The SENSIMED Triggerfish contact lens sensor for continuous 24-hour recording of ocular dimensional changes in people with or at risk of developing glaucoma

Introduction

Glaucoma is a disease in which the optic nerve becomes damaged, leading to progressive vision loss. Damage occurs because of raised pressure inside the eye (intraocular pressure, or IOP). Raised IOP (described as ocular hypertension, or OHT) is caused by a build-up of the fluid (aqueous humour) within the eye. This can happen when the drainage tubes within the eye (called the trabecular network) become blocked, so the aqueous humour cannot drain out of the eye properly. The most common form of glaucoma is chronic open-angle glaucoma (COAG), which develops slowly and can cause blindness (NHS Choices Glaucoma). Open-angle glaucoma is a chronic and progressive condition in which visual field loss is associated with morphological changes, which occur at the optic nerve head and retinal nerve fibre layer in the absence of other ocular disease (European Glaucoma Society, 2008).

COAG is usually asymptomatic until irreversible vision loss has occurred. The peripheral vision is lost first before the damage progresses inwards in the visual field. In England, approximately 480,000 people have COAG. Among those of white European family origin, around 2% over the age of 40 years and 10% of those aged over 75 years have COAG. People of African or African-Caribbean family origin have a higher risk of developing the disease. Other risk factors include short-sightedness, ocular hypertension and family history of glaucoma. People with diabetes may also be more at risk of developing glaucoma (NHS Choices Glaucoma).

OHT is defined as pressure in the eye consistently greater than 21 mm Hg, and it affects 5% of people aged over 40 years in the UK. This equates to around 1 million people (Burr et al. 2012).

People who have or who are at risk of developing COAG, including those with OHT, need regular monitoring to prevent their vision deteriorating any further. All COAG treatments aim to reduce IOP to a level low enough to limit disease onset or progression. Treatment can be in the form of medication (eye drops which either increase the flow of aqueous humour out of the eye or reduce its production), laser treatment to open up the trabecular meshwork (trabeculoplasty), or surgery (most commonly trabeculectomy to remove part of the meshwork, allowing the aqueous humour to drain properly). Some people's glaucoma continues to progress despite IOP-lowering treatment; indeed, 1 study suggests that glaucoma may progress in up to 45% of people, regardless of treatment (Heijl et al. 2002).