Physiology

A healthy 70-kg man has approximately 28 kg of muscle. Skeletal muscle is 80% water and 20% protein, accounting for about half of the total body protein stores, and is the largest organ in the human body.¹ A large amount of adenosine triphosphate (ATP) is required for muscle function, even at rest. ATP generation by muscle accounts for 30% of the body’s oxygen consumption at rest and up to 85% at extremes of physical activity.² Resting muscle uses fatty acids for ATP generation; muscle then draws on stored ATP for the first 8 seconds of activity, next using the phosphagen (creatine phosphate) stores for an additional 10 to 15 seconds; finally, muscle depends on anaerobic glycogen metabolism to lactate for enough ATP for an additional 30 to 40 seconds of activity. Aerobic ATP production provides the bulk of the energy needed for muscle activity but requires oxygen. Glucose, amino acids, and fatty acids are incorporated into the Krebs cycle to produce much larger quantities of ATP by energy-rich compounds, such as the reduced forms of nicotinamide adenine dinucleotide and flavin adenine dinucleotide.

Myoglobin, like hemoglobin, binds and releases oxygen and delivers it to active skeletal muscle. Unlike for hemoglobin, its ability to deliver oxygen is unaffected by pH, resulting in a relatively increased affinity for oxygen in comparison to hemoglobin and delivery of oxygen to cellular mitochondria at low partial pressures of oxygen.

The integrity of muscle cells is dependent on healthy cell membranes, which rely on ATP for proper membrane ion pump function. The sarcolemma, a thin membrane that encloses striated muscle fibers, contains numerous pumps that regulate electrochemical gradients. Under normal physiologic conditions, the sodium-potassium–adenosine triphosphatase (Na⁺,K⁺-ATPase) pump located in the sarcolemma maintains intracellular sodium concentrations of 10 mEq/L and intracellular potassium concentrations of 150 to 160 mEq/L by actively transporting sodium from the interior of the cell to the exterior, thereby making the interior of the cell more negative by the efflux of net positive charge (three sodium ions pumped out per two potassium ions pumped in). The electrical gradient then pulls sodium to the interior of the cell through a separate channel in exchange for calcium, effectively removing calcium from the cytoplasm. Low intracellular calcium levels are also maintained by an active calcium exchanger (Ca²⁺-ATPase pump) that promotes calcium entry into the sarcoplasmic reticulum and mitochondria. As their names indicate, these ATPase pumps depend on ATP as a source of energy.

Under normal physiologic conditions, the concentration of free ionized calcium in the extracellular space is approximately 10,000 times greater than that in the intracellular space.³ The high concentration of free calcium in the extracellular pool compared with the intracellular compartment and the resulting large electrochemical force on Ca²⁺ are particularly convenient to its role as an intracellular regulator. Even minor changes in the permeability of the plasma membrane to calcium will produce significant fluctuations in the cytosolic concentration with potentially unfavorable consequences for the integrity of the cell.^4,5

Several transmembrane proteins exist to regulate calcium homeostasis. The plasma membrane transmembrane proteins are the energy-consuming Ca²⁺ channels, the Na⁺-Ca²⁺ exchangers, and the Ca²⁺-ATPase pumps. The last removes Ca²⁺ from the intracellular space by transporting Ca²⁺ out of the cytosol into the extracellular space as well as into the sarcoplasmic reticulum. Plasma membrane Ca²⁺ channels bring Ca²⁺ into the cytosol when activated at the neuromuscular junction. Na⁺-Ca²⁺ exchangers are complicated, insofar that the direction of ion movement depends on the chemical and electrical gradients of each ion within the cell, which vary according to the contractile state of the myocyte3,6 (Fig. 127-1).

Pathophysiology

Whereas the causes of rhabdomyolysis are diverse, the pathogenesis appears to follow a final common pathway: increased cytoplasmic calcium concentration, leading to myocyte destruction with release of muscle components into the circulation. There are two primary mechanisms by which calcium pathologically accumulates in the cell: direct cell membrane damage and ATP depletion.

Membrane damage from trauma and genetic or biochemical factors results in a massive influx of extracellular calcium into the cytoplasm driven by both electrical and chemical gradients.⁵ ATP depletion results in failure of cellular transport and increased permeability to sodium ions. Any event that increases cytosolic sodium concentrations, whether it is from increased membrane permeability and inward Na⁺ traffic or decreased ATP-dependent pump removal of intracellular Na⁺, results in increased Na⁺-Ca²⁺ ion exchanger function and increased cytosolic calcium concentrations. ATP depletion results in dysfunction of energy-dependent ion pumps, such as Na⁺,K⁺-ATPase and Ca²⁺-ATPase in the sarcolemma. Na⁺,K⁺-ATPase pump dysfunction leads to increased intracellular Na⁺, causing a temporary increase in Na⁺-Ca²⁺ exchanger function and resultant increase in intracellular Ca²⁺. The Na⁺-Ca²⁺ exchanger requires ATP, however, and continued activity of the Na⁺-Ca²⁺ exchanger further deprives the cell of ATP, leading to increased Ca²⁺-ATPase dysfunction and rising intracellular Ca²⁺ levels. The sarcoplasmic reticulum and the mitochondria are also equipped with these same energy-dependent transmembrane calcium transport mechanisms (Ca²⁺-ATPase) as well as an ATP-dependent Ca²⁺ uniporter, further exacerbating the cell’s inability to remove intracellular Ca²⁺ in the low-energy conditions put forth by rhabdomyolysis.

An abrupt increase in cytoplasmic Ca²⁺ leads to a corresponding increase in mitochondrial Ca²⁺ because the mitochondria serve as the Ca²⁺ safety net. In addition to triggering apoptotic cell death by upregulating expression of proapoptotic factors, this mitochondrial Ca²⁺ overload leads to dysfunction of oxidative phosphorylation through structural and functional alterations, disrupting ATP production and further worsening the ATP debt and intracellular homeostasis of Ca²⁺.

Increased mitochondrial Ca²⁺ also leads to increased production of reactive oxygen species (ROS),⁷ which are a broad group of chemical substances that include free oxygen radicals (O₂⁻, OH^•) and powerful oxidants (hydrogen peroxide, nitric oxide) that lead to free radical production. Free radicals carry extra, unpaired electrons (e⁻) that have a strong tendency to pair off, causing oxidation by extraction of electrons from other chemical species. Under normal physiologic conditions, 2 to 5% of oxygen consumed by the mitochondria is transformed during electron transport to ROS that are neutralized by endogenous antioxidants such as glutathione. When the antioxidant capacity of the endogenous system is overwhelmed during times of intense, sustained physical activity or illness such that rhabdomyolysis results, a condition termed oxidative stress ensues. The resulting ROS-induced destruction of lipids and proteins injures the structure of membranes as well as DNA with mutations, leading to disruptions in cellular architecture and mitochondrial respiratory protein integrity. The cumulative effect exacerbates the inability to regulate intracellular Ca²⁺ homeostasis. Whereas ROS may have adaptive physiologic functions in exercising muscles, significant damage to muscle cells occurs when ROS are triggered by pathologic stimuli.

Damage of the sarcoplasmic reticulum and mitochondria during cell death results in the release of stored calcium ions into the cytoplasm. The decreased removal of Ca²⁺ into the extracellular fluid, along with inadequate intracellular storage and increased entry into the cytoplasm, leads to myocytic Ca²⁺ overload and initiation of the cascade of cellular death.

Calcium-induced activation of proteases, phospholipases, and other proteolytic enzymes degrades the protein structure of myofibrils, the cytoskeleton, and membrane composition, leading to further cellular damage. Increased Ca²⁺ leads to sustained contraction of the myofibrils, resulting in severe ATP depletion in the initial stages while leading to myofibrillar breakdown as rhabdomyolysis progresses. Breakdown of the myofibrillar network hastens the disintegration of the myocyte, whereas high Ca²⁺ concentrations within the mitochondria arrest cellular respiration and block ATP production. Diminished ATP production after Ca²⁺-mediated activation of degradative cellular enzymes, with large metabolic energy requirements of their own, exacerbates the preexisting energy crisis within already ischemic cells.⁸ This process releases free fatty acids and lysophosphates that cause direct toxic damage to the sarcolemma, intracellular organelles, and plasma membrane carrier proteins, effecting increased Ca²⁺ entry into the cytosol.

Microelectrode studies of intercostal myocytes show that intracellular calcium concentration rises to 1.27 µmol/L after induction of rhabdomyolysis by exhaustive physical exertion compared with 0.12 µmol/L in normal muscles.⁹ That is nearly an 11-fold increase in intracellular calcium concentration. In this study, dantrolene, a known inhibitor of Ca²⁺ release from the sarcoplasmic reticulum, was administered and led to an 83% reduction of cytosolic Ca²⁺ and an improvement in the clinical symptoms of muscle stiffness, rigidity, and pain, highlighting the essential role of increased cytosolic Ca²⁺ in the pathophysiologic process of rhabdomyolysis.

The end result in rhabdomyolysis is muscle cell breakdown, with the release of toxic contents into the extracellular space and damage to adjacent capillaries, resulting in local edema, increased compartmental pressures, and regional ischemia, which further induces energy depletion and destroys more capillaries. Circulating leukocytes adhere to these damaged capillaries, become activated, and transmigrate to the site of myocyte injury, where they release ROS and proteolytic enzymes that further injure the cell.

In instances in which ischemia is the inciting event (e.g., crush syndrome, arterial thromboembolism) of rhabdomyolysis, myocyte destruction primarily takes place not during ischemia but rather during reperfusion¹⁰ (Fig. 127-2).

Etiology

Rhabdomyolysis is classified into four basic pathophysiologic processes:

Box 127-1 lists the various causes of rhabdomyolysis.