Heat Cramps

Heat cramps are painful, involuntary muscle contractions that occur during or after prolonged exertion in a hot environment, usually in the muscles that are working hardest (calves, thighs, abdominal wall, shoulders), in a person who sweats heavily and replaces losses with water or low sodium fluids. They are the mildest heat related illness, with a normal core temperature and normal mental state.

• Mechanism: loss of sodium, chloride and water through sweat, together with muscle fatigue and altered neuromuscular control, causes spasm; replacing fluid losses with plain water dilutes sodium further.
• Occur in athletes, soldiers, outdoor and mine workers, farm labourers, and people exercising in heat; also in unacclimatised visitors, children, older adults, and people on diuretics.
• Risk factors: lack of acclimatisation, high humidity, previous cramps, poor fitness, dehydration, salty sweat, low dietary salt, diuretics, and large drops in electrolytes.
• If untreated or if exposure continues, it may progress to heat exhaustion and heat stroke.

• Sudden, severe muscle pain and spasm lasting 1 to 3 minutes, recurring, in the calf, quadriceps, hamstrings, abdomen or arms; the muscle feels hard and knotted; between spasms the muscle is sore.
• Heavy sweating, thirst, fatigue; temperature normal or slightly raised (under 38 degrees Celsius); alert and oriented; pulse may be fast.
• Distinguish from heat exhaustion and heat stroke: heat exhaustion has weakness, dizziness, headache, nausea, vomiting, pallor, sweating, rapid pulse, fall in blood pressure and core temperature of 38 to 40 degrees Celsius; heat stroke has core temperature over 40 degrees Celsius, confusion, collapse or seizures, and hot skin.
• Red flags: confusion, vomiting that prevents oral fluids, temperature of 38 degrees Celsius or higher, dark urine and severe muscle pain (rhabdomyolysis), chest pain, seizures, and cramps that do not respond to rest and fluids.

• Clinical, from heat exposure, exertion and typical muscle spasm with otherwise normal findings.
• Measure core (rectal or oral) temperature, pulse, blood pressure, mental state, hydration, and capillary glucose.
• Blood tests if not rapidly improving, or if severe: urea, creatinine and electrolytes (sodium, potassium, magnesium, calcium), creatine kinase, glucose, and urinalysis (dark urine with blood and no red cells suggests myoglobin).
• Exercise associated hyponatraemia (sodium under 135 mmol/L) occurs with large intakes of plain water; ensure it is excluded in cramping athletes who drank heavily.
• Differentials: heat exhaustion, heat stroke, exercise associated hyponatraemia, rhabdomyolysis, hypokalaemia, hypomagnesaemia, hypocalcaemia, medicine induced cramps (diuretics, statins, beta agonists), peripheral arterial disease, and compartment syndrome.

Immediate

• Stop the activity, move to a cool, shaded or air conditioned area, remove excess clothing, and rest.
• Gently stretch and massage the cramping muscle (for a calf cramp, straighten the knee and pull the toes toward the shin); hold the stretch for 20 to 30 seconds; apply warmth to a knotted muscle or ice for soreness.

Fluid and salt replacement

• Oral rehydration: sports drink or oral rehydration solution (sodium 45 to 90 mmol/L) 500 mL, then 250 to 500 mL every 15 to 20 minutes until symptoms settle, usually 1 to 2 litres in total; or a salt solution made with one quarter teaspoon (about 1.5 g) of table salt in 250 mL of water; salty snacks.
• Avoid plain water in large amounts, alcohol, caffeinated or sugary fizzy drinks.
• Salt tablets are not advised because of gastric irritation and hypernatraemia risk.

If cramps continue or oral fluids are not tolerated

• Intravenous 0.9 percent sodium chloride 1 litre over 1 hour, repeated according to response and weight, with monitoring of electrolytes; check sodium before giving large volumes if hyponatraemia is possible, and use hypertonic saline (3 percent 100 mL bolus over 10 minutes, up to 3 times) only for hyponatraemic encephalopathy.
• Correct potassium and magnesium deficits according to results.
• For severe, persistent spasm: diazepam 2 to 5 mg orally or intravenously once.

Observation and return to activity

• Observe until cramps resolve, vital signs are normal, and the patient is passing normal coloured urine.
• Avoid strenuous activity and heat for at least 24 to 48 hours; return gradually.
• Hospital assessment for core temperature of 38 degrees Celsius or higher, altered mental state, heat exhaustion features, dark urine, creatine kinase over 5 times normal, abnormal electrolytes, or older, frail and cardiac patients.

Prevention

• Acclimatise over 7 to 14 days with progressive increases in duration and intensity in the heat; schedule hard activity for cooler hours.
• Hydrate before (500 mL 2 hours before), during (150 to 250 mL every 15 to 20 minutes, matching sweat loss without overdrinking, with sodium 300 to 600 mg/L in the drink for long sessions) and after exercise.
• Include salt in meals on hot days; consider electrolyte drinks; wear light, loose clothing; maintain fitness and stretching.
• Review diuretics and other medicines before heat exposure.

Special groups

• Heart failure, hypertension, or kidney disease: do not give extra salt or large fluid volumes without medical advice.
• Older adults and children: lower threshold for medical assessment; children should have water and electrolyte fluid and rest in shade.