The myoglobinuria refers to the presence in the urine of myoglobin, a protein that is found in the muscles and fulfills the function of storing oxygen. This is a rare finding. The process begins with hemoglobinuria.
Hemoglobinuria occurs from muscle damage or injury. This causes the release of myoglobin into the blood. The blood is filtered and eliminated by the kidneys, which can be severely damaged due to the size of the molecule.
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Myoglobin, whose structure is similar to hemoglobin, is a muscle protein responsible for storing oxygen, necessary for muscles to fulfill their function.
It has a heme group, with a capacity to bind oxygen even greater than that present in hemoglobin, which facilitates the passage of oxygen from the blood to the muscle.
The Merrian-Webster Medical Dictionary defines myoglobinuria as "the presence of myoglobin in the urine," a simple definition for a complex problem..
Myoglobinuria is a clinical sign, evidenced by darkening of the urine, which occurs when a muscle is severely damaged, allowing myoglobin to pass into the blood. As it passes through the kidney, the blood will be filtered and circulating myoglobin will be eliminated, which produces the dark color of the urine..
Myoglobinuria is said to be a clinical sign and not a symptom because it is observable. This is usually accompanied by symptoms of the main cause: muscle injury.
Anything that causes considerable damage to the muscle can cause hemoglobinuria, and the causes are multiple.
According to the muscle injury mechanism, Nayak & Jindal (2015) divide the main causes of myoglobinuria into four groups: physical, hypoxic (lack of oxygen), chemical and biological. In turn, these can come from outside or inside the body.
The location of the muscles, the activity they develop and their oxygen requirement predispose them to physical damage, either from trauma or strenuous physical exercise.
A clear example is rhabdomyolysis, a clinical condition that causes skeletal muscle rupture and includes cell damage, the product of intense physical exercises or inappropriate for muscle development. Dehydration and poor oxygenation can make this condition worse.
Injuries from accidents or natural disasters also correspond to physical causes of myoglobinuria.
The mechanism by which a lack of oxygen causes muscle damage is relatively simple: in the absence of oxygen, muscle metabolism will occur anaerobically, producing lactic acid and free radicals..
There are medications that can be harmful to skeletal muscle. These include antipsychotics, statins, some anesthetics, alcohol, drugs of abuse, dietary supplements, and antibiotics..
Biological agents, such as viruses and bacteria, can cause varying degrees of muscle inflammation or myositis, with the potential for cell damage and myoglobin release.
Myoglobinuria is visible evidence of muscle damage. The symptoms that accompany the darkening of the urine will lead to suspect its origin.
In general, myoglobinuria is associated with the general symptoms and signs that indicate muscle injury or that derive from its causes: pain, limitation of movements, inflammation, weakness and fever, among others.
In the case of rhabdomyolysis, dark urine is part of the diagnostic triad of this syndrome, accompanying pain and muscle weakness.
The most serious complication that can occur from myoglobinuria is acute kidney damage. Knowledge of kidney complications from myoglobinuria has been known for a long time. The mechanisms of myoglobin-induced renal toxicity are:
- Renal blood vessel contraction.
- Renal cellular toxicity, caused by direct action of the heme group of myoglobin.
- Obstruction of the renal tubules due to accumulation of myoglobin.
As in any disease, the diagnosis is based on a detailed history and a thorough physical examination..
The estimation of serum and urinary myoglobin is less reliable in the diagnosis of rhabdomyolysis and myoglobinuria, therefore, in the opinion of Nayak & Jindal (2015), it should not be performed routinely.
It should be taken into account that complementary tests should be used to diagnose diseases that cause muscle injury, because muscle injury not only releases myoglobin, but other enzymes and intracellular potassium.
Some of these tests are routine tests, serum electrolytes, determination of Creatine PhosphoKinase (CPK), lactic dehydrogenase (LDH), among others..
Evidence of dark urine associated with symptoms of muscle injury should make one suspect the existence of myoglobinuria.
For this reason, the patient will require immediate hospitalization for intravenous hydration. Adequate physical rest must be guaranteed, the cause must be diagnosed, complications prevented, and the clinical picture monitored.
In general lines, the treatment will have as objectives:
- Treat the triggering cause of the muscle injury (physical, hypoxic, chemical, or biological).
- Prevent the potential damage posed by the passage of myoglobin through the kidney.
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