Mitochondrial Diseases

A Pulmonary and Critical-Care Medicine Perspective

The clinical spectrum of mitochondrial diseases has expanded dramatically in the last decade. Abnormalities of mitochondrial function are now thought to participate in a number of common adult diseases, ranging from exercise intolerance to aging. This review outlines the common presentations of mitochondrial disease in ICUs and in the outpatient setting and discusses current diagnostic and therapeutic options as they pertain to the pulmonary and critical-care physician. (CHEST 2001; 120:634-648)

Key words: critical care; exercise intolerance; exercise testing; hypoventilation; lactic acidosis; mitochondria; mitochondrial genome; mitochondrial myopathy; muscle biopsy; muscle weakness

Abbreviations: ADP = adenosine diphosphate; ATP = adenosine triphosphate; CoA = coenzyme A; CPT = carnitine palmitoyl transferase; DCA = dichloroacetate; [FADH.sub.2] = flavin adenine dinucleotide; KSS = Kearns-Sayre syndrome; LHON = Leber's hereditary optic neuropathy; MELAS = myoclonic epilepsy with lactic acidosis and stroke-like episodes; MM = mitochondrial myopathy; mtDNA = mitochondrial DNA; NADH = nicotinamide adenine dinucleotide reduced form; NARP = neurogenic muscle weakness ataxia and retinopathy; NMR = nuclear magnetic resonance; PCr = phosphocreatine; PD = pyruvate dehydrogenase; Pi = inorganic phosphate. HER = respiratory exchange ratio; RRF = ragged red fiber; TCA = tricarboxylic acid; [VO.sub.2]max = maximal oxygen consumption

Theodur Leber unknowingly described the first mitochondrial disease in his description of adult-onset blindness in 1871.[1] Luft's disease, a disease characterized in 1962 with hypermetabolism and elevated core temperature, was the first disease proposed to have a mitochondrial origin.[2] However, it was not until 1989 that the genetic bases for these diseases was discovered and their mitochondrial origin confirmed. Over the subsequent 11 years, there has been tremendous progress in understanding the genetics of mitochondrial disease, with [is greater than] 200 mutations of the mitochondrial genome having been described.[1] As knowledge has grown about these diseases, the clinical spectrum has expanded dramatically. Diseases with subtle clinical manifestations are now being described and relatively common diseases, such as Alzheimer's dementia[3-5] and Parkinson's disease,[4,6] are being investigated for mitochondrial involvement. Within the expanding spectrum of clinically significant diseases are common problems that present to the pulmonologist and intensivist, including unexplained dyspnea and exercise intolerance, respiratory failure, inability to wean from mechanical ventilation, and persistent lactic acidosis.

Mitochondrial dysfunction is devastating to the overall function of an organism. As the organelle responsible for energy production, pathologic changes in the mitochondria deprive cells of the adenosine triphosphate (ATP) that is essential for cellular functioning. Cells with high metabolic rates, such as those in the heart, brain, and skeletal muscle, are particularly vulnerable. Deprivation of ATP also facilitates alternative metabolic pathways, resulting in the accumulation of metabolic byproducts, such as lactate, which may be harmful to the organism.

Recognition of mitochondrial diseases requires an understanding of their common signs and symptoms as well as a basic understanding of mitochondrial biochemistry and genetics. The diseases are not always severe or life-threatening, and multiple organ systems do not need to be obviously involved. Skeletal muscle, the retina, or the pancreas may be the only organs affected. Diseases may present in the early and later years of life. DNA mutations with associated defects in mitochondrial function can be inherited with maternal, autosomal-dominant, or autosomal-recessive patterns, can occur spontaneously, or can be acquired as a result of environmental exposure or drug exposure. These mutations may be transient or persistent, or even may accumulate in the mitochondrial genome over a lifetime.

Several excellent reviews[1,2,7-12] have summarized common clinical syndromes and the large and expanding number of mitochondrial DNA (mtDNA) mutations that are implicated in disease pathogenesis. This review will focus on the presentation of mitochondrial disease to the pulmonary and critical-care physician and will discuss, in terms of pathophysiology, the relevant clinical features, strengths, and weaknesses of diagnostic tests and the available therapeutic options.

BIOCHEMISTRY

DNA replication and protein synthesis, which are essential for cellular functioning, are costly metabolic reactions that require the provision of an energy substrate. ATP produced by the mitochondria serves as this substrate. Mitochondria are dependent on several metabolic pathways within a cell to guarantee the supply of ATP during varying cellular conditions. Glycolysis and fatty acid oxidation are two of these pathways. Glycolysis is the principal pathway on which the mitochondria depend in the nutritionally replete state, while fatty acid oxidation is the principal pathway on which they depend during fasting.

In glycolysis (Fig 1, left, A), glucose is converted to pyruvate in the cytosol, is transferred into the mitochondria, and is converted into acetyl coenzyme A (CoA) by the action of pyruvate dehydrogenase (PD). Acetyl CoA enters the tricarboxylic acid (TCA) cycle, and the reduced form of nicotinamide adenine dinucleotide (NADH) is formed. NADH subsequently is utilized by the mitochondria to produce ATP. PD is an important regulatory step; NADH, acetyl CoA, ATP, and an anaerobic environment inhibit the enzyme.[13-15] When PD is inhibited, pyruvate accumulates and is metabolized into lactate and alanine. Lactate is made under physiologic conditions, such as exercise, when the anaerobic environment inhibits PD. Lactate production also is facilitated when NADH, acetyl CoA, or ATP accumulate and feedback inhibits PD, as can be seen in several diseases of the mitochondria. The subsequent metabolism of lactate results in the hydrolysis of ATP and the production of two hydrogen ions.[16] These ions are the etiology of the metabolic acidosis seen with increased levels of lactate. Thus, lactate also may accumulate when perfusion is limited or when hepatic function is impaired.[17]

[ILLUSTRATION OMITTED]

In fatty acid oxidation (Fig 1, middle, B), long-chain fatty acids are conjugated with carnitine to produce acylcarnitine, which is shuttled into the mitochondria, and are metabolized to acetyl CoA and flavin adenine dinucleotide ([FADH.sub.2]) by acyl dehydrogenases. [FADH.sub.2], like NADH, is subsequently utilized by the mitochondria to produce ATP. In the fed state, acetyl CoA enters the TCA cycle, resulting in additional NADH synthesis. During fasting, acetyl CoA is metabolized to [Beta]-hydroxy-butyrate, an essential source of energy for the brain.

ATP is generated by oxidative phosphorylation, a task carried out by five protein complexes that are located in the inner mitochondrial membrane (Fig 1, right, C). NADH from the TCA cycle is a substrate for complex I, and [FADH.sub.2] from fatty acid oxidation is a substrate for complex II (Fig 1, right, C). Electrons are transferred in succession from either complex I or II to the remaining complexes, resulting in a proton gradient. Complex V (ATPase) fuels the production of ATP by coupling this proton gradient to ATP production.[7,9,10,18]

Efficient cellular functioning requires ATP production and is contingent on the ability of the cell to utilize these different metabolic pathways, depending on the prevailing environmental conditions. An inability to depend on glycolysis, [Beta]-fatty acid oxidation, or oxidative phosphorylation threatens the energy supply to the cell and, thus, cellular viability.

For example, if the glycolytic enzyme PD is mutated, glucose cannot be used to produce ATP. Glucose will be metabolized to pyruvate, but pyruvate is not decarboxylated to acetyl CoA. Instead, pyruvate is metabolized to lactate. Under these conditions, a meal high in carbohydrates will result in excessive lactate and a drop in ATP production. Metabolically active cells, which depend on high levels of ATP, are stressed by these conditions.

Similarly, in the [Beta]-fatty acid oxidation pathway, deficiencies of carnitine or mutations in any essential enzymes of fatty acid oxidation, including medium-chain acyl dehydrogenase, long-chain acyl dehydrogenase, carnitine palmitoyl transferase (CPT) I or II, and carnitine translocase, compromise the use of long-chain and medium-chain fatty acids for ATP production (Fig 1, middle, B). Since the body relies on fatty acids during fasting or following a meal high in fat, patients with abnormalities in fatty acid oxidation are at risk for organ malfunction under these conditions. As an example, infants with disorders of fatty acid oxidation cannot tolerate decreased frequency of feeds. When the infant begins to fast during the night, metabolic derangements occur and may result in sudden infant death syndrome.[19,20]

In diseases involving only glycolysis or [Beta]-fatty acid oxidation, oxidative phosphorylation remains intact. ATP production continues if the body takes advantage of the unaffected metabolic pathways. As a result, some improvement may occur in patients simply by increasing the percentage of either carbohydrates or fats in their diet, or by carnitine supplementation. However, if oxidative phosphorylation is interrupted by mutations or deficiencies of any of the proteins that comprise complexes I through V, ATP production is severely impaired. As with the diseases of glycolysis and fatty acid oxidation, cellular functioning suffers due to a drop in ATP production. Additional metabolic derangements also may occur, and decreased use of NADH and [FADH.sub.2] by the oxidative phosphorylation chain results in the accumulation of these molecules. Increased levels of NADH inhibit PD activity, and lactate levels increase dramatically resulting in metabolic acidosis.

Oxidative phosphorylation also is affected by environmental conditions. Increased body temperature is a result of an uncoupling of the protein gradient and ATP production.[21] This uncoupling increases demand on oxidative phosphorylation and decreases ATP production. For patients with already impaired oxidative phosphorylation, the increased metabolic demands of fever may cause the unmasking or exacerbation of the underlying disease.[22] Additionally, drugs such as propofol[23] and nucleoside analogs[24,25] may transiently inhibit oxidative phosphorylation and also may precipitate disease. Physiologic stress of any kind, whether related to infection, fasting, heat, or cold, also may exacerbate mitochondrial disease.[26]

GENETICS

Mitochondrial disease may result from mutations in DNA encoding any of the proteins of glycolysis, fatty acid oxidation, and oxidative phosphorylation. Therefore, some understanding of the genetics of mitochondrial inheritance is fundamental in disease recognition. Mitochondrial function depends on proteins encoded by nuclear and mtDNA; mutations in either source can result in clinically significant disease.[1,7,9-12] mtDNA differs significantly from nuclear DNA and is at greater risk for mutations. mtDNA has no introns and is dependent on nuclear genes for replication, transcription, translation, and repair.[9,11,27] The mtDNA is also subject to more oxidative damage than nuclear DNA because it mutates up to 10 times faster and has fewer and less efficient repair mechanisms.[7,9-12,27] As a result, a significant number of mtDNA mutations can be acquired as one ages.

Nuclear DNA is inherited in a mendelian pattern; one copy of each gene is inherited from the mother and the father. If nuclear DNA encodes the mutated protein, the disease may be inherited in an autosomal-dominant or autosomal-recessive fashion.[28] Because the mtDNA is inherited almost exclusively from the mother, mitochondrial disease also may be inherited maternally.[1,7,12,29] mtDNA is governed by population genetics because each cell contains many mitochondria and each mitochondrium contains multiple copies of mtDNA. As a result, the genotype of each cell is not necessarily the same. Homoplasmy refers to cells in which there is a homogenous population of mtDNA.[1,7,9,10,30,31] Heteroplasmy refers to the state in which multiple populations of mtDNA are present.[3,9,10,12,18] Mutations may be present in all of the mtDNA or only in a subpopulation. Like all cells, oocytes have multiple copies of mtDNA. Since mitochondria are randomly sorted during meiosis, different oocytes have varying concentrations of mutated mtDNA. As a result, siblings from the same mother may have marked variations in the expression of mitochondrial disease.[1,32,33]

Whether cells can generate enough ATP depends on the amount of dysfunctional, mutated mtDNA relative to the total mtDNA within each cell. Since cells have different metabolic needs, some cells may be able to tolerate a greater burden of mutated mtDNA. The threshold is the quantity of the mutated mtDNA that is tolerated by the cell before its vital energy needs are compromised.[1,7,10,12] Neurons, cardiac cells, and skeletal muscle cells, which are highly dependent on oxidative phosphorylation, are most sensitive to the accumulation of the mtDNA defects and are organ systems that are commonly affected by mitochondrial disease.[9,10,28,33-35]

Because mtDNA is more prone than nuclear DNA to acquired mutations, mitochondrial disease also may be acquired. Transient mutations (most commonly deletions) may be induced by medications such as propofol and nucleoside analogs. Additionally, given the poor repair and replication mechanisms and the oxidative environment of the mitochondria, sporadic mutations of mtDNA occur in everyone. Since these mutations accumulate as one ages, people born with different burdens of mtDNA may reach the threshold for the expression of their disease during infancy, adolescence, or adulthood.[10,36,37]

The field of mitochondrial genetics is rapidly evolving. New mutations including deletions, duplications, and point mutations are being unraveled for nearly every manifestation of mitochondrial disease. Descriptions of these mutations are beyond the scope of this article but have been discussed elsewhere.[1,5,7,9,10,12,18,33,35-40] A reference of the currently known mitochondrial and nuclear mutations that are associated with different diseases may be found at the following Web site: http://www. gen.emory.edu/mitomap.html.[9]

CLINICAL PRESENTATION

Mitochondrial diseases can affect almost every organ system (Table 1). Ophthalmologic manifestations are a common presenting feature and include ptosis, ophthalmoplegia, optic atrophy, retinitis pigmentosa, cortical blindness, and cataracts. CNS and peripheral nervous system manifestations include seizure, ataxia, stroke, dementia, psychosis, migraines, peripheral neuropathies, and even severe depression. The heart may develop hypertrophic or dilated cardiomyopathy and/or cardiac conduction defects. Skeletal muscle may weaken or easily fatigue. Liver function test results may be markedly abnormal, renal tubular acidosis may occur, and even endocrinologic abnormalities such as diabetes mellitus or hypothyroidism may result. It is estimated that approximately 0.5 to 1.5% of adult-onset diabetes mellitus is the result of an mtDNA point mutation.[7,12] Excellent reviews have been written on some of the well-defined mitochondrial syndromes, including chronic progressive external ophthalmoplegia,[41] Kearns-Sayre syndrome (KSS),[2,34,42] myoclonic epilepsy with ragged red fibers (RRFs),[1,22,43] myoclonic epilepsy with lactic acidosis and stroke-like episodes (MELTS),[1,44] Leigh's disease,[6,29] Leber's hereditary optic neuropathy (LHON),[1,45-49] and mitochondrial myopathy (MM).[2,8,11,36,37]

MITOCHONDRIAL DISEASE IN THE PULMONARY AND CRITICAL-CARE SETTING

Several manifestations of mitochondrial disease may be presented to pulmonary critical-care physicians and deserve further discussion (Table 2).

Lactic Acidosis in the Absence of Hypoxia or Sepsis

This condition often presents with tachypnea or altered mental status. Additionally, it may be discovered incidentally by the presence of an anion gap metabolic acidosis. Patients with mitochondrial disease may have a chronic sustained, or intermittent, lactic acidosis. Several factors may precipitate or worsen acidosis including exercise, drug use, or physiologic stress such as fasting. Exercise, which increases the amount of lactate even in healthy individuals,[50] can markedly elevate lactate levels in patients with mitochondrial disease.[15,51-53] Exercise increases the demand for ATP production.[54] Patients with defects of oxidative phosphorylation are unable to meet this demand.[15,52,53] As the body attempts to provide the mitochondria with substrates for oxidative phosphorylation, NADH and [FADH.sub.2] levels increase and inhibit PD activity. As a result, lactate production is increased. Alcohol may precipitate lactic acidosis by increasing NADH levels by a mechanism that is different from exercise.[55]

Therapeutic agents also may cause or potentiate lactic acidosis. These drugs should not be given to patients with mitochondrial disease, and their use should be excluded before diagnosing mitochondrial disease in a patient. In HIV patients, nucleoside analogs may precipitate muscle weakness with lactic acidosis.[24,25,56] Prolonged use of propofol has been reported[23] to increase lactate levels by inducing transient abnormalities in oxidative phosphorylation (Fig 1). Both propofol and nucleoside analogs directly modify mtDNA, with the resultant mutations then affecting mitochondrial function.

Several other pharmacologic agents are known to directly affect lactic acid production and clearance, resulting in elevated lactate levels. Aspirin, dinitrophenol, cocaine, and the blue dye (FD&C blue No. 1) used to color tube feedings can uncouple oxidative phosphorylation, resulting in increased temperature and increased lactate levels.[57-60] A cyanide ion, from nitroprusside administration or smoke inhalation, directly inhibits oxidative phosphorylation.[16] Catecholamines (ie, pressors), theophylline, and cocaine can increase lactate production by decreasing perfusion to peripheral tissue and decreasing lactate clearance by the liver.[16,61,62] Metformin increases the production of lactate in the intestines and decreases hepatic clearance of lactate.[17,63] Valproate, in overdose, has been reported to inhibit fatty acid oxidation and, thereby, to increase lactate production.[64]

Respiratory Failure

Respiratory failure may be the initial sign of mitochondrial disease in adults and may present fulminantly or as an intermittent, relapsing problem.[65-67] Two presentations of respiratory failure have been reported in patients with mitochondrial disease. One presentation is respiratory muscle fatigue following a period of increasing dyspnea.[66,67] Tachypnea, intercostal muscle retractions, and a rise in Pa[CO.sub.2] in this setting may masquerade as an exacerbation of COPD.[66] The second presentation is hypoventilation as a result of an inciting event, including pneumonia,[65-67] extremely low doses of sedative-hypnotics (eg, clonazepam, meperidine, and secobarbital),[65,68,69] or high altitude.[68] Reported outcomes following mechanical ventilation are variable. Some patients recover but have fluctuating courses with recurrence of respiratory failure at a later time, other patients are partially dependent on mechanical ventilation, while others became totally ventilator-dependent.[66,67]

One mechanism of hypoventilation in patients with mitochondrial disease may be impaired ventilatory response to hypercapnia and hypoxia.[68,70] Two studies have investigated patients with previous episodes of hypoventilation by exposing them to low oxygen tension (ie, 40 mm Hg) and high [PCO.sub.2] tension (ie, 50 to 60 mm Hg). Impaired ventilatory responses to both stimuli were observed.[68] Muscle weakness may contribute to hypoventilation, and a restrictive pattern may be seen on spirometry.[65,66] Diaphragmatic paralysis also has been reported.[71]

Given its prevalence, respiratory failure would not necessarily prompt the consideration of mitochondrial diseases. However, the clinician should consider this diagnosis in the following settings: failure to wean from the ventilator when other causes such as electrolyte disturbances, hypothyroidism, myasthenia gravis, and critical illness polyneuropathy are ruled out; prolonged paralysis with atracurium, mivacurium, and succinylcholine in the absence of liver or kidney disease[1,72]; respiratory failure following minimal sedation; or respiratory failure with persistent lactic acidosis in the absence of hypoperfusion or drugs known to cause lactic acidosis.

Neurologic Abnormalities

Young adults or teenagers with migraines, focal neurologic signs, and evidence of stroke seen on head CT examinations may harbor mitochondrial abnormalities.[1,73-75] Stroke often involves the occipital or parietal lobes and usually follows a nonvascular distribution that is seen on a head CT scan.[1,9,12,41] Approximately 14% of the occipital strokes in patients [is less than] 30 years of age are due to an underlying mitochondrial point mutation.[1] Seizures, another neurologic feature of mitochondriopathies, should be treated cautiously in patients who have received a previous diagnosis. Phenobarbital and valproic acid may inhibit oxidative phosphorylation and lower the seizure threshold.[27]

Cardiac Abnormalities

Hypertrophic cardiomyopathies and cardiac conduction defects are the most frequently encountered heart diseases in mitochondrial disorders.[33,35,42,76] Hypertrophic cardiomyopathy may manifest as heart failure in adults who have few other manifestations of mitochondrial disease.[76] In contrast, cardiac conduction defects are present in patients with KSS[40,42] or MELAS,[35] both of which are diseases that produce severe multiorgan impairment. Right bundle branch block, left anterior fascicular block, and preexcitation syndromes are common.[33] Second-degree and third-degree heart block also may develop. Since conduction defects are a common cause of death in these patients, prophylactic pacemaker placement is recommended.[42] These patients also may develop dilated cardiomyopathy and heart failure with increased risk of thromboembolism.[35]

Several other clinical settings in the ICU may warrant further investigation. These include acute renal tubular acidosis of unknown etiology,[34,77] otherwise unexplained acute renal failure,[78,79] deafness in patients with exposure to aminoglycosides,[80,81] and an exaggerated response to anesthetics or paralytics in patients with no known history of liver or kidney disease.[1,27,72]

Exercise Intolerance and Unexplained Dyspnea

After exercise, patients may have dyspnea, tachycardia out of proportion to the degree of work,[39] and lactic acidosis.[38] Diplopia may occur in one third of cases, and in severe cases dysphagia may be present.[8] Severe exercise intolerance in the absence of other systemic features of mitochondrial disease has only recently been appreciated. Genetic screening in patients with exercise intolerance has resulted in the discovery of several new mtDNA deletions, suggesting that mitochondrial disease is more prevalent than previously has been thought.[38] In addition, exercise intolerance may be drug-related. Patients who have received lung transplants may experience exercise limitation as a result of cyclosporine-induced MM.[82]

Muscle Weakness

There is also increasing recognition of MM as a cause of acquired and/or late-onset weakness.[36-38,83] Muscle weakness may be acquired following long-term treatment with nucleoside analogs, which induce MM that presents as weakness and myalgia.[25,56,84] Additional evidence suggests that muscle weakness may result from acquired somatic mutations that accumulate over a lifetime. An analysis of mtDNA in a population of elderly patients with muscle weakness revealed multiple different mutations within each patient and classic findings of mitochondriopathy on muscle biopsy specimens, suggesting that this kind of myopathy may be the second-most common etiology of muscle weakness in those [is greater than] 69 years of age.[38]

Sleep Apnea

There are sporadic case reports[70,85-88] of sleep apnea in patients with mitochondrial disease, especially those with cytochrome c deficiency,[70] Leigh's encephalopathy,[86,87] or neurogenic muscle weakness, ataxia, and retinopathy (NARP).[85] Apneic events are most often central but may also be obstructive.[85] Respiratory arrest during sleep has been reported.[86,87] The treatment of sleep apnea with tracheostomy has resulted not only in improvements of sleep apnea, but also in improvements of general cerebral functioning.[85] Patients with mitochondrial disease are postulated to have central sleep apnea due to a reduced ventilatory response to Pa[CO.sub.2].[70,85-87] Metabolic abnormalities may contribute to the reduced ventilatory drive, as has been described for other diseases such as myxedema.[86,87] Muscle weakness and involvement of the phrenic nerve also may contribute to sleep apnea.[85,88] To date and to our knowledge, there have been no prospective studies evaluating patients with milder versions of mitochondrial disease, such as isolated exercise intolerance, for sleep apnea. Given the dramatic response to treatment in at least one patient, patients with known mitochondrial disease and features suggestive of sleep apnea should be evaluated by polysomnography and treated appropriately.

EVALUATION AND DIAGNOSIS

The importance of a complete history, including a detailed review of systems, medications, family history, and physical examination cannot be overemphasized. The history of present illness may elucidate important precipitating events, such as infection, fasting, exercise, or use of prescription drugs. A review of systems may detect a history of exercise intolerance, dyspnea, muscle weakness or fatigue, and visual changes. A family history may reveal pedigrees with maternal inheritance, or mendelian autosomal-dominant or autosomal-recessive inheritance. On physical examination, focal or diffuse neurologic signs including ptosis, ophthalmoplegia, proximal muscle weakness, or alteration in mental status may be found.

The important laboratory tests in the initial investigation include serum levels of pyruvate, lactate, alanine, acylcarnitine, and carnitine.[7,51,55,89,90] Lactate levels are increased when normal oxidative phosphorylation is disrupted and NADH concentration increases.[50,77] A lactate-to-pyruvate ratio of [is greater than] 20 is abnormal. However, normal lactate levels and a normal lactate-to-pyruvate ratio do not exclude mitochondrial disease.[7,14,74,77,90] Low levels of carnitine suggest carnitine deficiency or an abnormality of CPT I or II, which may be isolated or may occur concomitantly with diseases of oxidative phosphorylation or hyperlactemia.[2,14,89] Twenty-four-hour urine measurements of pyruvate, lactate, glucose, phosphate, and amino acids may detect defects in the renal tubular cells, which are highly dependent on oxidative phosphorylation.[7,14,77,90]

Pulmonary function test results may be abnormal with a reduced FVC, maximum minute ventilation, and inspiratory-expiratory pressures secondary to muscle weakness.[51] Spirometric maneuvers alone will rarely induce fatigue, and asymptomatic patients and patients with complaints of isolated fatigue will usually have normal spirometry results.[66,68]

Exercise testing and [sup.31]P nuclear magnetic resonance (NMR) spectroscopy are two additional noninvasive tests that may reveal metabolic abnormalities in patients who experience exercise intolerance.[54] These tests should be used in patients with an unrevealing initial laboratory evaluation and a high index of suspicion for disease. Exercise testing will reveal an elevated heart rate relative to the degree of work and a diminished maximal workload.[15,39,51-53,91] Maximal oxygen consumption ([VO.sub.2]max) and ventilatory threshold are reduced, and the respiratory exchange ratio is increased.[39,51,91-93] These findings are not specific to mitochondrial disease and can be seen in deconditioning or other myopathic conditions.[94]

[sup.31]P NMR spectroscopy is a method used to assess the metabolic state of muscle fibers.[50,54,94-96] Resting, exercise, and postexercise levels of phosphocreatine (PCr), adenosine diphosphate (ADP), and inorganic phosphate (Pi) are measured. By extrapolation, the ability of the muscle to generate ATP (ie, oxidative phosphorylation potential) is assessed. In mitochondrial diseases, the resting PCr level is reduced and rapidly declines during exercise,[37,50,54,94,96] and the postexercise recovery of PCr, ADP, and Pi levels is prolonged.[54,96]

When initial laboratory test results are abnormal or when the results of noninvasive testing suggest a mitochondrial disease, a muscle biopsy specimen often is used to confirm the diagnosis of the disease. Traditional findings on light microscopy include RRFs, which represent subsarcolemmal proliferation of mitochondria that can be detected with Gomori's trichrome and succinate dehydrogenase staining (Fig 2).[2,8,83,97] The absence of RRFs does not exclude disease[2,12]; certain mutations alter the likelihood of this finding (Table 2). Lipid deposition also may be seen on light microscopy, but increases of connective tissue are not. On electron microscopy, mitochondria may appear to be abnormal, with increased size, abnormal cristae, and/or paracrystalline inclusions (Fig 3).[2,12]

[ILLUSTRATIONS OMITTED]

In specialized centers, mitochondrial enzymology may be performed on muscle biopsy specimens to assess the metabolic capabilities of oxidative phosphorylation. Mitochondria are isolated from fresh muscle tissue, and the function of each individual complex is assessed using a variety of specific complex inhibitors and substrates. This test can locate specific abnormalities within the mitochondria, thus directing further diagnostic testing and treatment, but it is difficult to perform and is not widely available.[12,18,89,93]

The genetic analysis of mtDNA is useful when the above-described test results are normal but the clinical suspicion of mitochondrial disease remains high or when previous test results have been abnormal and a more specific diagnosis is desired. mtDNA is isolated from leukocytes or muscle. The genome may be screened for more common mitochondrial mutations using in situ hybridization, or the entire genome may be sequenced. For reasons of expense, in situ hybridization is often the initial test, although if results are negative and suspicion remains high, the entire mitochondrial genome should be sequenced. Although genetic analysis is extremely sensitive for detecting mutations, unfortunately a negative test result does not completely rule out the presence of disease.[12] mtDNA mutations are more easily detected in postmitotic tissue because the mutations remain in each cell. In rapidly dividing cells such as leukocytes, mitochondria are randomly assorted with each division, with the concentration of mutated mtDNA being diluted with each cell division.[18,33,35] Therefore, mutations may not be present in the tissue used for analysis[7,14,90]; if leukocyte analysis is initially performed and the results are negative, the analysis should be repeated on skeletal muscle.[7,14,18] Unfortunately, these test results may be negative even when the mitochondria are abnormal. Mitochondrial function may be compromised as a result of nuclear mutations or from previously undescribed mtDNA mutations.[7,9] An analysis of 340 adult and pediatric specimens from patients with known mitochondrial disease at Emory University using in situ hybridization for selected point mutations and mtDNA rearrangements detected only 30% of adult patients and 5% of pediatric patients.[12]

An evaluation of patients for the diagnosis of a mitochondrial disease is obviously difficult and is complicated both by the availability and reliability of each diagnostic test. Table 3 is a list of the available tests and factors affecting the sensitivity and specificity of each test. Figure 4 is a diagnostic algorithm that may serve as a guide to establishing the diagnosis of a mitochondrial disease.

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TREATMENT

Treatment options are limited, although prevention and behavioral modification can be helpful. Patients must be warned to avoid precipitating factors, such as fasting, exposure to cold, alcohol, tobacco, sedatives or anesthesia, and infections. Stress can also worsen the disease.[26] Pacing and the avoidance of overexertion are important. In patients with CPT deficiencies, a diet high in carbohydrates may improve the disease by shifting metabolism toward glycolysis.

Exercise may benefit patients with mitochondrial disease in two ways. First, it prevents deconditioning, which exacerbates preexisting exercise intolerance and fatigability.[15,52,53] Second, exercise may have a direct effect on the population of mtDNA within the muscle. Regular aerobic exercise improves muscle oxidative metabolism; lactate levels are decreased, ATP production increases, and the half-life of ADP as measured by [sup.31]P NMR is reduced.[15,52,53] Metabolic improvements may result from increased capillary density and blood flow, as well as mitochondrial size and density following exercise. Exercise may place positive selective pressure on those mitochondria with a normal pheno-type.[9,53,98] Submaximal exercise has been shown to improve exercise tolerance, heart rate, anaerobic threshold, and quality of life in such patients.[15,52,53] Anecdotal reports also suggest a symptomatic benefit from supervised aerobic exercise in combination with carnitine, riboflavin, and/or coenzyme Q.[9,67,99] Exercise programs should be initiated with care in patients with mitochondrial disease, especially those with cardiomyopathies or cardiac conduction defects. Nearly all the studies evaluating aerobic training in these patients limited exercise to 60 to 80% of the heart rate reserve.[53]

Several drugs have been used with limited success in the treatment of mitochondrial disease.[100,101] Drug therapy attempts to improve ATP production by providing protein components of oxidative phosphorylation and/or by inhibiting lactate production. Treatment with riboflavin or coenzyme Q has increased [VO.sub.2]max and the maximal amount of workload tolerated and has reduced lactate levels during exercise.[91,99] Coenzyme Q also has improved cerebellar signs[102] and has improved respiratory function in a patient with diaphragmatic paralysis and respiratory failure.[71] Dichloroacetate (DCA) has been used to treat severe lactic acidosis. DCA inhibits the inactivation of PD, thus decreasing the accumulation of pyruvate and the production of lactate. In mitochondrial disease with lactic acidosis, a double-blind, placebo-controlled trial of DCA (25 mg/kg bid) resulted in reductions in lactate, pyruvate, and alanine levels. Patients also showed improvements on the [sup.31]P NMR scans of their brains but did not have alterations in their muscle spectroscopy and did not report symptomatic improvement.[13] However, in a study combining DCA and aerobic training, improvements were found in symptoms and on [sup.31]P NMR muscle spectroscopy.[15] In patients with carnitine deficiency, carnitine supplementation has markedly ameliorated muscle fatigue.[95]

Unfortunately, to our knowledge, no large randomized controlled trials have been performed. It does seem clear that behavior modification and exercise are helpful. Some of the other proposed treatments could be viewed as nutritional supplements with an apparently low risk of adverse consequences and with the potential for symptomatic improvement.

In conclusion, the clinical diversity and prevalence of mitochondrial disease have only recently been appreciated. And as our knowledge of these abnormalities increases, additional clinical syndromes, particularly those that present in adulthood, will be described. Diagnosis will require a high level of suspicion and a familiarity with the biochemical and genetic abnormalities that help to guide the diagnostic testing and therapy.

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(*) From the Department of Internal Medicine (Dr. Clay), Division of Pulmonary, Allergy, Critical Care, and Occupational Medicine (Dr. Behnia), Indiana University School of Medicine, Indianapolis, IN; and the Department of Pulmonary Sciences and Critical Care Medicine (Dr. Brown), University of Colorado Health Sciences Center, Denver, CO.

Manuscript received August 2, 2000; revision accepted March 13, 2001.

Correspondence to: Kevin K. Brown, MD, FCCP, Director, Clinical Interstitial Lung Disease Program, National Jewish Medical and Research Center, 1400 Jackson St, F108, Denver, CO 80222; e-mail: brownk@njc.org

COPYRIGHT 2001 American College of Chest Physicians
COPYRIGHT 2001 Gale Group