Emergent Presentation of a Cat with Dystrophin-Deficient Muscular Dystrophy

Introduction

The purpose of this article is to discuss the clinical presentation of a cat with a novel gene mutation resulting in muscular dystrophy, and to review veterinary literature as it relates to this disorder. It is believed that the phenotypic presentation of this patient is related to the novel dystrophin gene mutation and up-regulation of utrophin in the skeletal muscles.

Case Report

A 7 mo old castrated male indoor domestic shorthair presented to the emergency room at the Angell Animal Medical Center. The presenting complaint was vomiting and open-mouth breathing for 3 days despite a normal appetite. Slight weight loss was reported by the owner, with a body weight of 2.05 kg and a body condition score of 4 out of 9. Physical examination at the time of presentation revealed an enlarged tongue without an obvious oral foreign body. Abdominal discomfort, mild dehydration, increased bronchovesicular lung sounds, and tachypnea were noted. The gait was within normal limits, without proprioceptive deficits in any limb. Cranial nerves were assessed to be normal. Appendicular muscle hypertrophy was not observed. Preliminary differential diagnoses included gastrointestinal foreign body with obstruction, electrocution, and intoxication.

Blood work obtained at the time of admission showed hyperglycemia (15.7 mmol/L; reference range, 4–7.33 mmol/L), hyperlactatemia (6.2 mmol/L; reference range, 1.1–3.5 mmol/L), mild hypochloremia (113 mmol/L; reference range, 117–125.3 mmol/L), and mild hyperkalemia (4.9 mmol/L; reference range, 3.41–4.71 mmol/L). The hyperglycemia was attributed to stress, the hyperlactatemia was attributed to either decreased perfusion or hypoxemia, and the hypochloremia was attributed to proximal gastrointestinal loss from regurgitation. Serum osmolality was elevated, with a calculated osmolality of 330.2 mmol/L (reference range, 290–300 mmol/L).

Initial treatment included crystalloid boluses^a totaling 68.4 mL/kg, dolasetron mesylate^b (0.5 mg/kg IV q 24 hr), and ampicillin^c (22 mg/kg IV q 8 hr). Hydromorphone^d (0.07 mg/kg IV) was administered in the event tachypnea was a sign of pain. Fluids were continued after the initial crystalloid bolus at a daily rate of 105 mL/kg/day. A complete blood cell count, serum biochemical analysis, lead level, and creatine kinase (CK) were submitted.

Whole-body radiographs were taken for a suspected foreign body. The thorax was included to evaluate a cause for altered lung sounds and tachypnea (Figure 1). Diffuse megaesophagus was identified, the diaphragm had a scalloped appearance, and the tracheal lumen was narrowed and had ventral displacement of the trachea by the esophagus. The stomach was empty. No signs of intestinal obstruction were observed, and the colon contained formed stool. Hepatomegaly was also present, and abdominal detail was within normal limits for age and body condition. An amended list of differential diagnoses included esophageal foreign body, esophageal stricture at the level of the diaphragm, esophagitis, congenital neuromuscular disease, and lead toxicity. Myasthenia gravis was not strongly considered (despite the fact that reports of focal myasthenia gravis with megaesophagus in the cat exist) because the feline esophagus contains skeletal muscle.^1,2

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Endoscopy was performed to evaluate the esophagus and upper gastrointestinal tract. Anesthesia was induced with a combination of fentanyl^e (5 μg/kg), ketamine^f (3.3 mg/kg), and midazolam^g (0.16 mg/kg) as an IV injection. Anesthesia was maintained with isoflurane^h. At the time of intubation, white plaques were present on the ventral tongue. Aspiration occurred at the time of intubation, and a large volume of tan to brown particulate material was suctioned. No strictures, ulcerations, or foreign bodies were documented during endoscopy. Partial thickness biopsies obtained from the stomach and duodenum and were consistent with mild gastric lymphofollicular hyperplasia and mild lymphoplasmacytic and eosinophilic duodenitis with mild mucosal fibrosis. An endotracheal wash was performed due to observed aspiration and the potential for either aspiration pneumonia or pneumonitis. Chronic-active inflammation was found with hemosiderophages. No phagocytized bacteria were observed. A culture of the tracheal wash fluid grew Edwardsiella hoshinae and Enterococcus faecalis. A nasogastric tube was placed to initiate enteral feeding after recovery from anesthesia. Postoperative radiographs (Figure 1) were taken to confirm placement of the nasogastric tube and evaluate the lungs. Improved tracheal position and lumen diameter were noted (Figure 1). All lung lobes appeared well inflated. The scalloped appearance of the diaphragm persisted.

Recovery from anesthesia was uneventful. A constant rate infusion of metoclopramideⁱ was initiated (1 mg/kg/day). Enteral feedings^j were started at one-third of the resting energy requirement. IV fluids were continued at 90 mL/kg/day. Enrofloxacin^k (5 mg/kg IV q 24 hr) was started following aspiration. Gastroprotectants were prescribed, including sucralfate^l (400 mg per os [PO] once then 250 mg PO q 6 hr), famotidine^m (0.5 mg/kg IV q 24 hr), and pantoprazoleⁿ (0.5 mg/kg IV q 24 hr). Feedings were well tolerated overnight. A recheck lactate was normal (2.3 mmol/L; reference range, 1.1–3.5 mmol/L).

The next morning, the condition of the tongue remained unchanged, and cervical ventroflexion was present. An electrocardiogram was performed at that time, showing sinus bradycardia with sinus arrhythmia at a heart rate of 100 beats/min. The sinus bradycardia was attributed to high vagal tone. Abdominal ultrasound was also performed, and the only abnormality present was hepatomegaly. No aspirates of the liver were performed. An amended list of differential diagnoses included gastroesophageal reflux, congenital neuromuscular disease, lead toxicity, and gastroenteritis.

At that time, results of blood work submitted at the time of admission became available. Aspartate aminotransferase (AST) was markedly elevated at 61.18 μkat/L (reference range, 0.29–0.73 μkat/L). Alanine aminotransferase (ALT) was also elevated, albeit to a lesser degree (6.78 μkat/L; reference range, 0.54–1.39 μkat/L). The serum CK activity was markedly elevated at > 510 μkat/L (reference range, 1.38–4.52 μkat/L). Blood lead level was 0.5 μmol/L (reference range, < 0.97 μmol/L). Lymphocytosis (5.6 × 10⁹/L; reference range, 6–5.5 × 10⁹/L) was present, with a subpopulation of morphologically abnormal lymphocytes consistent with immune stimulation.

The marked elevation in AST and CK (without history of trauma) in addition to tongue enlargement, white tongue plaques, megaesophagus, hepatomegaly, cervical ventroflexion, and a scalloped diaphragm were strongly suggestive of feline muscular dystrophy (FMD).^3–8 Radiographic abnormalities in FMD reportedly include megaesophagus, hepatomegaly, splenomegaly, cardiomegaly, scalloping of the diaphragm, renomegaly, and adrenal mineralization.⁸ Multiple radiographic abnormalities consistent with FMD were present in this cat, despite the lack of appendicular muscle hypertrophy. FMD is a progressive congenital myopathy with neither specific treatment nor cure.^3,9 Muscle relaxants may be prescribed to improve quality of life.^9,10 Prognosis for this disease was discussed with the owners, and humane euthanasia was elected.

At necropsy, gross findings included diaphragmatic thickening, megaesophagus, enlarged tongue with white plaques, hepatomegaly, and diffuse muscular pallor. No stricture of the esophagus at the level of the diaphragm was identified. Samples of the esophagus, liver, skeletal muscle, and diaphragm were obtained and immersed in 10% neutral buffered formalin for general histopathology. Histopathologic findings in muscle specimens collected at the time of necropsy included excessive variability in myofiber size, with diameters ranging from 15 μm to 100 μm (Figure 2). Degenerative changes, including myonecrosis and phagocytosis by macrophages, and rare basophilic fibers consistent with regeneration were noted. Many degenerate myofibers contained basophilic, granular material consistent with mineralization. Additional unfixed muscle biopsy specimens were shipped under refrigeration by an overnight service to the Comparative Neuromuscular Laboratory at the University of California-San Diego. Following flash freezing in isopentane precooled in liquid nitrogen, 8 μm sections were cut and incubated with either monoclonal or polyclonal antibodies against dystrophy associated proteins as previously described (Figure 2).^7,8 Compared with the control tissues, staining was absent using antibody against the C-terminus of dystrophin, confirming a dystrophin-deficient muscular dystrophy (DDMD). Staining for utrophin was increased, and regenerative fibers were noted using an antibody against developmental myosin heavy chain. Utrophin is a protein that is similar in amino acid sequence and secondary structure to dystrophin.¹¹ In mouse models of dystrophin deficient muscular dystrophy, the up-regulation of utrophin may be responsible for a milder phenotype by preventing repeat damage and regeneration of myofibers.¹¹

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Genomic DNA was purified from whole blood and sent to Cornell University for mutational analysis by polymerse chain reaction. The dystrophin mutation identified in this case was different from previously described gene mutations, with loss of the cortical neuronal promoter and first exon and the skeletal muscle promoter and first exon.^3,7 In the current case, a similar but nonidentical promoter region deletion was identified by polymerase chain reaction from genomic DNA. That deletion was large and involved regions of the dystrophin gene that are poorly characterized, including the cortical neuronal promoter and its first exon and the skeletal neuronal promoter and its first exon. The Purkinje neuronal promoter and first exon remain intact, and expression driven by that promoter may account for low levels of Purkinje neuronal type dystrophin in either muscle or nonmuscle tissues.

Discussion

FMD was considered in this case due to the age, gender, markedly elevated CK, presence of radiographic abnormalities associated with muscular dystrophy, and presenting clinical signs. Males are more commonly affected than females, as the dystrophin gene is carried on the X chromosome.^9,10 Female carriers could be affected; however, it would be considered rare and would involve random X chromosome inactivation. The hypertrophic form of DDMD has been previously described in domestic shorthairs, with genetic mutations identified.^3,7,12 In the cat described in this case, typical signs of appendicular muscle hypertrophy and gait abnormalities were lacking, making diagnosis difficult. Other forms of muscular dystrophy have been previously described in cats, including laminin α-2 deficiency in the domestic shorthair, Siamese, Persian, and Maine Coon; α-dystroglycan deficiency in the Sphynx and Devon Rex; and muscular dystrophy with reduced β-sarcoglycan in the domestic shorthair.^13–17 In DDMD, initial clinical signs can include gait abnormalities, regurgitation or dysphagia, and an enlarged tongue with white plaques secondary to mineralization of the tongue muscle.^3,9,10 Regurgitation can easily be confused with vomiting by the owner, particularly if there is either forceful movement or loud noises. It is important to distinguish regurgitation from vomiting in any patient. In FMD, regurgitation may occur if progressive diaphragmatic hypertrophy results in stricture of the esophagus as it passes through the diaphragm, although a stricture was not identified in the cat described in this report.

Muscular dystrophy is rare in cats, and clinical presentation can be variable. Even within a colony of cats bred to study the cardiac effects of muscular dystrophy, with all members arising from the same affected individual, clinical signs of disease varied among individuals.¹² In the case described herein, the clinical presentation was for vomiting, but megaesophagus was identified radiographically, and the authors believe the owner was observing regurgitation. The authors believe the regurgitation was the result of megaesophagus, not an esophageal stricture. The expected signs of appendicular muscle hypertrophy and gait abnormalities were lacking. The lack of appendicular muscle hypertrophy, but the presence of diaphragmatic abnormalities, may be the result of utrophin up-regulation in the appendicular muscles. Genetic studies comparing dystrophin-deficient mice to dystrophin- and utrophin-deficient mice have shown that the diaphragmatic changes are present in both genotypes of mice, but the appendicular changes are much more severe in the dystrophin- and utrophin-deficient mice.¹¹ It appears that in dystrophin-deficient mice, the up-regulation of utrophin is protective against appendicular muscle pathology, and the same may be true for the patient in this case report.¹¹

Abnormalities on serum biochemical analysis in FMD include marked and persistent elevation of AST, ALT, and CK.^3,9 Dramatic elevation of those enzymes were present in the cat described in this report, but that information was not immediately available at the time of presentation. The lack of myofiber and membrane stability in dystrophin deficiency may lead to necrosis of the myofibers, which can also result in AST leakage.⁶ Previous reports have attributed ALT elevation to hepatic congestion in heart failure.³ Although the cat described in this case did not have clinical signs of heart failure, he did have hepatomegaly and diffuse hepatic congestion at the time of necropsy, which may explain the elevation in ALT. However, elevations in ALT have been found in the absence of liver congestion in other cases of FMD, and previous authors have concluded that serum ALT elevations can to be muscular in origin.⁶ Plasma osmolality is often increased, possibly as a result of decreased enteral fluid intake, with values exceeding 310 mmol/L.⁶ Radiographs may show megaesophagus, an irregular diaphragmatic silhouette, adrenal calcification, and cardiomegaly.^8,10,12 Diaphragmatic scalloping is the result of progressive diaphragmatic muscle hypertrophy.⁸ Echocardiography can be performed to further characterize cardiomegaly and may show atrial dilation, left ventricular wall thickening, and papillary muscle hypertrophy.¹² Although cardiac changes are reported, clinical signs of heart disease are variable.^7,12 Signs of cardiomegaly are reported to develop in affected cats between 6 mo and 9 mo of age.¹² Those changes may also be present in carrier cats.¹² A report of colony-bred cats with hypertrophic FMD resulted in clinical signs of heart disease in only 2 of the 12 affected cats in the colony.¹² An echocardiogram was not performed in the cat described in this report. A cause for sinus bradycardia in the current case was not identified, but may be related to either the use of opioids or high vagal tone.

Although a high index of suspicion can be obtained with signalment, clinical presentation, radiographic findings, and marked elevation of CK activity, a definitive diagnosis of DDMD requires histopathologic evaluation of muscle biopsy specimens. Pathologic features of muscular dystrophy on either paraffin or frozen sections stained with hematoxylin and eosin include degenerative and regenerative changes, variable fibrosis, and calcific deposits in muscle fibers.⁹ It is important to note that tissue collected for histochemistry and immunohistochemistry should be handled differently than tissue collected for routine paraffin sections stained with hematoxylin and eosin. Cryosections of muscle biopsies are typically used for histochemical and immunohistochemical staining to identify the specific type of muscular dystrophy, and most available antibodies used for detection of dystrophy-associated proteins do not work on paraffin-embedded fixed tissues.⁷

It is important to recognize the increased anesthetic risk present in cats FMD.⁵ Although not yet proven, it is hypothesized that cats with DDMD have an increased sarcoplasmic reticulum membrane sensitivity to volatile anesthetics.^5,7 That sensitivity may result in either acute or peracute rhabdomyolysis, leading to life-threatening hyperkalemia, cardiac arrhythmias, hyperlactatemia, metabolic acidosis, or acute kidney injury as a result of pigment nephropathy.⁵ The adverse anesthetic events reported in humans with Duchenne muscular dystrophy are a separate entity from malignant hyperthermia and have occurred with the use of halothane, isoflurane, and sevoflurane. Adverse events can occur in the postoperative period, even after uneventful surgical intervention.¹⁸ Cases of rhabdomyolysis have also been reported with the use of succinylcholine, a depolarizing neuromuscular blockade agent.¹⁸ Rhabdomyolysis may also occur in those patients when they are either stressed or restained.⁵ Avoidance of succinylcholine, the use of anesthetic protocols that reduce the use of volatile anesthetics, use of minimal restraint, and consideration for the use of dantrolene may improve safety of muscular dystrophy patients under anesthesia.¹⁸ The use of oral dantrolene has been reported in a dog with acute rhabdomyolysis at a dose of 1.5 mg/kg PO q 8 hr and is believed to act as a muscle relaxant by inhibiting Ca release from the sarcoplasmic reticulum.¹⁹ The patient described in this report underwent anesthesia for diagnostic testing with no associated problems. Retrospectively, muscle biopsies should have been collected during anesthesia. Recently, a case of anesthetic death in a 4 yr old castrated male domestic shorthair cat with DDMD was reported.²⁰

This case involves a dystrophin-deficient form of muscular dystrophy. Over 30 different forms of muscular dystrophy are documented across species, which can lead to significant variation in case presentation.^9,10,13 Dystrophin deficiency is the most common type of muscular dystrophy in humans and appears to be the most common type in dogs and cats as well.¹⁰ Dystrophin is a large cytoskeletal protein and is present in skeletal, cardiac, and smooth muscle, including vascular smooth muscle.^6,9 It is believed that dystrophin stabilizes the specialized sarcoplasmic reticulum present in muscle cells.^6,7,9 The high frequency of DDMD has been attributed to the large size of the dystrophin gene with a propensity for spontaneous mutation.⁶ Dystrophin-deficient hypertrophic FMD has been correlated to Duchenne muscular dystrophy in humans due to similar dystrophin deficiency.⁶ Patients with Duchenne muscular dystrophy can have diaphragmatic changes similar to those reported in cats, with hypertrophy and thickening.^6,9,11,21 Golden retrievers are the most completely studied breed of dog with DDMD, although that form of dystrophy has been reported in many other dog breeds.^9,10 Dogs with DDMD display muscle atrophy with isolated muscle hypertrophy similar to humans with Duchenne muscular dystrophy.^6,9,10

In the case described above, DNA was obtained to determine the mutation responsible for DDMD. Previous reports from several years ago described two apparently unrelated domestic shorthair cats from the Boston area with hypertrophic FMD and dystrophin deficiency.^3,7 A causative mutation was not identified in the earliest reported case; however, a deletion of the skeletal muscle first exon region was ruled out by Southern blot analysis.³ In the second case, a deletion encompassing the dystrophin skeletal muscle and Purkinje neuronal promoters and their respective first exons was identified by reverse transcription polymerase chain reaction and Southern blotting.⁷ The region containing the cortical neuronal promoter and first exon remained intact in that animal, and the promoter was shown to drive low-level expression of cortical neuronal-type dystrophin in skeletal, but not cardiac, muscle tissue.⁷ Although mutational studies show that the cats in those reports are unrelated, it is possible that Boston represents an area with an increased incidence of FMD because the subjects of multiple reports have been from the Boston area.

Conclusion

Although rare, it is important to consider congenital neuromuscular disease even when patients present in emergent situations. This case highlights the importance of including CK as part of a minimum database for emergency cases. A marked elevation in CK, had it been available at the time of presentation (along with the presence of an enlarged tongue containing calcific deposits), could have better focused the diagnostic and therapeutic plan and allowed a more rapid diagnosis of muscular dystrophy. Since the presentation of this case, CK has been added to the chemistry and presurgical panels at the authors’ institution. Formation of a complete differential diagnosis list is important to direct diagnostic and therapeutic plans. Special precautions should be exercised in patients with suspected DDMD, including avoidance of stress, muscle exertion, and possibly anesthesia. Prevention of complications, like either rhabdomyolysis or aspiration of esophageal contents can be attempted with proper understanding of muscular dystrophy. This case illustrates the importance of a complete diagnostic workup, muscle biopsies, and necropsy to confirm diagnosis and gain further information about rare neuromuscular diseases.