Signalment:  

Eight-year-old male golden retriever, Canis familiarsThis dog presented in respiratory distress with a left sided systolic heart murmur and a lifelong history of generalized weakness, difficulty eating, and abnormal stance with hyperextended carpi, hock flexion, and abduction of all 4 paws. Frequent multiform ventricular premature complexes were noted during cardiology exam, as were periods of non-sustained ventricular tachycardia. This animal had survived previous bouts of both aspiration pneumonia and congestive heart failure and was treated for both based on echocardiographic and radiographic findings. The animal died during hospitalization.

Gross Description:  

The animal exhibited marked symmetrical atrophy of the masticatory, paravertebral, and proximal limb muscles resulting in pronounced bony prominences. The right middle and ventral aspects of both caudal lung lobes were red-purple, firm, and rubbery. Sections from the right middle lobe sank in 10% formalin. The heart exhibited marked dilation of both ventricles and atria and multifocal, white-tan streaks within the myocardium, especially within the interventricular septum near the apex. The muscular portions of the diaphragm were markedly thickened up to 1 cm in cross section. The proximal third of the stomach was displaced cranially into the thorax through a thick, fibrous, lax, tendinous portion of the diaphragm. The liver was enlarged and dark red with rounded margins.

Histopathologic Description:

Heart: The myocardium contains broad swaths of degenerating myofibers that contain abundant granular, basophilic material (mineral) and is multifocally replaced by coalescing bands of fibrous connective tissue with variable numbers of admixed fibroblasts and lesser amounts of mature adipose tissue. Myofibers in areas of mineralization are typically hypereosinophilic with flocculent to fragmented sarcoplasm and pyknotic nuclei (degeneration and necrosis). Small numbers of neutrophils and macrophages are occasionally present at the periphery of these mineralized areas.

Diaphragm: The diaphragm is markedly thickened. Myofiber numbers are diffusely and markedly reduced while remaining myofibers are individualized by marked fibrosis and adipose tissue deposition. Myofibers vary markedly in diameter and frequently exhibit loss of cross striations, hyalinization, and fragmentation (degeneration). Scattered myofibers contain abundant granular basophilic material (mineral) within the sarcoplasm.

Morphologic Diagnosis:  

1. Heart: myocardial mineralization and necrosis, multifocal to coalescing, moderate to marked, with moderate fibrosis
2. Diaphragm: myocyte necrosis and loss, multifocal to coalescing, marked with marked fibrosis and fat replacement.

Condition:  

Duchenne muscular dystrophy

Contributor Comment:  

The patients widespread muscle necrosis with replacement by adipose tissue and fibrous connective tissue is consistent with Golden Retriever Muscular Dystrophy (GRMD), an animal model for the human condition, Duchenne Muscular Dystrophy (DMD)(10-12). Both DMD and GRMD are the result of X-linked deficiencies in the cytoplasmic protein dystrophin. This protein is responsible for linking cytoplasmic actin to the transmembrane complexes of the sarcoglycans and dystroglycans, which interact with extracellular matrix components. Absence or dysfunction of dystrophin results in necrosis and regeneration of myocytes with progressive replacement by fibrous and adipose tissue. In young pups, death is thought to occur from respiratory failure following necrosis of diaphragmatic myofibers, while in older animals death frequently follows dysphagia and subsequent aspiration pneumonia or congestive heart failure(11). This dog had evidence of aspiration pneumonia and congestive heart failure at necropsy. The role of the recent onset of ventricular arrhythmias in the animals death was also considered. In young men with DMD, death from a combination of pneumonia and cardiovascular decompensation is common(3).

X-linked dystrophin deficiencies have been identified in cats and a variety of dog breeds, including Irish Terrier, Samoyed, Rottweiler, Dalmation, Shetland Sheepdog, Labrador Retriever, Brittany Spaniel, Rat Terrier, Belgian Groenendael Shepherd, Schnauzer, and Spitz, but are best studied in the mdx mouse, German Shorthair Pointer, and Golden Retriever(11). This particular patient was a member of the breeding colony at the National Center for Canine Models of Duchenne muscular dystrophy (NCDMD) at the University of North Carolina-Chapel Hill. As the murine models fail to show severe clinical signs or cardiac lesions comparable to those observed in humans, these canine models fill an important niche in therapeutic studies(5,12).

Muscular dystrophies have been identified in people resulting from deficiencies or alterations of more than 30 different proteins(10) and may be X-linked, autosomal recessive, or autosomal dominant8. Many of these deficiencies have not yet been observed in veterinary species outside of the laboratory. Examples of spontaneous muscular dystrophies that have been identified in animals are summarized in table 1. Other references contain a more extensive list of murine musculodystrophy models(2,10).

Table 1. Spontaneous muscular dystrophies in veterinary species^{modified from 8}

Deficient Protein (Human Disease name)	Protein Location	Species
Laminin α2 (Congential muscular dystrophy type 1A) (RC)	Extracellular Matrix	Dog(8) Cat(4, 6, 8) dy/dy mouse(8, 9)
α-, β-, γ-, or δ-sarcoglycan (Limb-girdle muscular dystrophy types 2C-F)	Transmembrane	Dog(1, 10) Cat(7) BIO14.6 hamster(10)
Dysferlin (Limb-girdle muscular dystrophy type 2B)	Transmembrane	SJL-Dysf mouse(9)
Dystrophin (Duchenne muscular dystrophy, Becker muscular dystrophy)	Cytoskeleton	Dog Cat mdx mouse(8, 11)

JPC Diagnosis:  

1. Heart, myocardium: Degeneration, necrosis, and loss, multifocal, with myocardial fibrosis and mineralization.
2. Diaphragm: Myofiber atrophy and loss, diffuse, moderate with myocyte atrophy and marked fatty infiltration.

Conference Comment:  

Dystrophin-dependent muscular dystrophy is confirmed immunohistochemically by the absence of dystrophin staining, or by Western blot analysis. Although muscle atrophy is the usual course in dystrophin-dependent muscular dystrophy, marked muscular hypertrophy of unknown cause is seen in mice, cats, and rat terrier dogs. Clinical pathology changes include a marked increase in serum creatinine kinase (CK) and aspartate aminotransferase (AST) in neonates; these continue to rise until 6 months of age. Serum CK and AST levels in older dogs are elevated to a lesser extent. Clinical signs of neuromuscular weakness progress until 8-12 months of age before stabilizing. Affected dogs exhibit a stiff-legged gait, thickened muscles at the base of the tongue, excessive drooling, abdominal breathing, and ribcage deformities due to contracture of the diaphragm. In some breeds, such as the rat terrier, there is paradoxical muscle hypertrophy of the thighs, neck, and shoulders. Female carriers are clinically normal but have elevated serum CK and AST(13).

Gross lesions include severe degeneration indicated by pale streaks in the diaphragm, trapezius and sartorius muscles. In chronic cases, the diaphragm is thickened, contracted, and fibrotic. In the heart, the left ventricular wall and right side of the interventricular septum are most severely affected by necrosis, mineralization, and fibrosis. Common histologic findings not seen in this case are the presence of swollen and dark-staining fibers (large dark fibers), which represent the earliest stages of degeneration. Muscle fiber necrosis is due to the influx of calcium through defects in the sarcolemma. The basal lamina of necrotic fibers is preserved, leaving an empty sarcolemmal tube capable of fully regenerating the myofibers(13).

Other common examples of muscular dystrophy in veterinary species include X-linked muscular dystrophy in mixed-breed cats, which have similar clinical, gross, and histologic findings to those seen in dogs. Cats with muscular dystrophy are also prone to develop malignant hyperthermia associated with restraint or general anesthesia. Unlike the canine condition, cardiac involvement in cats is not common. Merino sheep have an autosomal recessive muscular dystrophy, and skeletal muscle of affected sheep expresses normal dystrophin. The major gross feature is the replacement of the intermedius, soleus, anconeus, and medial head of the triceps with mature adipose tissue. This disorder only affects type 1 muscle fibers, with initial lesions of type 1 fiber hypertrophy followed by myofibril loss and formation of sarcoplasmic masses at the center or periphery of the cell. Some fibers develop peripheral annular fibrils known as ringbinden. This condition is a result of loss of alpha-actinin and desmin proliferation, which forms the sarcoplasmic masses. It is important to distinguish true muscular dystrophies, which are inherited progressive myopathies, from other muscle disorders such as secondary nutritional degenerative myopathies(13).

There are four categories of reaction to muscle injury. Focal monophasic reactions result from an isolated single event; multifocal monophasic reactions result from a single injurious event that causes widespread lesions in the same phase of injury; focal polyphasic reactions are due to repeated mechanical injury at the same location; and multifocal polyphasic reactions are the result of continuous injury over a prolonged period of time such that lesions are widespread and exhibit various pathologic changes ranging from degeneration to necrosis to regeneration(13). This case is an example of a multifocal polyphasic (multiphasic) muscle lesion.

References:

1. Deitz K, Morrison JA, Kline K, Guo LT, Shelton GD: Sarcoglycan-deficient muscular dystrophy in a Boston terrier. J Vet Intern Med 22:476-480, 2008.
2. Durbeej M, Campbell KP: Muscular dystrophies involving the dystrophin-glycoprotein comples: an overview of current mouse models. Curr Opin Genet Dev 12:349-61, 2002.
3. Emery AE: The muscular dystrophies. Lancet 359:687-695, 2002
4. Gaschecn F, Jaggy A, Jones B: Congenital diseases of feline muscle and neuromuscular junction. J Feline Med Surg 6:355-366, 2004.
5. Khurana TS, Davies KE: Pharmacological strategies for muscular dystrophy. Nature Reviews: Drug Discovery 2:379-390, 2003.
6. Poncelet L, Resibois A, Engvall E, Shelton GD: Laminin α2 deficiency-associated muscular dystrophy in a Maine coon cat. J Small Anim Pract 44:550-552, 2003.
7. Salvadori C, Vattemi G, Lombardo R, Marini M, Cantile C, Shelton GD: Muscular dystrophy with reduced β-sarcoglycan in a cat. J Comp Path 140:278-282, 2009
8. Shelton GD, Engvall E: Muscular dystrophies and other inherited myopathies. Vet Clin North Am Small Anim Pract 32:103-124, 2002.
9. Shelton GD, Engvall E: Canine and feline models of human inherited muscle disease. Neuromuscul Disord 15:127-138, 2005
10. Vainzof M, Ayub-Guerrieri D, Onofre PC, Martins PC, Lopes VF, Zilberztajn D, Maia LS, Sell K, Yamamoto LU: Animal models for genetic neuromuscular diseases. J Mol Neurosci 34:241-248, 2008
11. Valentine BA, McGavin MD: Skeletal muscle. In: Pathologic Basis of Veterinary Disease, eds. McGavin MD, Zachary JF, 4th ed., pp. 1026-1029, 1035-1037. Elsevier, St. Louis, MO, 2007
12. Valentine BA, Winand NJ, Pradhan D, Moise NS, deLahunta A, Korneygay JN, Cooper BJ: Canine X-linked muscular dystrophy as an animal model of Duchenne muscular dystrophy: a review. Am J Med Genet 42:352-356, 1992
13. Van Vleet JF, Valentine BA. Muscle and tendon. In: Maxie MG, ed. Jubb, Kennedy and Palmers Pathology of Domestic Animals. vol. 1, 5th ed. Philadelphia, PA: Elsevier; 2007:198-9, 210-16.

Click the slide to view.

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2-1. Lateral Radiograph

2-2. Heart

2-3. Heart

2-4. Diaphragm

2-5 Diaphragm

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