Analysis of organic acids, amino acids, and carnitine in dogs with lipid storage myopathy

A Nonsense Variant in the ACADVL Gene in German Hunting Terriers with Exercise Induced Metabolic Myopathy

Several enzymes are involved in fatty acid oxidation, which is a key process in mitochondrial energy production. Inherited defects affecting any step of fatty acid oxidation can result in clinical disease. We present here an extended family of German Hunting Terriers with 10 dogs affected by clinical signs of exercise induced weakness, muscle pain, and suspected rhabdomyolysis. The combination of clinical signs, muscle histopathology and acylcarnitine analysis with an elevated tetradecenoylcarnitine (C14:1) peak suggested a possible diagnosis of acyl-CoA dehydrogenase very long chain deficiency (ACADVLD). Whole genome sequence analysis of one affected dog and 191 controls revealed a nonsense variant in the ACADVL gene encoding acyl-CoA dehydrogenase very long chain, c.1728C>A or p.(Tyr576*). The variant showed perfect association with the phenotype in the 10 affected and more than 500 control dogs of various breeds. Pathogenic variants in the ACADVL gene have been reported in humans with similar myopathic phenotypes. We therefore considered the detected variant to be the most likely candidate causative variant for the observed exercise induced myopathy. To our knowledge, this is the first description of this disease in dogs, which we propose to name exercise induced metabolic myopathy (EIMM), and the identification of the first canine pathogenic ACADVL variant. Our findings provide a large animal model for a known human disease and will enable genetic testing to avoid the unintentional breeding of affected offspring.

Metabolic Encephalopathy and Lipid Storage Myopathy Associated with a Presumptive Mitochondrial Fatty Acid Oxidation Defect in a Dog

A 1-year-old spayed female Shih Tzu presented for episodic abnormalities of posture and mentation. Neurological examination was consistent with a bilaterally symmetric multifocal encephalopathy. The dog had a waxing-and-waning hyperlactemia and hypoglycemia. Magnetic resonance imaging revealed bilaterally symmetric cavitated lesions of the caudate nuclei with less severe abnormalities in the cerebellar nuclei. Empirical therapy was unsuccessful, and the patient was euthanized. Post-mortem histopathology revealed bilaterally symmetric necrotic lesions of the caudate and cerebellar nuclei and multi-organ lipid accumulation, including a lipid storage myopathy. Malonic aciduria and ketonuria were found on urinary organic acid screen. Plasma acylcarnitine analysis suggested a fatty acid oxidation defect. Fatty acid oxidation disorders are inborn errors of metabolism documented in humans, but poorly described in dogs. Although neurological signs have been described in humans with this group of diseases, descriptions of advanced imaging, and histopathology are severely lacking. This report suggests that abnormalities of fatty acid metabolism may cause severe, bilateral gray matter necrosis, and lipid accumulation in multiple organs including the skeletal muscles, liver, and kidneys. Veterinarians should be aware that fatty acid oxidation disorders, although potentially fatal, may be treatable. A timely definitive diagnosis is essential in guiding therapy.

Dystrophin-deficient muscular dystrophy in a Norfolk terrier

A six-month-old male entire Norfolk terrier was presented with a 3-month history of poor development, reluctance to exercise and progressive and diffuse muscle atrophy. Serum creatine kinase concentration was markedly elevated. Magnetic resonance imaging of the epaxial muscles revealed asymmetrical streaky signal changes aligned within the muscle fibres (hyperintense on T2-weighted images and short-tau inversion recovery with moderate contrast enhancement on T1-weighted images). Electromyography revealed pseudomyotonic discharges and fibrillation potentials localised at the level of the supraspinatus, epaxial muscles and tibial cranialis muscles. Muscle biopsy results were consistent with dystrophin-deficient muscular dystrophy. The dog remained stable 7 months after diagnosis with coenzyme Q10 and l-carnitine; however after that time, there was a marked deterioration and the owners elected euthanasia. This case report describes the clinical presentation, magnetic resonance imaging, electrodiagnostic and histopathological findings with immunohistochemical analysis in a Norfolk terrier with confirmed dystrophin-deficient muscular dystrophy, which has not been previously described in this breed.

Cannabinoid receptor antagonist-induced striated muscle toxicity and ethylmalonic-adipic aciduria in beagle dogs

Ibipinabant (IBI), a potent cannabinoid-1 receptor (CB1R) antagonist, previously in development for the treatment of obesity, causes skeletal and cardiac myopathy in beagle dogs. This toxicity was characterized by increases in muscle-derived enzyme activity in serum and microscopic striated muscle degeneration and accumulation of lipid droplets in myofibers. Additional changes in serum chemistry included decreases in glucose and increases in non-esterified fatty acids and cholesterol, and metabolic acidosis, consistent with disturbances in lipid and carbohydrate metabolism. No evidence of CB1R expression was detected in dog striated muscle as assessed by polymerase chain reaction, immunohistochemistry, Western blot analysis, and competitive radioligand binding. Investigative studies utilized metabonomic technology and demonstrated changes in several intermediates and metabolites of fatty acid metabolism including plasma acylcarnitines and urinary ethylmalonate, methylsuccinate, adipate, suberate, hexanoylglycine, sarcosine, dimethylglycine, isovalerylglycine, and 2-hydroxyglutarate. These results indicated that the toxic effect of IBI on striated muscle in beagle dogs is consistent with an inhibition of the mitochondrial flavin-containing enzymes including dimethyl glycine, sarcosine, isovaleryl-CoA, 2-hydroxyglutarate, and multiple acyl-CoA (short, medium, long, and very long chain) dehydrogenases. All of these enzymes converge at the level of electron transfer flavoprotein (ETF) and ETF oxidoreductase. Urinary ethylmalonate was shown to be a biomarker of IBI-induced striated muscle toxicity in dogs and could provide the ability to monitor potential IBI-induced toxic myopathy in humans. We propose that IBI-induced toxic myopathy in beagle dogs is not caused by direct antagonism of CB1R and could represent a model of ethylmalonic-adipic aciduria in humans.

Routine and specialized laboratory testing for the diagnosis of neuromuscular diseases in dogs and cats

The diagnosis of neuromuscular diseases can be challenging. The first step is recognition that the disease involves the neuromuscular system (muscle, neuromuscular junction, peripheral nerve, and ventral horn cells of the spinal cord). Many neuromuscular diseases share clinical signs and cannot be distinguished based on clinical examination. Routine laboratory screening, including a CBC, biochemical profile, and urinalysis, can identify some of the most common systemic abnormalities that cause muscle weakness and myalgia, such as hypo- and hyperglycemia, electrolyte disorders, or thyroid abnormalities, and may suggest a specific diagnosis, such as diabetes mellitus, hypo- or hyperadrenocorticism, renal failure, or hypothyroidism. Increased creatine kinase activity, increased cardiac troponin I concentration, and myoglobinuria are useful in detecting skeletal and cardiac muscle damage. Identification of acetylcholine receptor antibodies is diagnostic for acquired myasthenia gravis. For primary muscle or peripheral nerve diseases, tissue biopsy is the most direct way to determine specific pathology, correctly classify the disease, and determine the course of additional laboratory testing. For example, inflammatory, necrotizing, dystrophic, metabolic, or congenital myopathies require different laboratory testing procedures for further characterization. Many neuromuscular diseases are inherited or breed-associated, and DNA-based tests may already be established or may be feasible to develop after the disorder has been accurately characterized. This review focuses on both routine and specialized laboratory testing necessary to reach a definitive diagnosis and determine an accurate prognosis for neuromuscular diseases.

Evaluations of labrador retrievers with exercise-induced collapse, including response to a standardized strenuous exercise protocol

Clinical and metabolic variables were evaluated in 14 Labrador retrievers with exercise-induced collapse (EIC) before, during, and following completion of a standardized strenuous exercise protocol. Findings were compared with previously reported variables from 14 normal Labrador retrievers that participated in the same protocol. Ten of 14 dogs with EIC developed an abnormal gait during evaluation, and these dogs were significantly more tachycardic and had a more severe respiratory alkalosis after exercise compared to the normal dogs. Muscle biopsy characteristics and sequential lactate and pyruvate concentrations were normal. Genetic testing and linkage analysis excluded malignant hyperthermia as the cause of EIC. Common causes of exercise intolerance were eliminated, but the cause of collapse in EIC was not determined.

Refractory seizures associated with an organic aciduria in a dog

A 6-month-old, female Cavalier King Charles spaniel exhibited seizures that were difficult to control with standard anticonvulsants over a 12-month period. The diagnosis of an organic aciduria with excessive excretion of hexanoylglycine was determined when the dog was 20 months old. Recurrent and cluster seizures were eventually controlled with the addition of levetiracetam to potassium bromide and phenobarbital.

Pyruvate dehydrogenase deficiency in a Sussex spaniel

A two-year-old, intact female Sussex spaniel was presented with signs of exercise intolerance. Pre- and post-exercise serum lactate and pyruvate concentrations and urinary organic acid screening supported a diagnosis of pyruvate dehydrogenase deficiency, as previously reported in this breed. Dietary therapy was initiated for six months, during which time there was no reported clinical deterioration. A full neurological examination and repeat evaluation of lactate and pyruvate concentrations before and after exercise was conducted one year after diagnosis, at which time the patient had been without dietary modification for six months and had developed more severe exercise intolerance along with evidence of central nervous system dysfunction.

L-2-Hydroxyglutaric aciduria in Staffordshire Bull Terriers

L-2-Hydroxyglutaric aciduria is an inborn error of metabolism, which has been recognized in humans since 1980. The metabolic defect responsible for the disease is unknown, but the disorder can be diagnosed in humans by elevations of the organic acid, L-2-hydroxyglutaric acid in the cerebrospinal fluid (CSF), plasma, and urine of affected patients. The disorder produces a variety of clinical neurological defects in humans including psychomotor retardation, seizures, and ataxia. There have previously been no recognized animal models of the disease. However, 6 Staffordshire Bull Terriers were recently identified with the disorder. The animals presented with a variety of clinical signs, most notably seizures, ataxia, dementia, and tremors. They were all screened for organic acid abnormalities in urine, and CSF and plasma (when available). Levels of L-2-hydroxyglutaric acid were elevated in all body fluids evaluated. The clinical, clinicopathologic, and magnetic resonance imaging (MRI) characteristics associated with L-2-hydroxyglutaric acid in Stafforshire Bull Terriers is reported herein and represents the first veterinary model of this inborn error of metabolism.

Muscle and nerve biopsy

Biopsy of muscle and nerve is an essential component of the diagnostic plan for animals with suspected neuromuscular disease. This article includes descriptions of the biopsy procedures and information regarding appropriate biopsy site selection and correct handling and processing of the tissues. The normal and pathological appearance of muscle and nerve using routine histochemical and histological techniques are described and illustrated emphasizing the basic myopathic and neuropathic responses of the neuromuscular system.

Neuromuscular complications in endocrine and metabolic disorders

Many of the endocrine and metabolic myopathies have no unique features, and for most clinicians, it is not possible to remember the clinical nuances of all the specific abnormalities and deficiencies responsible for these myopathies. This can make this group of diseases difficult to suspect. It is more important to recognize the general features of myopathic disease and to consider muscle biopsies as a preliminary diagnostic technique, with the potential for further investigation if a myopathy is confirmed.

Lipomatosis, proximal myopathy, and the mitochondrial 8344 mutation. A lipid storage myopathy?

Multiple symmetric lipomatosis (MSL) has been related in some cases to the 8344 point mutation of the tRNA-lysine gene of the mitochondrial DNA, mainly in the context of families with classic myoclonic epilepsy with ragged-red fibers (MERRF) and exceptionally in patients with proximal myopathy as the only manifestation of mitochondrial disease. We report on two families harboring the 8344 mutation. The patients presented with MSL and myopathy, expressed as limb girdle weakness in index cases and as exercise intolerance in the others. All muscle biopsies performed showed lipid storage apart from RRF and respiratory chain complexes deficiency. A possible explanation for both adipose proliferation and lipid storage myopathy in these cases is a disturbance in intermediary lipid metabolism secondary to mitochondrial respiratory chain deficiency that could be related via carnitine deficiency.

Lipid storage myopathy in a cocker spaniel

A six-year-old male cocker spaniel was presented to the Veterinary Medical Teaching Hospital, University of Florida, with a three-week history of generalised weakness and myalgia. Electrodiagnostic evaluation, cerebrospinal fluid analysis and thoracolumbar myelography were unremarkable. Biopsies from vastus lateralis and triceps muscles revealed numerous large lipid droplets within type 1 fibres and to a lesser degree within type 2 fibres. The resting plasma lactate was mildly increased and there was elevated urinary excretion of lactic, pyruvic and acetoacetic acids, increased urinary excretion of carnitine esters, and increased plasma alanine. This pattern of metabolite excretion is consistent with an, as yet undefined, block in oxidative metabolism.