Type 1 Diabetes Mellitus

Type 1 Diabetes Mellitus (T1DM) is an organ-specific autoimmune disease resulting from the T-cell mediated destruction of the insulin-secreting beta cells in the Islets of Langerhans. This leads to a state of absolute insulin deficiency, rendering the patient dependent on exogenous insulin for survival. In the UK, it accounts for approximately 10% of all diabetes cases. The incidence is rising, particularly in children under five. Diagnosis is based on clinical presentation and biochemical evidence of hyperglycaemia: a fasting plasma glucose ≥ 7.0 mmol/L or a random plasma glucose ≥ 11.1 mmol/L. While HbA1c is a standard monitoring tool, it should not be used for diagnosis in T1DM as the onset is often too rapid for the glycation to reflect the true level of hyperglycaemia. Patients often present with the '4 Ts': Toilet (polyuria), Thirsty (polydipsia), Tired (lethargy), and Thinner (unintentional weight loss). If left untreated, the metabolic shift toward fatty acid oxidation leads to the production of ketone bodies, resulting in life-threatening Diabetic Ketoacidosis (DKA). The long-term goals of management are to achieve glycaemic control (typically a target HbA1c of 48 mmol/mol or 6.5%) to minimise the risk of complications while avoiding severe hypoglycaemia. Modern management in the UK increasingly utilizes technology, including continuous glucose monitoring (CGM) and insulin pumps, as recommended by NICE (NG17).

The pathophysiology of T1DM involves a complex interplay between genetic susceptibility and environmental triggers. The strongest genetic association is with the HLA-DQ and HLA-DR loci on chromosome 6, specifically HLA-DR3-DQ2 and HLA-DR4-DQ8. However, 90% of children with T1DM have no first-degree relative with the condition. The process begins with an environmental trigger (e.g., enteroviruses like Coxsackie B, dietary factors, or toxins) occurring in a genetically predisposed individual. This initiates a T-cell mediated (Type IV hypersensitivity) autoimmune response. Activitated CD8+ T-lymphocytes, macrophages, and autoantibodies (anti-GAD65, anti-islet cell, anti-zinc transporter 8, and anti-insulin) infiltrate the islets, a process known as 'insulitis'. Clinical symptoms only appear when approximately 80-90% of the beta-cell mass has been destroyed. In the absence of insulin, GLUT4 transporters remain sequestered within cells, preventing glucose uptake into muscle and adipose tissue. This leads to extracellular hyperglycaemia. Simultaneously, the lack of insulin's inhibitory effect on lipolysis and proteolysis causes a catabolic state. The liver undergoes unrestrained gluconeogenesis and ketogenesis. Ketones (acetoacetate and beta-hydroxybutyrate) are acidic; their accumulation overwhelms the body's buffering systems, leading to metabolic acidosis.

The clinical presentation is typically acute or subacute. Symptoms include polyuria (due to osmotic diuresis when blood glucose exceeds the renal threshold of ~10 mmol/L), polydipsia, weight loss, and extreme lethargy. Red flags include abdominal pain, vomiting, and Kussmaul respiration (deep, sighing breaths), which indicate DKA. Long-term complications are categorised into microvascular and macrovascular. Microvascular complications include retinopathy (leading cause of blindness in working-age adults), nephropathy (characterised by albuminuria and declining GFR), and neuropathy (distal symmetrical 'glove and stocking' distribution, autonomic neuropathy, or mononeuritis). Macrovascular complications include accelerated atherosclerosis, leading to ischaemic heart disease, cerebrovascular disease, and peripheral arterial disease. Regular screening is essential: annual retinopathy screening from age 12, annual albumin:creatinine ratio (ACR), and annual foot checks to assess for 'at-risk' feet. Psychological impact is significant, and patients should be screened for 'diabetes distress' and eating disorders (diabulimia).

Bedside: Urine dipstick (glucose and ketones), Finger-prick capillary blood glucose, Capillary ketones (if unwell). Bloods: Venous plasma glucose (fasting or random), HbA1c (for baseline/monitoring, not diagnosis), Urea & Electrolytes (renal function), Thyroid Function Tests and Coeliac screen (autoimmune association). Special Tests: C-peptide (low in T1DM, high/normal in T2DM) and Diabetes-specific autoantibodies (GAD, IA-2, ZnT8) if the diagnosis is uncertain (e.g., in older adults or LADA). Imaging: Not routinely required unless investigating complications.

Acute: Management of DKA requires IV fluids, fixed-rate insulin infusion, and electrolyte replacement. Chronic: NICE recommends a multiple daily injection (MDI) basal-bolus regimen as first-line. Basal: Long-acting insulin analogue (e.g., Levemir twice daily or Tresiba/Lantus once daily). Bolus: Rapid-acting insulin analogue (e.g., NovoRapid, Humalog, Fiasp) before meals, adjusted based on carbohydrate counting. Technology: Real-time CGM or intermittently scanned CGM (Freestyle Libre) should be offered to all. Continuous Subcutaneous Insulin Infusion (CSII/Insulin pump) is considered if glycaemic targets aren't met or with frequent hypoglycaemia. Education: Structured education (e.g., DAFNE - Dose Adjustment For Normal Eating) is mandatory. Monitoring: HbA1c every 3-6 months. Complication management: ACE inhibitors for microalbuminuria; statins for cardiovascular risk (offered to all adults >40 or with 10+ years duration).