Very high blood sugar (HHS): a case-based approach

The presentation

Imagine an older adult with type 2 diabetes who has had a chest infection for several days. They have been drinking little, feeling more and more tired, passing large amounts of urine, and are now drowsy and confused with a very dry mouth and skin. A finger-prick test shows a very high blood sugar. This slow slide over days — rising thirst, frequent urination, increasing sleepiness and dehydration — is the classic picture of HHS. It differs from the faster, hours-long illness of diabetic ketoacidosis and does not usually cause the deep, sighing breathing or fruity breath of ketoacidosis. Recognising this gradual pattern in someone with diabetes who is becoming dehydrated and drowsy is the key to acting in time.

Why it happens

In HHS, blood sugar climbs to extremely high levels, often triggered by another illness such as an infection, a heart problem, or missed diabetes medicines. As sugar spills into the urine, it drags large amounts of water with it, so the person passes lots of urine and becomes severely dehydrated. The blood becomes thick and concentrated. Unlike diabetic ketoacidosis, there is usually still enough insulin activity to prevent large amounts of ketones and acid building up, which is why HHS tends to develop more slowly and without the same fruity breath. But the extreme dehydration and thickened blood are dangerous in their own right, raising the risk of clots, kidney injury and a fall in consciousness. It most often affects older people whose thirst signals may be blunted.

The warning signs

The signs of HHS build up over days rather than hours. Watch for extreme thirst and a very dry mouth, passing large amounts of urine at first, then passing less as dehydration worsens, and a general sense of weakness and tiredness. As it progresses, the person becomes drowsy, confused or hard to rouse, and the skin and tongue look and feel very dry. There may be signs of the illness that triggered it, such as a cough, fever or burning on passing urine. Sometimes there is blurred vision, leg cramps or, in severe cases, seizures. In anyone with diabetes who is becoming increasingly sleepy, confused and dehydrated during another illness, HHS should be suspected and blood sugar checked without delay.

Sick-day awareness

Many episodes of HHS can be prevented with good sick-day awareness. When someone with diabetes becomes unwell, blood sugar often rises even if they are eating less, so it is important to keep taking diabetes and other medicines unless advised otherwise, to check blood sugar more often, and above all to keep drinking plenty of sugar-free fluids to stay hydrated. Never stop insulin without medical advice. A written sick-day plan from the diabetes team, and knowing the personal thresholds at which to seek help, make a real difference. Carers of older relatives with diabetes should be alert during infections and offer regular drinks. Prompt treatment of the triggering illness and staying hydrated are the best defences against a slide into HHS.

The safe pathway

The practical rule is straightforward. Mildly raised blood sugar during a short illness, in someone who is alert, drinking well and following their sick-day plan, can often be managed at home with more frequent monitoring and advice from the diabetes team or NHS 111. But persistently very high blood sugar, especially with heavy thirst, marked dehydration, or any drowsiness, confusion or reduced consciousness, is a medical emergency. Call 999 or go to A&E straight away if someone with diabetes becomes drowsy, confused, very dehydrated or hard to wake, or has very high blood sugar that will not come down. HHS is treated in hospital with careful fluids and insulin, and outcomes are far better when help is sought early.