Recurrent nocturnal hypoglycaemia after basal insulin increase and exercise programme

Mrs. Smith, a 30-year-old woman with type 1 diabetes mellitus for 12 years, presents to the diabetes clinic with a three-week history of recurrent nocturnal hypoglycaemia. She reports waking at approximately 3 am with sweating, palpitations, and confusion. Capillary blood glucose measured on these occasions has been between 2.4 and 3.0 mmol/L. She treats these episodes with oral glucose.

Medication history: She has been on a stable insulin regimen for the past year, comprising insulin glargine (Lantus) 20 units subcutaneously once daily at bedtime, insulin lispro (Humalog) 6 units before breakfast, 8 units before lunch, and 6 units before dinner. She also takes levothyroxine 50 mcg once daily for hypothyroidism.

Two months ago, she started a jogging programme, jogging for 30 minutes in the evening, three times a week. One month later, her HbA1c was 8.2%, and her insulin glargine was increased to 24 units once daily. The nocturnal hypoglycaemic episodes began three days after the dose increase, occurring two to three times per week. Her clinician reduced the insulin glargine back to 20 units after one week. The hypoglycaemic episodes resolved within two days of the dose reduction and did not recur during the following two weeks. She later mistakenly took 24 units for two nights (thinking she was still on the higher dose) and the nocturnal hypoglycaemia recurred.

On examination during an episode, she was pale and sweaty with a heart rate of 110 bpm. Between episodes, physical examination was unremarkable. Recent HbA1c after the dose reduction was 7.8%. Thyroid function tests were normal. The patient did not change her exercise schedule during the dose increase or reduction.