Two construction workers manage to dislodge a large boulder from the path of a new water pipe installation. As the boulder begins rolling, they suddenly become aware of a pickup truck parked below them at the bottom of the hill. The boulder smashes through the side window and lands on the driver's lap. The injured 44-year-old man has multiple contusions from the blunt trauma to his thighs and lower abdomen. Radiographs reveal no bony fractures. A paracentesis yields no blood. That evening in hospital, the injured man's urine output begins to drop.

Questions:

1. How do you explain the macroscopic findings in view of the microscopic findings? How does the history fit with this?

The urine is brownish and the test for blood is markedly positive, but there are no RBC's visualized. Perhaps this is hemoglobinuria or myoglobinuria (the dipstick test for blood is even more sensitive for myoglobin). The history suggests a crush injury to soft tissue, including muscle.

2. What further laboratory studies would be of help?

Laboratory tests for serum and urine myoglobin are available. More likely, the patient had serum chemistries ordered on admission, one of which should have been creatine kinase (CK) in view of the history.

3. Explain the appearance of the casts and epithelial cells.

The damage to the kidney from the excessive myoglobin excretion leads to acute tubular necrosis.

4. What are some other causes for this condition?

Muscle ischemia from laying in one position for a long time (comatose patient or alcoholic "found down"), hyperthermia, cocaine or phencyclidine intoxication.