Relative Elevations of Serum Alanine and Aspartate Aminotransferase in Muscular Dystrophy

Introduction Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) are enzymes important in the intermediary metabolism of amino acids and gluconeogenesis. Their concentration in liver is high relative to other tissues. ALT is found primarily in the hepatocyte cytosol, while AST is found in both cytosol and mitochondria. Serum concentrations of ALT and AST are measured in clinical laboratories by their enzymatic activity. The low levels normally recorded probably represent a constant leak of enzyme from tissues to serum, although the sources are not known. Processes causing hepatocyte death or injury result in loss of the enzymes into the plasma compartment thus elevating the serum concentrations. Elevated serum ALT and AST are sensitive for detecting acute and chronic hepatitis and many other liver diseases; therefore they are among the first tests obtained in an evaluation of suspected liver disease (1). Because they are included in many serum chemistry panels, however, they are sometimes measured in children in whom there is no clinical suspicion of liver disease. Because of the strong association of elevated ALT and AST with liver disease, the incidental discovery of elevated serum ALT and AST usually leads to an evaluation focused on the liver ignoring the possibility of other pathology. Both ALT and AST are found in tissues other than liver. Increased serum concentrations of the enzymes resulting from injury to non-hepatic tissue is a function of the total enzyme content of the tissue. The absolute values of muscle enzyme activities are reported to be (in units/gram wet tissue) 3281 for creatine kinase (CK), 4800 for ALT and 99,000 for AST. By activity, CK is 1000 times more concentrated in muscle than liver (2,3) whereas ALT is 10 times more concentrated in liver than muscle and AST only 1.5 times more concentrated in liver than muscle (4). Because of their concentration in liver, ALT and to a lesser extent AST are considered “liver specific” enzymes, and CK is considered more muscle specific. However, given that an average adult male skeletal muscle mass is 60 kg with an average liver mass of 1.5 kg, four times more ALT, 26 times more AST and 40,000 times more CK are contained in muscle than in liver. Thus, it is clear that muscle disease should be considered early in the evaluation of children with elevated serum aminotransferases. An elevated serum CK is unlikely to result from liver disease. Serum concentrations of ALT, AST and CK are determined by their rate of entry into the plasma compartment, their distribution in the extracellular fluid compartment and their rate of catabolism or clearance. Their relative concentration in tissue partly determine their rates of entry into plasma as a result of tissue damage. This situation has led to some confusion over the interpretation of elevated serum aminotransferases. In liver disease, the ratio of AST to ALT has little meaning except in alcoholic liver disease, where the AST/ALT ratio often exceeds 3 because of depressed hepatic ALT synthesis secondary to pyridoxal 5′-phosphate deficiency (5). In muscle disease, one could assume from the relative muscle concentrations that 20 times more AST than ALT would enter the plasma space per unit of time and incorrectly conclude that a high serum AST/ALT should be a characteristic finding of muscle disease. We report four children with congenital myopathies who presented with otherwise unexplained aminotransferase elevations and with ALT and AST at approximately equal concentrations. A representative case is presented in detail, and all four cases are collectively summarized. CASE REPORT A 16-year-old previously healthy and completely asymptomatic competitive female athlete was informed that she had elevated liver enzymes when donating blood. Her pediatrician confirmed that her enzymes were elevated: ALT, 94 IU/L (normal range, 5 to 35 IU/L) and AST, 76 IU/L (normal range, 8 to 39 IU/L). Other liver test results were normal: total serum bilirubin, 0.3 mg/dL; total protein, 7.8 g/dL; albumin, 4.6 g/dL; alkaline phosphatase, 74 IU/L and international normalized ratio, 1.04. Testing for mononucleosis was consistent with past infection. Two months later the ALT was 157 U/L and AST was 152 U/L. Further testing by a gastroenterologist failed to identify any acute or chronic liver disease. Serology for hepatitis A, hepatitis B and hepatitis C was negative; serum ceruloplasmin level was normal (29.3 mg/dL); serum alpha-1-antitrypsin level was normal (138 mg/dL); and autoimmune markers were negative (anti-nuclear antibody, liver kidney microsomal antibody and anti smooth muscle all ≤1:40). A liver biopsy showed normal liver morphology, and abdominal ultrasound showed no abnormalities of the liver and bile ducts. She was referred to the authors for evaluation of suspected liver disease. In the medical history there was nothing to suggest liver or muscle disease. She denied jaundice, change in appetite, acholic stools, nausea and skin rash. She had been a healthy child with active participation in school athletic competitions and had never experienced chronic weakness or fatigue. She had no family history of liver, muscle or autoimmune disease. Her father had died as a result of complications from Zollinger-Ellison syndrome. Her physical examination was normal, including a normal liver span of 9 cm. There was no palpable spleen and no visible stigmata of chronic liver disease. Suspecting occult muscle disease, her serum CK was found to be 4358 IU/L (normal range, 22 to 145 IU/L). She was referred to a specialty clinic for evaluation of muscular dystrophy. A detailed neurologic examination showed normal power, strength and muscle tone. The only abnormal findings were mildly hypertrophied calf muscles bilaterally and macroglossia. A deletion was found within one of her dystrophin genes which suggested she was heterozygous for Duchenne muscular dystrophy. Case Summaries The findings of this case and the other three cases are presented in Table 1. All of these patients were referred to pediatric gastroenterologists for asymptomatic transaminase elevations. Two girls heterozygous for Duchenne muscular dystrophy had simultaneously measured serum CK, AST and ALT. The ratios of CK/AST/ALT were 29/1/1 and 11/1.2/1, respectively. Two boys with muscular dystrophy had much higher absolute values for CK, AST and ALT and their CK/AST/ALT ratios were 57/0.9/1 and 38/0.9/1, respectively.TABLE 1: Case summariesDISCUSSION The findings in these four children demonstrate that elevated serum ALT and AST in the absence of signs and symptoms of liver disease should lead to a consideration of occult muscle disease as a probable source. A simple next step in evaluating such children is to examine a muscle-specific enzyme such as CK before embarking on a full-scale evaluation for liver disease. Of interest, these cases show that the serum concentrations of CK, AST and ALT in muscle disease do not reflect their relative concentrations in muscle. Indeed, in these cases the serum ALT equaled or exceeded the AST, which confused the evaluation and led to unnecessary testing, including liver biopsy in two patients. Clinical findings in the early stages of congenital muscular dystrophies may be subtle. It is well known that these conditions also cause chronically elevated transaminases as well as elevated intrinsic muscle enzymes such as CK (6). Small series of congenital myopathies presenting with elevated aminotransferases have been reported (7-14). Despite these reports, congenital myopathy is often a late consideration in the differential diagnosis of elevated aminotransferases. Sibley et al. first measured serum enzyme levels in muscular dystrophy in 1949 and hypothesized that enzymes entered the blood from damaged muscle cells (15). The difference in liver and muscle tissue concentrations of ALT and AST has been extrapolated to a common misconception that the serum AST level in muscular dystrophy should be significantly greater than ALT. If it is assumed that tissue injury leads to the entry of AST, ALT and CK into the plasma compartment at rates equal to their concentrations in muscle and that the rates of entry determine serum concentrations, then there should be a one-to-one correlation between tissue concentrations and serum enzyme levels. It appears that this is not the case. Several studies have shown that patients with muscular dystrophy have equally elevated serum concentrations of AST and ALT but not the relative elevation of AST that would be expected according to the common perception (6,16,17). Other factors determining serum enzyme concentration include volume of distribution and rate of catabolism or clearance. The volume of distribution is not a factor in the steady state, as would be expected in chronic, low-grade myopathy, leaving only the rate of catabolism or clearance to explain our findings. In canine experiments ALT clearance is much slower than AST clearance (18,19). The half-life of serum ALT in the rat (42 ± 11 hours) is more than twice that of AST (17 ± 11 hours) (20). Assuming a constant rate of entry into the plasma space that is proportionate to the tissue concentration of enzyme (i.e., AST's rate of entry would be 20-fold greater in muscle and threefold greater in liver than ALT) and assuming catabolic half-lives in humans that are similar to those in dogs and rats, it is possible to show mathematically that low-grade muscle disease can produce steady state concentrations of ALT higher than those of AST. Furthermore, making the same assumptions, it can be concluded that the clearance half-life of CK is much shorter than that of either AST or ALT to achieve the observed serum ratios of CK/AST/ALT. Trends for AST and ALT clearance rates have been seen in studies of acute liver injury secondary to hypoxia related to protracted seizures (21). Such cases show that with acute injury, the degree of insult relates to the magnitude of serum enzyme elevation, whereas the clearance half-lives and the time elapsed from the time of injury relate to the ratio of AST to ALT measured in serum. All of our patients had ALT/AST ratios ≥1. Our review of the literature revealed 20 cases of myopathy presenting with elevated aminotransferases; 12 patients had measurements of ALT ≥ AST (Table 2). In these patients the AST concentration was not uniformly significantly higher than ALT concentration, but the increase in CK was much greater than either AST or ALT, confirming the muscle origin of the enzymes (22). We conclude that muscle disease must be considered as a cause for elevated serum aminotransferases even when ALT equals or exceeds AST. CK is an easy, specific and inexpensive marker of muscle disease. We suggest CK should be measured in any child with persistently elevated transaminases in the absence of other clinical or biochemical evidence of liver disease. This will prevent the unnecessary risk and expense of complete evaluation for liver disease.TABLE 2: Summary of previously published case series