Dry eye disease (DED) is thought to affect upwards of 30–40 million people within the US.1 Officially, the definition and classification of dry eye was updated in 2007 during the Tear Film and Ocular Surface (TFOS) Dry Eye WorkShop (DEWS).2
‘Dry eye is a multifactorial disease of the tears and ocular surface that results in symptoms of discomfort, visual disturbance and tear film instability with potential damage to the ocular surface. It is accompanied by increased osmolarity of the tear film and inflammation of the ocular surface.’
Whether initiated by ageing, androgen deficiency, contact lens wear, refractive surgery or an autoimmune disease,1,3 dysfunctions of the lacrimal and meibomian glands result in hyposecretion and increased evaporation of tear fluid, which promote instability of the tear film.2,4 This leads to significant fluctuations in vision, loss of lubrication, inflammation, an increase in wear (epitheliopathy) and varying levels of neuropathy and corneal sensitisation. Thus, while diagnosis of DED may be thought to be straightforward in more severe examples of disease, determining the severity of the disease – especially at early stages, requires a deeper understanding of the potential aetiologies and clinical presentation of the disease to reconcile both the statistical and subjective aspects of severity.
Biomarkers – surrogates or substitutes for variables of clinical disease – are often used in the assessment of disease states, particularly in clinical trials of therapeutics and diagnostic devices.5
The terms (bio)markers, surrogates, endpoints, outcomes and others ave been used to describe a metric that is either objective or subjective, which accurately reflects the characteristics of disease. In medicine, common examples of this are the use of serum cholesterol or blood pressure for cardiovascular disease,6 or in ophthalmology intraocular pressure for glaucoma.7 A surrogate outcome has been defined as a “laboratory measurement or a physical sign used as a substitute for a clinically meaningful endpoint that measures directly how a patient feels, functions or survives”.8 The value of such a metric lies in how accurately it can capture important aspects of the disease. Such measures are typically used clinically in diagnosis, assessment of disease severity and response to therapy.
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