were observed after exercise compared with healthy people.17 In our study, the blood lactate concentrations after exercise in horses with moderate and severe RAO were also higher compared with healthy horses and horses with mild RAO. In humans with RAO, early activation of anaerobic glycolysis, a decrease in oxidative enzyme capacity, and lactic acidosis with muscle fatigue are discussed as a reason for this finding.17 We did not examine this possibility in our study, and additional research with examination of oxidative enzyme capacity at rest and after exercise will be necessary to prove this hypothesis for horses with RAO.

Primary myopathy such as exercise-induced rhabdomyolysis or tying-up could be ruled out in the present study by evaluating muscle enzyme activity at rest and after exercise.16,18,19 Normal resting CK activity ranges from 10 to 350 U/L.20,21 All CK results (resting and after exercise) measured in our study were <300 U/L. Nevertheless, we found a mild increase in CK 4 hours after exercise in horses with moderate and severe RAO compared with resting activity. We considered these results normal because the CK activity 24 hours after exercise was comparable to resting activity and the activity of other muscle enzymes was normal.

Typical muscle fiber necrosis, muscle fiber fibrosis, neuropathologic changes, or inflammatory infiltrations that occur in horses with primary or neurologic myopathy6,16,19–25 were also not observed in the muscle biopsies of the RAO horses in our study. With PAS staining with prior incubation by diastase, no diastase-resistant polysaccharide storage, as observed in polysaccharide storage myopathy,6 could be found in any of the examined horses. Because primary or neurogenic myopathy or extreme inactivity could be excluded, the present study shows that the muscles in RAO horses without loss of muscle mass are myopathic as a probable peripheral effect of the RAO.

Poor pulmonary ventilation and diffusion are associated with respiratory insufficiency and decreased oxygen content in peripheral blood.25 In horses with chronic pulmonary disease, deteriorating function may have negative effects on oxygen uptake by muscle. In humans with RAO, histologic muscle examinations include oxidative and glycolytic mitochondrial enzyme profiles in addition to characterization of the interactions of individual fiber types and a determination of fiber diameter in muscles.1,4 Additional studies are warranted to determine typical changes of the muscular mitochondrial enzyme profiles and decreased mitochondrial fractional area contribute to decreased oxidative capacity of the muscles in horses with RAO, as observed in humans with RAO. Changes of mitochondrial activation observed in the present study (visible as a broad irregular subsarcolemmal red rim, occasionally in combination with ragged red fibers) might be a first indication because in humans ragged red fibers are considered to be the result of an increase in mitochondrial enzyme activity.26

In the horses with RAO in our study, decreased glycogen content in muscle cells was an interesting result, especially because we did not find it in the control horses. At the beginning of exercise, large amounts of energy in the form of energy-rich phosphate compounds (eg, adenosine triphosphate, ATP) are immediately available for muscle contraction for a short period of time.25 The extensive glycogen storage of the musculature of the horse then serves as the main source of energy for short-term, strong, anaerobic performances.27,28 In endurance performance, the energy derived from the glycogen pool is used to bridge the time until the slower aerobic system takes over energy provision.25 In human patients with RAO, the muscle fiber shift from oxidative to glycolytic fibers leads to increased muscle oxidative stress, resulting from an inability of the antioxidant systems to cope with increased oxidant production, especially during exercise, and decreased peripheral muscle endurance.9,29 Examinations of the muscle metabolic enzyme profile of patients with RAO showed decreased activity of enzymes involved in oxidative energy metabolism (eg, cytochromoxidase, citrate synthetase, 3-hydroxyacyl-CoA dehydrogenase) compared with healthy people.1,2,30 In horses with RAO, an increase in oxidative stress, especially during exercise, has also been described.31 Whether or not there is a connection between RAO and muscle glycogen storage or depletion is unclear. Several factors may affect glycogen depletion (and were not examined in our study), and this possibility deserves further evaluation.

Horses with RAO usually have undergone several treatment regimens, sometimes over years, before being referred. Drugs used for the treatment of chronic equine pulmonary diseases are β-2-mimetic agents and glucocorticoids. Both classes of drug are known to influence muscle (ie, anabolic or catabolic effect).32,33 Histopathologic changes seen in humans with steroid myopathy or horses with hyperadrenocorticism caused by pituitary pars intermedia dysfunction include atrophy of type 2 muscle fibers, changes of oxidative enzyme capacity, myofibrillar degeneration, mitochondrial changes, and subsarcolemmal glycogen accumulation.4,24,34 To avoid medication-induced effects on musculature, we only examined horses that were untreated 4 weeks before examination. Therefore, it seems unlikely that the changes in skeletal muscle observed in this study were the result of previous treatment. Possible effects of previous medications (month or years before) on musculature, which are unknown in horses, unfortunately could not be excluded in our study but seem unlikely.

In humans, RAO-induced changes in skeletal muscle can be positively influenced and even compensated for by exercise.4 In horses, exercise has been reported to decrease clinical signs of pulmonary disease.23,31,35 It remains unclear whether improvement of performance after therapy is solely attributable to improved lung function or can also be a result of improved skeletal muscle structure and function. In our study, we found no differences in muscle histology or histochemistry between trained and untrained RAO horses. The differences between trained (regularly ridden) and untrained (not ridden) warmblood horses may not be substantial enough to identify. Additional studies are needed to determine how exercise can influence the structure of skeletal muscle in horses with RAO. Oxidative and glycolytic enzymes such as citrate synthetase, 3-hydroxy-CoA-dehydrogenase, and hexokinase35–37 have been examined in horses without pulmonary disease under varying training regimens.5,38,39 It may be useful to examine these enzymes in combination with histomorphologic evaluations to better understand the effects of RAO on skeletal muscle.

In conclusion, we have shown evidence for marked structural alterations of skeletal muscle in horses with RAO; thus pulmonary diseases can affect the microstructure of the skeletal muscle in horses. The relevance of these changes and the pathologic consequences are not yet clear. It should also be clarified in additional studies to what extent such muscular changes contribute to or are associated with the frequently observed exercise intolerance in horses with RAO.


aDomosedan, Pfizer Tiergesundheit, Karlsruhe, Germany

bDepartment of Neuropathology, University of Düsseldorf, Germany

cMerck, Darmstadt, Germany

dSAS Institute, Cary, NC