Cataract is characterised by an opacity in the lens or lens capsule, which can vary in size, shape and location within the lens. Classification is based on the time of onset, location, aetiology and maturity. Cataract is a common complication of diabetes in dogs; a study involving 200 dogs reported that 50 percent of the general diabetic population developed cataracts by 170 days post-diagnosis of diabetes mellitus. Furthermore, between 75 and 80 percent of diabetic dogs developed diabetic cataracts within 370 and 470 days of diagnosis, respectively (Beam et al., 1999).
Pathophysiology of diabetic cataract
Diabetic cataract develops due to increased blood glucose levels and, subsequently, increased glucose levels in the aqueous humour and lens. Normally, glucose is metabolised in the lens via an anaerobic pathway catalysed by the enzyme hexokinase. However, in the presence of large amounts of glucose, this pathway becomes overwhelmed, leading to the conversion of glucose into sorbitol via an alternative pathway involving the aldose reductase enzyme. This results in the accumulation of sorbitol inside the lens. Hygroscopic sorbitol creates osmotic forces that draw water from the aqueous humour into the lens, causing swelling and the rupture of lens fibres, visible as cataract formation. A complete cataract and vision loss can develop within days.
Diabetic cataract typically exhibits bilateral sudden onset and symmetry, often described as intumescent with characteristic “waterclefts” along the lens suture lines
Diabetic cataract typically exhibits bilateral sudden onset and symmetry, often described as intumescent with characteristic “waterclefts” along the lens suture lines (Figure 1). The anterior-posterior diameter is around 10mm, compared to approximately 7mm in a normal lens (Williams, 2004; Wilkie et al., 2006; Samuelson, 2013) (Figure 2).
Ongoing lens swelling leads to the stretching of the lens capsule which can rupture, resulting in the exposure of lens protein to the eye, causing lens-induced phacoclastic uveitis (Wilcock and Peiffer, 1987; Wilkie et al., 2006). Less commonly, lens protein can leak through an intact lens capsule and cause phacolytic uveitis (Wilcock and Peiffer, 1987). Intraocular inflammation is usually severe and causes irreversible damage to the ocular structures.
Clinical signs
Dogs with early diabetic cataract often present with reduced vision and opaque lenses, but their eyes are usually comfortable. As the condition progresses and lens-induced uveitis develops, clinical signs may include epiphora, blepharospasm, conjunctival hyperaemia, perilimbal and corneal neovascularisation, aqueous flare, fibrin, keratic endothelial precipitates, miosis or dyscoria due to the formation of posterior synechiae (Paulsen et al., 1986; Fischer and Meyer-Lindenberg, 2018; Michau, 2021) (Figure 3). Later, the iris becomes thickened and hyperpigmented, with the formation of pre-iridal fibrovascular membranes and ectropion iridis.
Pigment dispersion on the anterior lens capsule, hyphaema and phthisis of the globe can also occur (Figure 4). Vitreal degeneration (16 to 32 percent), hyalitis, lens subluxation/luxation, retinal degeneration or detachment (9 to 19 percent) and, subsequently, irreversible loss of vision can also develop (van der Woerdt et al., 1993; Adkins and Hendrix, 2005; Heldt, 2006) (Figure 5).
The intraocular pressure is initially reduced or can be normal (Leasure et al., 2001). However, secondary glaucoma, which is associated with pain and the irreversible loss of vision, develops with the progression and development of lens-induced uveitis (Gelatt and MacKay, 2004).
Diabetic patients appear to be predisposed to keratoconjunctivitis sicca (Cullen et al., 2005).
Diagnostic procedures
A thorough history should be obtained, followed by a comprehensive physical examination and a complete ophthalmic and neuro-ophthalmic examination to determine if the patient is a suitable candidate for cataract surgery.
Prior to surgery, additional diagnostic tests should be carried out. These include electroretinography to assess retinal function and ocular ultrasound to rule out spontaneous lens capsule rupture, retinal detachment and other intraocular abnormalities (Figures 6 and 7). Gonioscopy is used to evaluate the iridocorneal angle and pectinate ligaments, which helps the surgeon to decide on the best approach and prognosis for the patient (Figure 8).
How to deal with diabetic cataract in general practice
When should I refer?
Diabetic cataract can occur and progress even with good control of diabetes and can lead to permanent blindness if not addressed early (Salgado et al., 2000). If the cataract is not significantly impairing vision, close monitoring for progression and signs of uveitis is recommended. However, the surgical success rate decreases and the complication rate increases with maturity, so early referral is crucial (Barnett, 1971; Paulsen et al., 1986; Lim et al., 2011; Fischer and Meyer-Lindenberg, 2018). Lens rupture can occur spontaneously in a mature cataract and is particularly common in diabetic cataract (Wilkie et al., 2006). Therefore, due to this rapid progression and high risk of lens rupture, diabetic patients should be referred to an ophthalmologist as early as possible after cataract is detected (Wilkie et al., 2006).
Pre-referral tests and treatments
When referring a diabetic patient to a veterinary ophthalmologist, it is advisable to inform the receptionist that an early/urgent appointment is needed. If possible, you should also discuss the case with your local ophthalmologist, who can guide you on pre-referral ocular treatment and systemic diagnostic tests, which may vary from case to case.
Discuss the case with your local ophthalmologist, who can guide you on pre-referral ocular treatment and systemic diagnostic tests, which may vary from case to case
In general, a complete systemic health examination, including haematology, biochemistry, urine/blood ketones and/or fructosamine tests should be performed. Additional tests may be conducted when the animal is seen for an ocular examination and shortly before surgery (for instance, venous blood gases).
Prior to referral, diabetic cataracts should be treated with topical NSAIDs to control lens-induced uveitis twice to three times daily (or more often if uveitis is severe) (Wilcock and Peiffer, 1987). The choice of NSAIDs usually depends on the ophthalmologist’s preference and should not be started without consulting them, especially when there is any concurrent corneal disease, including keratoconjunctivitis sicca. Lacrimomimetics can also be considered if there is any tear film quality imbalance. If intraocular pressure is high or borderline high, topical antiglaucoma medication should also be initiated, and topical carbonic anhydrase inhibitors are a good option.
Any pre-existing ocular disease, such as keratoconjunctivitis sicca, should also be addressed early through discussion with an ophthalmologist.
What happens next?
Surgical removal is the gold-standard treatment for blinding cataracts (Ofri, 2007). However, patients with complete retinal detachment or significantly reduced retinal function are generally excluded from cataract surgery (Keil and Davidson, 2001). Surgery is not recommended if the eye is unlikely to regain vision.
If cataract removal surgery is not a possibility, dogs usually adapt well to blindness. Their eyes should be monitored throughout their lives for signs of uveitis, secondary glaucoma, retinal detachment and lens subluxation or luxation. General ocular health also needs to be monitored.
Surgery for diabetic cataract
Pre-surgical considerations
Appropriate patient selection is essential for a successful surgical outcome. Very bouncy, uncooperative or aggressive patients that are challenging or difficult to medicate and cannot be kept settled in the immediate post-operative phase may not be suitable candidates. If owners opt for surgery, a specific individualised plan must be created. Owners should be counselled on post-operative care, including transient exercise restriction, frequent topical eye drops, oral medication with potential side effects and regular lifelong re-examinations. Owners should receive concise information about the surgery, complications and expected outcome.
If surgery is delayed and lens-induced uveitis develops, the risk of complications is higher, and the prognosis for vision after cataract surgery is guarded to poor
The timing of diabetic cataract surgery is crucial (Wilkie et al., 2006). If surgery is performed early, the results are comparable to those for non-diabetic cataract patients (Bagley and Lavach, 1994; Bouhanna, 2003). However, if surgery is delayed and lens-induced uveitis develops, the risk of complications is higher, and the prognosis for vision after cataract surgery is guarded to poor (Lim et al., 2011). Delayed surgery and preoperative complications can make the procedure challenging or even impossible (Lim et al., 2011).
If the eye is painful and not responding to medical treatment, then it should be enucleated.
Phacoemulsification
Cataract surgery, or phacoemulsification, is performed under general anaesthesia and non-depolarising neuromuscular blockade with the aid of an operating microscope and specific microsurgical instruments.
Initially, a 2.8 to 3.2mm full-thickness corneal or corneo-limbal incision is made, followed by the creation of a continuous curvilinear rhexis of the anterior lens capsule (capsulorhexis) (Figure 9). Ophthalmic viscosurgical devices (OVDs), also known as ophthalmic viscoelastic devices, either dispersive or cohesive, are used in combination to protect the intraocular tissue from damage and create space for instruments, respectively. A second corneal incision for another instrument may also be created if indicated or deemed necessary. Some ophthalmologists routinely opt for two corneal incisions.
The phacoemulsification probe is then carefully inserted into the anterior chamber via corneal incision, stopping at the level of capsulorhexis. The lens material is then carefully emulsified by ultrasonic energy and aspirated from the lens capsule (Figure 10). Multiple techniques can be used; the type depends on surgeon preference, cataract status and complications.
Next, the lens cortex is removed via automated irrigation and aspiration. The lens capsule is left in place and gently filled by OVDs, and an artificial intraocular lens (IOL) with the power of 41 dioptres is then implanted into the lens capsule bag to achieve post-operative emmetropia (or close to it). The remaining cortex and OVDs are removed via automated irrigation and aspiration.
The corneal incision is then sutured watertightly using absorbable suture material ranging from 9-0 to 10-0 (such as polyglactin 910 or polyglycolic acid), employing a simple interrupted suture or other suitable suture pattern based on the surgeon’s preference (Figure 11).
The intraocular lens replaces the refractive properties of the lens and, therefore, helps with focused vision. When the posterior lens capsule is severely torn or completely ruptured, or when significant lens instability exists, placement of an IOL is not possible, and dogs remain aphakic and hyperopic (farsighted) with approximately 14 dioptres (Wilkie and Colitz, 2009). However, dogs with aphakia can adjust well and have fair to good vision post-operatively.
Post-operative care after cataract surgery
The main post-operative complications of diabetic cataract surgery include corneal ulcers, uveitis and glaucoma. Therefore, close monitoring of corneal health, intraocular pressure and intraocular inflammation is essential. Patients are typically hospitalised for 24 to 48 hours post-operatively. Diabetic dogs receive careful monitoring, with minimal disruption of their feeding regimen.
In general, all dogs receive frequent topical and systemic anti-inflammatory medication, which is gradually tapered over several months after the surgery. Systemic corticosteroids are contraindicated in diabetic dogs; therefore, oral NSAIDs are chosen unless contraindicated. Topical NSAIDs are initiated, and some ophthalmologists also add topical corticosteroids while corneal and systemic health is closely monitored. A recent study showed no differential effect on diabetic regulation in dogs treated with topical prednisolone acetate versus diclofenac sodium (Rankin et al., 2019). Lacrimomimetics are initiated to control corneal health, and prophylactic topical and systemic antibiotics are also administered (Michau, 2021). Topical antiglaucoma medication is started if the intraocular pressure does not settle down post-operatively. Insulin is continued.
In general, all dogs receive frequent topical and systemic anti-inflammatory medication, which is gradually tapered over several months after the surgery
Diabetic canine eyes are more likely to develop complicating post-operative uveitis; therefore, lifelong treatment with topical NSAIDs is required (Michau, 2021). Topical antiglaucoma medication and/or lacrimomimetics are also continued long-term if indicated.
The animal is re-examined shortly after surgery and every few weeks until the operated eye is stable. Future re-examinations are usually scheduled every three to four months, or even every six months, for the rest of the animal’s life, as long as the operated eye remains stable.
Complications associated with cataract surgery
Intraoperative complications include, for example, ocular haemorrhage, posterior lens capsule rupture, dislocation of lens material into the posterior segment, retinal detachment, damage to the corneal endothelium or corneal ulceration (Michau, 2021).
Short-term post-operative complications include uveitis, endophthalmitis, post-operative ocular hypertension, wound dehiscence, aqueous humour leakage, corneal ulceration, corneal oedema, hyphaema, retinal detachment and blindness (Michau, 2021).
The most common mid- and long-term complications include corneal disease, peripheral ocular neuropathies, ongoing intraocular inflammation, posterior capsule opacification, retinal detachment and glaucoma, which can lead to a painful eye and/or irreversible blindness (Michau, 2021). Subsequently, blind, uncontrolled and painful eyes may require enucleation.
Conclusion
The risk of developing diabetic cataract should be discussed early with owners of diabetic dogs. Sudden onset of diabetic cataract should prompt early referral to an ophthalmologist regardless of the current diabetic control of the patient. Lens-induced uveitis should be prevented and controlled with the goal of performing cataract surgery as soon as possible. If not addressed early, diabetic cataract can result in permanent blindness.
Overall, the success rate of cataract surgery is high, and most patients will experience significant improvement in their vision and quality of life (Wilkie et al., 2006; Michau, 2021). However, it is important to recognise that surgery and post-operative care can be demanding, and potential complications should be carefully considered.
