Exertional Rhabdomyolysis after a First Indoor Cycling Class: A Case Report

Introduction

Rhabdomyolysis is a clinical syndrome caused by the breakdown of skeletal muscle and the release of intracellular contents, including creatine kinase (CK), myoglobin and potassium, into the circulation (1,2). Its causes include trauma, drugs, infections and inherited myopathies, but strenuous exercise remains one of the most common triggers in otherwise healthy young adults (3).

Exertional rhabdomyolysis typically follows eccentric or repetitive exercise that the person is not trained for. Indoor cycling ("spin") classes combine sustained high-intensity effort, heat and peer pressure, and several case series have described rhabdomyolysis after a first class (7). Because a degree of post-exercise CK rise is normal, the challenge for clinicians is to recognise when muscle injury is severe enough to threaten the kidneys (8).

We report a case of exertional rhabdomyolysis after a first indoor cycling class, with the aim of highlighting its typical presentation, the time course of laboratory recovery and practical advice for prevention.

Case Presentation

A 26-year-old woman attended the emergency department with severe pain in both thighs that began the evening after her first indoor cycling class. The class lasted 45 minutes at high intensity, and she reported drinking little fluid before or during the session. She had no past medical history, took no regular medication or supplements, and had no family history of muscle disease.

The next morning she noticed swelling of both thighs, difficulty climbing stairs and dark, cola-coloured urine, which prompted her to seek care (Figure 2).

Examination

She was alert, with a heart rate of 96 beats/min, blood pressure of 124/78 mmHg and temperature of 37.1 °C. Both quadriceps were swollen and markedly tender, with pain on passive knee flexion. Distal pulses and sensation were normal, and there were no signs of compartment syndrome.

Investigations

Initial laboratory results were as follows:

• Creatine kinase: 48,500 U/L (reference < 170 U/L)
• Serum creatinine: 1.42 mg/dL (baseline 0.80 mg/dL from a test one year earlier)
• Potassium: 4.6 mmol/L; calcium and phosphate within normal limits
• Urine dipstick strongly positive for blood, with no red cells on microscopy, consistent with myoglobinuria

Electrocardiography showed sinus rhythm with no features of hyperkalaemia.

Management and Outcome

She was admitted and treated with intravenous isotonic saline, adjusted to maintain a urine output of 200–300 mL/h, with daily monitoring of CK, creatinine and electrolytes (4). CK peaked at 62,000 U/L on day 1 and then fell steadily, while creatinine returned to her baseline by day 5 (Figure 1). Intravenous fluids were tapered from day 3 once her urine had cleared.

She was discharged on day 5 with advice to drink plenty of fluids and to avoid strenuous exercise. At clinic review on day 10, her thigh pain had resolved and CK was 420 U/L. She was advised on a graded return to exercise, starting with low-intensity sessions.

Discussion

This case shows the typical pattern of exertional rhabdomyolysis: muscle pain out of proportion to the exercise, starting within 24 hours, followed by dark urine and a very high CK (2,5). The diagnosis is usually made when CK is more than five times the upper limit of normal in the right clinical setting, although there is no single threshold that predicts kidney injury (6,8).

Indoor cycling places a high, sustained load on the quadriceps, often in a warm room and at an intensity set by the class rather than by the individual. First-time participants may therefore exceed their tolerance without realising it (7). Dehydration, heat and lack of conditioning further increase the risk (5).

The main danger of rhabdomyolysis is acute kidney injury caused by myoglobin, which is toxic to the renal tubules, particularly when urine is concentrated and acidic (1). Early and generous fluid replacement is the cornerstone of treatment and, as in our patient, usually prevents the need for dialysis (3,4). Routine bicarbonate or mannitol has not been shown to improve outcomes and was not used (4).

Practical Points

• Suspect rhabdomyolysis in anyone with severe muscle pain or dark urine after unaccustomed exercise.
• Measure CK, creatinine and electrolytes, and test the urine for blood.
• Start intravenous fluids early and monitor urine output and kidney function until CK is falling.
• Advise new exercisers to start at a lower intensity and to stay well hydrated.

Recurrent episodes, rhabdomyolysis after only mild exertion, or a family history of muscle disease should prompt referral for investigation of an underlying metabolic myopathy (6).

Conclusion

Exertional rhabdomyolysis after a first indoor cycling class is preventable and, when recognised early, has an excellent outcome. Clinicians should ask about recent exercise in young adults with muscle pain or dark urine, and fitness instructors should encourage newcomers to build up their intensity gradually.