Metabolic Cataract

Metabolic cataracts arise from systemic metabolic or endocrine disease that alters lens osmotic balance, protein structure or lens energy metabolism.

• Diabetes mellitus: the commonest. High glucose is converted to sorbitol by aldose reductase, causing osmotic swelling of lens fibres, and advanced glycation damages lens proteins. Cataracts occur 2 to 5 times more often and at younger ages, and surgery is needed earlier.
• Galactosaemia: galactose is converted to galactitol (galactose-1-phosphate uridyltransferase deficiency, or galactokinase deficiency which presents with isolated cataract); oil droplet cataract in the first weeks.
• Hypocalcaemia (hypoparathyroidism, pseudohypoparathyroidism): punctate cortical flecks and posterior subcapsular opacities.
• Wilson disease: sunflower cataract (copper deposition) with Kayser-Fleischer rings.
• Fabry disease: posterior spoke like (wedge shaped) cataract and whorl like corneal opacity.
• Myotonic dystrophy: iridescent (Christmas tree) posterior cortical crystals followed by posterior subcapsular cataract.
• Others: Lowe syndrome, mannosidosis, cerebrotendinous xanthomatosis (juvenile cataract with tendon xanthomas), hypothyroidism, Down syndrome, Refsum disease, galactokinase deficiency.

• Diabetic: snowflake cataract (acute, bilateral, dense cortical white opacities in young patients with very high glucose, which can reverse when glucose is lowered) or accelerated senile cataract (posterior subcapsular and cortical); myopic shift with refractive fluctuation with glucose changes (hyperglycaemia gives myopia, rapid lowering gives hypermetropia).
• Galactosaemia: oil droplet central lens opacity in neonates, with vomiting, jaundice, hepatomegaly, failure to thrive, sepsis (Escherichia coli).
• Hypocalcaemia: carpopedal spasm, tetany, seizures, paraesthesia, positive Chvostek and Trousseau signs.
• Wilson: hepatic, neuropsychiatric (tremor, dysarthria), Kayser-Fleischer ring.
• Myotonic dystrophy: grip myotonia, frontal balding, ptosis, cardiac conduction defects, cataract in the third and fourth decades.

• Slit lamp lens morphology, dilated pupil, and screening for the specific systemic disease.
• Diabetes: random and fasting glucose, HbA1c (6.5 percent or more diagnostic), screening for retinopathy.
• Galactosaemia: urine non-glucose reducing substances (Clinitest positive, glucose oxidase negative), red cell galactose-1-phosphate uridyltransferase activity, galactokinase activity, and genotyping.
• Hypocalcaemia: serum corrected calcium, phosphate, magnesium, parathyroid hormone, 25-hydroxyvitamin D, ECG for QT prolongation.
• Wilson: serum caeruloplasmin (low), 24 hour urinary copper (over 100 micrograms), liver copper, ATP7B genetic testing.
• Fabry: alpha galactosidase A activity, GLA gene analysis. Myotonic dystrophy: DMPK CAG repeat analysis, electromyography.

• Treat the metabolic disorder:
  • Diabetes: optimise glycaemic control (HbA1c target about 7 percent), gradual lowering of glucose to avoid refractive and retinopathy worsening; metformin, insulin as required. Snowflake lens opacities may clear with control.
  • Galactosaemia: immediate galactose and lactose elimination (soy or elemental formula); early restriction can reverse early lens changes.
  • Hypocalcaemia: acute symptoms need calcium gluconate 10 percent 10 to 20 mL intravenously over 10 minutes with ECG monitoring; maintenance oral calcium carbonate 1 to 2 g elemental calcium daily and calcitriol 0.25 to 1 microgram daily.
  • Wilson: D-penicillamine 750 to 1500 mg daily in divided doses with pyridoxine 25 mg daily, or trientine 750 to 1500 mg daily, followed by zinc acetate 150 mg elemental zinc daily for maintenance.
  • Fabry: enzyme replacement therapy (agalsidase beta 1 mg/kg every 2 weeks) or migalastat 123 mg every other day for amenable variants.
• Cataract surgery when visual function is affected: phacoemulsification with a hydrophobic acrylic lens. Diabetic patients need preoperative HbA1c under 8 percent where feasible, dilated retinal examination with treatment of proliferative disease or macular oedema before or at surgery, topical NSAID (nepafenac 0.1 percent three times daily or 0.3 percent daily, started 1 to 3 days before and continued 4 to 12 weeks), and close follow up for cystoid macular oedema and posterior capsule opacification.
• Pupils dilate poorly in diabetes, so use a pupil expansion device when needed.
• Paediatric galactosaemic and metabolic cataracts: surgery when visually significant, with amblyopia treatment.