Severe diaphragmatic necrosis in 4 horses with degenerative myopathy

Abstract

Severe diaphragmatic necrosis occurred in horses with degenerative myopathy due to polysaccharide storage myopathy (n = 2), nutritional myopathy (n = 1), and vasculitis (n = 1). Blood gas analysis performed in 1 horse indicated development of respiratory acidosis. Respiratory muscle necrosis can be severe in horses with degenerative myopathy and can lead to respiratory failure.

Degenerative myopathy affecting respiratory muscles in horses is not commonly reported. Diaphragmatic necrosis can occur in horses with nutritional myopathy (1), the atypical myoglobinuria that occurs in pastured horses in Europe (2), and ionophore toxicity (3). Other degenerative myopathies in horses include equine polysaccharide storage myopathy (EPSSM) (4,5,6) and Streptococcus equi-associated myopathy (7,8). Characteristic changes have been found in the diaphragm of horses with EPSSM (9). Young quarter horses appear to be predisposed to a syndrome of immune-mediated degenerative myopathy with vasculitis, following exposure to Streptococcus equi (8). But death due to diaphragmatic necrosis and respiratory failure associated with these disorders has not been reported previously. This report describes 4 horses that died or were euthanized due to various degenerative myopathies that resulted in diaphragmatic necrosis. Respiratory failure was confirmed by venous blood gas analysis in 1 horse and was the suspected cause of death or reason for euthanasia in the other 3 horses.

A 4-y old Westfalean gelding (horse 1) with a history of recurrent exertional rhabdomyolysis was presented with a history of severe rhabdomyolysis following turnout on a new pasture. The horse had a temperature of 37.2°C and heart rate of 56 beats/min. The semitendinosus and semimembranosus muscles were firm on palpation. Results of serum chemical analyses obtained on admission were normal, except for high serum activity of creatine kinase (CK; 29 870 U/L; reference range, 0 to 224 U/L) and aspartate aminotransferase (AST; 7430 U/L; reference range, 226 to 365 U/L). Venous blood gas analysis at admission was normal (Table 1). The horse was treated with 6 L of IV polyionic fluids (Lactated Ringer's solution (LRS); Butler, Dublin, Ohio, USA), dimethylsulfoxide (DMSO, 500 mL, IV, q12h; Fort Dodge, Fort Dodge, Iowa, USA), methocarbamol (Robaxin-V, 100 mL, IV; Fort Dodge), acepromazine (0.02 mg/kg bodyweight [BW], IV; Butler), flunixin meglumine (Banamine, 0.25 mg/kg BW, IV, q12h; Schering Plough, Union, New Jersey, USA), and dantrolene sodium (Dantrium, 4 mg/kg BW, PO, q12h; Procter and Gamble, Cincinnati, Ohio, USA). Over the following 12 h, the horse was observed to be in apparent discomfort, with alternating periods of recumbency and standing, and with excessive sweating. Normal manure was passed. At 12 h after admission, the horse had a temperature of 37.4°C, a heart rate of 64 beats/min, and a respiratory rate of 35 breaths/min, with grayish mucous membranes and labored breathing. Respiratory distress progressed, and results from a serum chemical analysis revealed no evidence of electrolyte abnormalities or renal failure, but there was evidence of on-going muscle injury, with CK activity of 30 160 U/L and AST activity of 14 550 U/L. A repeated venous blood gas analysis was indicative of severe respiratory acidosis (Table 1). Mucous membrane color progressed to purple and the owner elected to have the horse euthanized, due to the poor prognosis for recovery. There were no gross abnormalities at postmortem examination. Samples of diaphragm and intercostal muscles, myocardium, kidney, and liver were fixed in 10% buffered formalin and submitted for histologic evaluation. Examination of microscopic sections revealed massive acute to subacute myofiber necrosis of the diaphragm and intercostal muscles, characterized by hypercontraction, lysis, coagulation necrosis, and macrophage infiltration. Intact fibers exhibited excessive fiber size variation and numerous fibers contained 1 or more internal nuclei. With periodic acid-Schiff (PAS) stain for glycogen, many fibers were observed to contain 1 or more subsarcolemmal to intracytoplasmic aggregates of PAS-positive, amylase-sensitive material, consistent with glycogen. A moderate number of fibers contained multiple intracytoplasmic “lakes” of PAS-positive, amylase-resistant material, consistent with complex polysaccharide. There were numerous acutely necrotic renal tubular epithelial cells and scattered tubules contained proteinaceous and granular casts that did not contain pigment. No significant lesions were found in heart or liver samples. The diagnoses were moderate to severe chronic myopathic change with glycogen and complex polysaccharide storage, characteristic of EPSSM, with severe acute to subacute myofiber necrosis of respiratory muscles and moderate acute renal tubular necrosis due to hypoxia.

Table 1.

Horse 2 was an 8-year-old Haflinger mare presented with acute rhabdomyolysis following a 3-hour trailer ride. Physical examination revealed pain on palpation of the gluteal, quadriceps femoris, and semitendinosus and semimembranosus muscles. Abnormal findings on a complete blood cell (CBC) count and serum chemical panel were markedly increased activities of CK (110 000 U/L; reference range, 93 to 348 U/L) and AST (3800 U/L; reference range, 212 to 426 U/L); all other values were within normal limits. Urine was discolored, indicative of myoglobinuria. Treatment included IV LRS with potassium chloride (4 L/h), flunixin meglumine (0.25 mg/kg BW, IV, q12h), butorphanol (Torbugesic, 0.01 mg/kg BW, IV; Fort Dodge), phenylbutazone (2.2 mg/kg BW, IV, q12h; Vetus Animal Health, Farmers Branch, Texas, USA), methocarbamol (50 mL, IV), and acepromazine (0.02 mg/kg BW, IV). Detomidine hydrochloride (Dormosedan, 5 mg/kg BW, IV; Pfizer Animal Health, Exton, Pennsylvania, USA) and meperidine hydrochloride (Demerol, 1.1 mg/kg BW, IV; Sanofi-Synthelab, New York, New York, USA) were administered, as needed, for relaxation and control of pain. The mare appeared to respond to therapy and was more alert and appeared less pained. Urine returned to normal color. However, repeat serum chemical analyses at 24 h after presentation revealed evidence of on-going muscle injury, with CK activity increased to 480 000 U/L and AST activity increased to 10 000 U/L. Treatment was continued, as previously described, with reduction of rate of IV fluid administration to 2–3 L/h. The mare spent increased time in recumbency but would rise to eat, drink, urinate, and defecate. At 40 h following presentation, her heart rate rose to 80 beats/min, she appeared uncomfortable and in respiratory distress, and she died. A complete postmortem examination revealed multifocal areas of pallor within the gluteal, biceps femoris, semitendinosus, semimembranosus, quadriceps femoris, and diaphragm muscles. The lungs and all other organs examined were normal. Toxicologic testing of liver for ionophores yielded negative results. Samples of heart, diaphragm, and various skeletal muscles were fixed in 10% buffered formalin. Findings on histologic examination of cardiac muscle were normal. Examination of gluteal and diaphragm muscle revealed moderate (gluteal) to severe (diaphragm) acute to subacute myofiber necrosis. Intact fibers exhibited increased variation in fiber size and vacuolar degeneration, and a moderate number of fibers contained 1 or more internal nuclei. Fibers with inclusions of blue-grey material were seen on hematoxylin and eosin-stained sections, and storage of both glycogen and complex polysaccharide was confirmed with PAS stain for glycogen. The histopathologic diagnosis was moderate chronic myopathic change due to EPSSM, with moderate to severe acute to subacute degenerative myopathy of gluteal and diaphragm muscle.

Horse 3 was a 5-year-old Haflinger stallion with a history of acute onset of tachycardia, tremors, weakness, depression, and recumbency. The horse was 1 of a group of 5 horses showing similar signs over a 2-week period during which 3 horses, including it, died. All affected horses had serum evidence of muscle necrosis, with CK activities ranging from 3200 U/L to 15 000 U/L (reference range, 100 to 300 U/L). The diet of the initial case in the group of 5 horses was hay only, whereas the other 4 horses, including horse 3, were fed on pasture only. No concentrated feeds or supplements were fed. Analysis of serum from horse 3 revealed a CK activity of 3240 U/L; other serum chemical values and results of a CBC count were within normal limits. Clinical signs were progressive, despite aggressive treatment with IV fluid therapy, and the horse was euthanized following development of respiratory distress, 24 h after the onset of signs. On complete postmortem examination, no significant gross lesions were found. Analysis of stomach contents for ionophores was negative. Histologic evaluation of samples of diaphragm revealed massive acute myofiber necrosis. Less severe myofiber necrosis was seen in samples from gluteal muscle. There were no chronic myopathic changes, and staining with PAS stain did not reveal glycogen or complex polysaccharide storage. No evidence of necrosis was found in sections of myocardium, and no significant histologic lesions were seen in sections of liver, kidney, lung, and spleen. Analysis of blood obtained antemortem revealed serum vitamin E concentration of 0.348 μmol/L (15 μg/L) (normal greater than 0.0464 μmol/L [20 μg/L]) and selenium concentration of 0.635 μmol/L (50 μg/L) (reference range, 1.778 to 3.175 μmol/L [140 to 250 μg/L]). The diagnosis was acute degenerative myopathy due to deficiencies of selenium, vitamin E, or both. Two of the affected horses in this group recovered following treatment with vitamin E and selenium. Vitamin E (1000 IU, PO, q24h) and selenium (1 mg, PO, q24h) were added to the diet of all the horses, and no similar cases occurred in the following 2 y.

Horse 4 was a 1.5-year-old quarter horse gelding with an acute onset of stiff gait and dark red-brown urine, thought to represent myoglobinuria. Serum chemical analyses revealed markedly increased activity of AST (> 1200 U/L; reference range, 200 to 400 U/L); CK was not analyzed. Treatment with IV LRS was instituted, but the horse developed progressive dyspnea and died. Samples of semimembranosus, semitendinosus, temporal, masseter, diaphragm, and heart muscle were fixed in 10% buffered formalin and submitted for histologic examination. A sample of liver was frozen and submitted for vitamin E and selenium analysis. Significant lesions were found only in the diaphragm, in which there were locally extensive areas of severe acute to subacute myofiber necrosis with multifocal hemorrhages and fibrinoid necrosis of blood vessels. There was no evidence of chronic myopathic change or of glycogen or complex polysaccharide storage. Vitamin E levels in liver were normal (5.15 μg/g; reference range, 3.5 to 15.5 μg/g), as were liver selenium levels (348 ng/g; reference range, 300 to 1000 ng/g). The diagnosis was necrotizing vasculitis and associated hemorrhagic degenerative myopathy, consistent with immune complex disease following exposure to Streptococcus equi.

Horses 1 and 2 had acute onset of rhabdomyolysis, a common manifestation of EPSSM (4,5,6), and were confirmed to have characteristic chronic myopathic changes and inclusions of complex polysaccharide within the diaphragm, as well as in other muscles. The abnormality of acid-base equilibrium that has been associated with exertional rhabdomyolysis in horses is metabolic alkalosis (3,6,10), but results of blood gas analyses in horse 1 were indicative of development of respiratory acidosis, consistent with respiratory muscle failure. None of these horses had a history of ionophore exposure, and results of toxicologic analysis for ionophores in samples from horses 2 and 3 were negative. Nutritional myopathy due to deficiency of selenium, vitamin E, or both was the suspected cause of rhabdomyolysis in the group of 5 horses that included horse 3. Although neither bacterial culture nor serum analysis for streptococcal A antibodies was carried out in horse 4, the histopathologic findings were consistent with immune complex vasculitis due to exposure to Streptococcus equi. (8).