Diabetes mellitus is one of the most common systemic diseases in the world and it occurs when the pancreas does not produce enough insulin or when the body cannot effectively use that insulin. Hyperglycaemia (elevation of blood glucose concentration) is a common effect of uncontrolled diabetes and over time this leads to damage to, and dysfunction and failure of many of the body’s organs including the eye. It is classified into two main types: type 1 (previously known as insulin dependent) diabetes and type 2 (formerly known as noninsulin dependent) diabetes. The prevalence of diabetes is increasing and, in 2019, an estimated 9.3% of adults aged 20 years and older had diabetes ( ). Type 1 diabetes accounts for approximately 8% of primary diabetes, whereas type 2 diabetes accounts for 90%, with rarer types accounting for the rest.
Although the retinal complications of diabetes are well documented, what is perhaps less well appreciated is that poorly controlled diabetes can adversely affect all ocular tissues. In order to understand the possible effects of diabetes on the eye during contact lens wear, it is necessary to consider how the anterior eye of diabetic patients may differ from that of nondiabetic patients.
The Anterior Eye
Retinopathy is the ocular complication that most commonly causes loss of vision in diabetic patients ( Fig. 32.1 ); however, various abnormalities affecting the anterior eye have also been described (see ; ) for review.
Patients with poorly controlled diabetes are more susceptible to bacterial and fungal infections of the orbit than are nondiabetic individuals. Mucormycosis is a rare but potentially fatal fungal infection and those affected may present with ophthalmoplegia, loss of vision, pain and proptosis.
Xanthelasma is a condition in which yellow deposits appear on the eyelid and is thought to be due to abnormalities in lipid metabolism. Xanthelasmata can occur at an earlier age and more frequently in patients with diabetes than in nondiabetic patients ( ) ( Fig. 32.2 ).
Ptosis can be an early sign of undiagnosed diabetes, the underlying cause being chronic tissue hypoxia. Blepharitis, styes and meibomian cysts are also more commonly found in the diabetic population. Recent research supports the hypothesis that insulin resistance/deficiency and hyperglycaemia are key factors in the pathogenesis of meibomian gland dysfunction in diabetic patients ( ).
Studies suggest that diabetic patients complain of dry eyes more frequently than nondiabetic individuals do ( ). Reduced tear secretion is often found in association with diabetes and is perhaps related to the impairment of corneal sensitivity and dysfunction of the autonomic nervous system serving the lacrimal gland ( ). reported that tear break-up time is significantly lower in diabetic patients who have decreased corneal sensitivity. Increased levels of secretory immunoglobulin A, lysozyme and glucose ( ) have been observed in the tear film of patients with diabetes. It has been suggested that increased tear glucose concentration could promote lens spoilation and the growth of microorganisms in diabetic contact lens wearers. The influence of high tear glucose levels may contribute to the altered biomechanical properties of the cornea observed in diabetic patients (see for review).
Microaneurysms, capillary proliferation and vessel dilations have been noted in the conjunctivae of diabetic patients. These features are not unique to diabetes; they can occur in association with other systemic diseases such as hypertension and arteriosclerosis ( ). Conjunctival oxygen tension may be reduced in patients with advanced diabetic eye disease compared with those with milder retinal changes ( ). showed that diabetic patients have an increased risk of developing conjunctivitis and noted a reduction in goblet cell density and conjunctival squamous metaplasia in diabetic eyes associated with peripheral neuropathy, poor metabolic control and decreased corneal sensitivity.
The ultrastructural and biochemical status of the cornea is altered in patients with diabetes ( ) and some of the changes that have been described are summarized in Table 32.1 .
Below is an account of alterations to the structure and function of the cornea that are of more direct clinical relevance to contact lens wear.
The corneal epithelium is known to be particularly susceptible to mechanical trauma in diabetic patients ( ). Recurrent corneal epithelial defects, superficial punctate keratitis and prolonged epithelial healing rates have also been reported, particularly after ophthalmic surgery. There is a need to better understand the impact of diabetes on limbal stem cells and the delayed corneal re-epithelialization that is often reported in diabetes ( ). Inadequate adhesion of cells to an abnormal underlying basement membrane, reduced corneal oxygen consumption and reduced corneal sensitivity have been implicated in many of the corneal epithelial complications observed.
Diabetes can cause a significant reduction in corneal sensitivity ( ) and this sensory deficit is thought to occur owing to a diffuse polyneuropathy ( ). The extent of the reduction appears to relate to the duration of the diabetes, the age of the patient and the degree of diabetic metabolic control. Corneal nerves play an important role in the maintenance of a healthy cornea by providing a trophic influence and warning of corneal insult. Reduced corneal sensitivity has been linked with the development of corneal ulcers in diabetic patients ( ), and it is generally accepted that contact lens wear should be approached with extreme caution in any patient with corneal hypoaesthesia. In view of the increased risk of keratitis in diabetic patients compared to controls ( ), diabetic patients should be advised to remove lenses and to seek advice if they experience any discomfort.
Corneal confocal microscopy has been used to evaluate the alterations that occur in patients with diabetes ( ). Researchers have reported structural changes to corneal nerves ( ), and others have related these changes to the reductions observed in corneal sensitivity ( ). The alterations in nerve structure appear to be more pronounced in patients with diabetic polyneuropathy ( ) and patients with diabetic retinopathy ( ). showed that corneal confocal microscopy can quantify small-fibre damage rapidly and noninvasively and can detect earlier stages of nerve damage compared with more established techniques, suggesting this potentially makes it ideal to diagnose and assess the progression of diabetic neuropathy.
Studies of corneal endothelial morphology in diabetic humans and animals have revealed increased levels of polymegethism and pleomorphism (see for review) ( Fig. 32.3 ).
Most researchers have failed to demonstrate significant reductions in endothelial cell density among diabetic patients ( ), which contrasts with the findings of .
The appearance of folds in Descemet’s membrane has been reported in diabetic patients ( ) ( Fig. 32.4 ). When associated with diabetes, vertical folds often appear in the central corneal endothelium of both eyes. Folds due to diabetes appear to be enduring, unlike folds due to contact lens-induced corneal oedema, which disappear after lens removal ( ).
Corneal Hydration Control
Corneal thickness is often increased in diabetic patients ( ). Abnormal function of the corneal endothelium has been postulated as the cause, perhaps as a direct result of the accumulation of glucose and sorbitol ( ).
Increased corneal epithelial and endothelial permeability to fluorescein has been reported in patients with diabetes compared with nondiabetic control individuals. It has been suggested that the limiting layers of the diabetic cornea function normally during everyday conditions, but may have a reduced ability to cope with the stress induced by ophthalmic surgery or long-term contact lens wear ( ). In a study analysing the effects of soft contact lens wear in diabetic and nondiabetic control individuals, concluded that corneal thickness and endothelial cell density are more affected by diabetes mellitus whereas corneal endothelial cell morphology is more affected by contact lens usage.
Investigators have used pachymetry to monitor the recovery response to oedema induced by contact lenses of low oxygen transmissibility. This approach has demonstrated reduced absolute corneal swelling ( ) and prolonged recovery times from oedema in diabetic patients compared with normal controls. Since the rate of recovery from corneal oedema is reduced in diabetic patients, it has been suggested that care should be taken when prescribing contact lenses for such patients. However, it could be argued that these studies do not identify any obvious problems arising in diabetic patients wearing contact lenses with high oxygen transmissibility on a daily-wear basis.
found that diabetic patients using contact lenses have an increased risk of developing microbial keratitis, although the magnitude of the increased risk was not stated. evaluated the clinical signs, symptoms and the ocular and systemic comorbidities in a large series of contact lens-related microbial keratitis. Their study showed that microbial keratitis was more common in those with poor general health. All contact lens wearers should be advised to seek immediate advice if they develop a painful red eye or other symptoms suggestive of corneal ulceration.
Iris changes associated with diabetes include the deposition of glycogen vacuoles in the iris pigment epithelium, pigment dispersion, iris atrophy and iris neovascularization.
Pupil size and pupil reactions may be altered in diabetic patients with poor metabolic control. The pupil may be more difficult to dilate in diabetic patients with neuropathy.
It has been suggested that diabetes-induced changes occurring in the aqueous humour, cornea and crystalline lens could play a role in refractive fluctuations although there is uncertainty about the exact mechanism of such changes. Changes in refractive error in uncontrolled or undiagnosed diabetic patients have been reported. showed that short-term fluctuation in blood glucose levels did not induce short-term changes in refractive error, ocular aberrations or the anterior ocular biometric parameters, although another study showed that diabetic patients demonstrated smaller anterior chamber depths, more curved lenses, greater lens thickness and lower lens equivalent refractive index ( ). A reduced amplitude of accommodation has also been shown in individuals with diabetes when compared with age-matched controls, suggesting that individuals with diabetes will experience presbyopia earlier in life than people without diabetes ( ). Cataracts are a well-known cause of visual impairment in people with diabetes.
Corneal complications such as reduced corneal sensitivity, corneal erosions and keratitis have been reported following panretinal photocoagulation (PRP). (This procedure has also been associated with other complications such as visual field loss, persistent mydriasis, iritis, iris atrophy, lens opacities, shallowing of the anterior chamber and accommodative palsy.) Corneal endothelial cell loss after PRP has also been documented.
Ocular Response to Contact Lenses
Given the alterations to the anterior segment that accompany diabetes, an important issue to be addressed is whether or not contact lenses should be prescribed for diabetic patients for correction of refractive error or for cosmetic reasons. Reports of ocular complications in diabetic patients who wear contact lenses do exist; however, a number of these concern patients with advanced diabetic eye disease using extended-wear contact lenses ( ).
The results of a prospective, controlled study have demonstrated that daily-wear soft contact lenses can be a viable mode of vision correction for patients with diabetes. and suggest that practitioners should not expect to see adverse clinical signs in diabetic contact lens wearers that are any different from those seen in nondiabetic lens wearers. If adverse signs are detected in a diabetic lens wearer, they should not be attributed solely to the fact the patient has diabetes and the predisposition of the diabetic patient to corneal infection should always be borne in mind.
Roughening of the fingertips caused by home blood glucose monitoring could lead to damage to the lens surface during cleaning or handling, and patients should therefore be reminded to inspect contact lenses for damage prior to lens application. Fingernails should be kept short and smooth to reduce the risk of corneal erosion ( ).
Prescribing Contact Lenses
Although there are established trends in contact lens prescribing for the general population, clinical opinion is divided as to which type of contact lens is most appropriate for diabetic patients. In view of the fragility of the corneal epithelium, rigid corneal lenses were previously not recommended for diabetic patients since corneal abrasion occurs more frequently with these lenses. However, rigid corneal lenses do have certain advantages over soft lenses. The likelihood of toxins and potential pathogens becoming trapped beneath the lens is reduced with a rigid lens compared with a soft lens, and rigid lenses are also more durable and less prone to tearing or splitting.
The results of a survey conducted in the UK indicated that practitioners believed that rigid lenses are the safest form of contact lens for diabetic patients ( ). However, this survey was conducted prior to the availability of silicone hydrogel contact lenses. The reason for rigid lenses being favoured for diabetic patients was a perceived reduced risk of infection with these lenses compared with soft lenses. Practitioners stated that important factors to be considered when fitting contact lenses to diabetic patients include the degree of metabolic control and the contact lens wear time.
investigated the notion that diabetic contact lens wearers may represent a special group displaying higher levels of compliance with their lens care regimen as a result of learned behaviour relating to the maintenance of their diabetic condition. To test this hypothesis, a prospective, single-centre, controlled, masked study was performed whereby 29 diabetic contact lens patients and 29 nondiabetic control individuals were issued with disposable contact lenses and a multipurpose lens care regimen. All participants were given identical instruction on lens care and maintenance. Compliance levels were assessed at a 12-month aftercare appointment by demonstration and questionnaire. Twenty-four different aspects of compliance were scored, 12 by observation and 12 by questionnaire report, of which only 2 showed a significant difference between the diabetic and control groups. Although the combined population of contact lens wearers was generally compliant, there were examples of noncompliance in both groups. Neither the duration of diabetes nor the degree of metabolic control appeared to have a significant effect on compliance. The results suggest that eye care practitioners cannot assume that diabetic patients will be more compliant with contact lens care and maintenance than nondiabetic patients.
Glucose Sensing in the Anterior Eye
Since tear glucose concentration has been shown to mirror blood glucose concentration, it has been proposed that the measurement of tear glucose concentration could be used to monitor metabolic control noninvasively in diabetic patients (see for review). As contact lenses are bathed in tears, it has been suggested that measurement of tear glucose concentration could be carried out using contact lenses with glucose-sensing properties. However physiological factors mitigate against the use of tear glucose measurement as a viable surrogate for blood glucose measurement. First, the concentration of glucose in the tears is much less than that in the blood, which in broad terms means that a tear glucose sensor must be much more sensitive than a blood glucose monitor. Second, changes in tear glucose concentration lag behind changes in blood glucose concentration, and delay times between detection and read-out of 20 minutes or so have been recorded with some devices ( ). This is problematic in relation to acute glycaemic control where the almost instantaneous determination of blood glucose levels may be required to implement timely strategies to avoid hypoglycaemia.
Notwithstanding the limitations outlined above, there have been many attempts to develop contact lens glucose monitoring devices over the past decades. described the concept of using a scleral lens to determine aqueous humour glucose concentration as a surrogate measure of blood glucose concentration. They conceived of a scleral lens that houses a light source, polarizers, electro-optic units and a light detector which measures the optical rotation of the aqueous humour ( ). used glucose-sensitive compounds in a hydrogel lens. An array of 488 nm light-emitting diodes in a hand-held device was used to illuminate the contact lens and the magnitude of the resultant fluorescent light was measured by a detector in the device. A correlation between the contact lens fluorescence and blood glucose concentration was demonstrated, albeit with the expected time lag.
investigated the possibility of using an intraocular lens impregnated with fluorescent compounds to monitor blood glucose concentration. used a holographic sensor embedded in a daily disposable hydrogel lens. When glucose is bound to components in the device, the colour of the light reflected off the hologram changes when illuminated by a laser. A trial of one patient showed a correlation between blood glucose concentration and the holographic signal. developed and tested an electronic biosensor embedded in a rigid contact lens on a rabbit eye. The biosensor showed a good correlation between the output current and glucose concentration. This concept was limited by a need for the lens to be wired to provide input voltage and output reading. developed graphene-based sensing for the simultaneous detection and monitoring of glucose and intraocular pressure. The biosensor comprised graphene and silver nanowires incorporated into contact lenses, enabling the measurement of molecular binding and structural changes for glucose and intraocular pressure monitoring respectively.
Although many of these examples demonstrate proof of concept, none of the devices has been commercialized, although the possibility of a contact lens sensor that is powered externally and permits wireless read-out using an auxiliary device-generated considerable interest from Google, Novartis and Microsoft ( ). Google later reported that there was insufficient consistency in their measurements of the correlation between tear glucose concentration and blood glucose concentration to meet the requirements.
Ultimately more research is needed to bring such devices from theory to practice and certainly, clinical acceptance will rely on data evidencing that the considerable challenges outlined have been overcome.
Contact Lens Wear in Patients with Other Systemic Disease
Patients with any systemic disease known to adversely affect the anterior eye (see for review) potentially face problems during contact lens wear. Conditions such as thyroid deficiency, hyperthyroidism, rheumatoid arthritis, atopic eczema, psoriasis and acne rosacea may affect the suitability for contact lenses. In general, these conditions, when managed, do not contraindicate contact lens wear but may influence the lens type or the wear schedule selected. Patients with allergies can achieve success with contact lenses but may require more-frequent lens replacement or more-frequent aftercare examinations, as these patients are more susceptible to lens-induced discomfort and lid problems. have suggested that daily disposable contact lenses can offer a barrier to airborne antigens. Furthermore, researchers have shown that contact lens delivery of antiallergy medications provides results comparable to direct topical drug delivery ( ).
There is no contraindication to fitting contact lenses to patients who are human immunodeficiency virus positive, provided that the anterior eye is healthy. Practitioners should wear protective gloves if they have open skin lesions. If the patient has progressed to acquired immunodeficiency syndrome, the increased risk of opportunistic infection should be considered and contact lens fitting approached with extreme caution.
Corneal Scarring or Thinning
Where a patient suffers from any condition resulting in an irregular corneal surface, contact lenses may prove to be the only practical method of vision correction. In such cases, rigid lenses are usually the first choice. Careful slit-lamp examination together with a detailed history will alert the practitioner to those patients susceptible to corneal perforation.
Patients with systemic conditions resulting in the dry eye may require ocular lubricants, tear supplements, dietary and environmental modifications and ultimately punctal occlusion. As the successful wearing of any form of contact lens requires a well-lubricated surface, contact lens materials offering good wettability should be selected.
Restrictions of mobility, as in rheumatoid arthritis or ‘diabetic hand syndrome’, may make the handling of contact lenses difficult ( Fig. 32.5 ).