A. Overview of Bartter and Gitelman Syndromes

Classification: Autosomal recessive salt-wasting tubulopathies characterized by: - Hypokaliemia (low serum potassium) - Metabolic alkalosis - Hyperreninemia and hyperaldosteronemia (secondary to volume depletion) - NORMAL blood pressure (distinguishes from pseudohyperaldosteronism) - Normal renal function (GFR preserved early; may decline with age)

Historical Context: - Bartter syndrome first described 1962; Gitelman 1966 - Originally called “normotensive hyperreninemic hypokalemic alkalosis” - Genetic characterization began 1990s with identification of mutations in genes encoding ion transporters and signaling molecules

B. Bartter Syndrome Types I-V

Genetic Basis and Pathophysiology:

Pathophysiology (All Types): 1. Defect in salt reabsorption in thick ascending limb (TAL) of loop of Henle 2. → Volume depletion → activation of renin-angiotensin-aldosterone system (RAAS) 3. → Increased sodium reabsorption in distal tubule via epithelial sodium channel (ENaC) 4. → Increased potassium and hydrogen ion secretion in collecting duct 5. → Hypokalemia and metabolic alkalosis 6. → Renal vasoconstriction from angiotensin II (GFR relatively preserved)

Clinical Presentation:

Prenatal/Neonatal (Type I and II particularly): - Polyhydramnios (increased fetal urine output) - Preterm delivery (50% of cases) - Neonatal presentation: Lethargy, poor feeding, muscle weakness, seizures (severe hypokalemia)

Infancy/Early Childhood (Type III typically presents here): - Failure to thrive - Polyuria, polydipsia (impaired concentrating ability; acquired nephrogenic diabetes insipidus) - Recurrent episodes of hypokalemia with muscle weakness, cramps, tetany - Developmental delay if untreated - Craving for salt (salt-wasting state)

Childhood/Adolescence: - Growth failure - Musculoskeletal manifestations: Muscle weakness, fatigue, cramping - Developmental impairment if inadequately treated - Progressive renal insufficiency (Types I-III more likely; incidence increases with age) - Nephrolithiasis risk (alkalosis, hypercalciuria in Types I and II) - Hearing loss (Type IV particularly; associated with mutations affecting chloride channel expression in cochlea)

Type I-Specific Features: - Most severe in utero presentation (polyhydramnios, preterm birth common) - Neonatal onset with severe symptoms - Nephrogenic diabetes insipidus (difficulty concentrating urine) - Hypercalciuria and nephrolithiasis risk

Type II-Specific Features: - Neonatal presentation - ROMK channel mutations → severe hyperkalemia possible early (unlike other types) - Rapid progression to severe symptoms

Type III-Specific Features: - Later childhood onset (often age 2-6 years) - Milder symptoms early; progressive - Hearing loss not associated - Somewhat slower progression than Types I-II

Type IV-Specific Features: - CASR mutations → associated hypercalcemia - Hearing loss (sensorineural, progressive) - Polyuria from hypercalcemia - May present with seizures from hypokalemia

Type V-Specific Features: - Gain-of-function CASR mutation - Mimics primary hyperparathyroidism picture - Hypercalcemia, hypercalciuria - Suppressed PTH level (distinguishes from hyperparathyroidism)

Laboratory Findings (All Types):

Diagnostic Approach: 1. Confirm hypokalemic metabolic alkalosis with normal BP 2. Assess renin-aldosterone axis (elevated = consistent with diagnosis) 3. Genetic testing: NKCC2, ROMK, ClC-Kb, CASR gene sequencing 4. Exclude other causes: vomiting, diuretic abuse, hyperaldosteronism 5. Prenatal diagnosis possible with early genetic testing (very early onset polyhydramnios suspicious)

Management:

Potassium Replacement: - Dose: 1-4 mEq/kg/day (in divided doses; see target below) - Target serum K+: 3.5-4.5 mEq/L (higher goals—closer to 4.5—reduce hypokalemia-related symptoms) - Forms: Potassium chloride (preferred, replaces both K+ and Cl⁻), potassium acetate - Monitoring: Check serum K+ every 2-4 weeks initially; then every 1-3 months once stable

NSAIDs: - Mechanism: Inhibit prostaglandin synthesis → reduce renal renin release → reduce secondary aldosteronism → reduce K+ wasting - Drugs: Indomethacin (first-line), ibuprofen, naproxen - Dosing: Indomethacin 0.5-1.0 mg/kg/day (divided, max 75 mg/day) - Efficacy: Raises serum K+ by 0.5-1.5 mEq/L in majority of patients - Monitoring: Monitor renal function, GFR; risk of chronic kidney disease with long-term NSAID use - Relative contraindications: Renal insufficiency, GI bleeding, platelet dysfunction - Duration: Often lifelong therapy needed; some reduction possible with maturity/improved dietary compliance

ACE Inhibitors/Angiotensin Receptor Blockers: - Mechanism: Reduce angiotensin II generation/signaling → reduce aldosterone secretion - Less effective alone than NSAIDs but additive when combined - Use: Particularly if NSAID contraindicated or insufficient response - Caution: May cause hyperkalemia if renal function declines (monitor K+)

Salt Supplementation: - Rationale: Restore intravascular volume; reduce RAAS activation - Approach: Encourage dietary salt intake; may need oral salt supplementation - Risk: Must balance against hypertension risk if renal function declines with age

Dietary Management: - High sodium diet (3-4 g sodium/day) - Adequate fluid intake (address polyuria) - Nutritional support (growth failure prevention) - Calcium intake: Assess for nephrolithiasis risk; evaluate urinary calcium

Specific Management by Type: - Types I-II (severe): More aggressive K+ replacement, NSAIDs, may need ACE inhibitor added - Type III: Often lower initial severity; respond better to NSAIDs alone; slower progression - Type IV: Management of hypercalcemia (thiazide diuretics, adequate hydration)

Long-Term Complications: - Progressive renal insufficiency (30-40% reach ESRD by adulthood in Types I-II; less common in Type III) - Nephrolithiasis - Growth failure if inadequately treated - Hearing loss (Type IV) - Hypertension develops in some patients with aging (paradoxical; mechanism unclear)

Outcome: - Early diagnosis and aggressive replacement therapy improves growth and development - Neurodevelopmental outcome excellent if treated before age 2-3 years - Many patients achieve normal adult height with adequate therapy - Prognosis for renal function variable; progression to ESRD possible but not inevitable

C. Gitelman Syndrome

Genetics: - Gene: SLC12A3 (chromosome 16q13) - Protein: Thiazide-sensitive sodium-chloride cotransporter (NCCT) in distal convoluted tubule (DCT) - Inheritance: Autosomal recessive - Incidence: 1 in 40,000 live births (more common in some populations: Japan, Turkey)

Pathophysiology: 1. Defect in NCCT → impaired NaCl reabsorption in DCT 2. → Volume depletion → RAAS activation 3. → Increased K+ and H+ secretion in collecting duct (via aldosterone effect) 4. → Hypokalemia + metabolic alkalosis 5. Unique feature: Hypocalciuria (increased urinary magnesium, decreased urinary calcium)

Clinical Presentation:

Age of Onset: - Usually age 6-16 years (later than Bartter; varies widely) - Some present in infancy; others not until adulthood

Symptoms: - Muscle weakness, cramps, tetany (from hypokalemia) - Paresthesias - Polydipsia, polyuria (from hypokalemia-induced nephrogenic DI) - Growth retardation (if untreated) - Fatigue, decreased exercise tolerance - Periodic paralysis (rare; severe hypokalemia)

Clinical Features (Distinguishes from Bartter): - Hypomagnesemia (more severe and clinically significant than Bartter) - Hypocalciuria (low urinary calcium; distinguishes from Bartter types with hypercalciuria) - Normal to low serum calcium (hypokalemia-induced, mild) - Chondrocalcinosis (calcium pyrophosphate deposition in joints; occurs from hypomagnesemia) - Absence of hearing loss (unlike Bartter Type IV)

Laboratory Findings:

Diagnostic Approach: 1. Confirm hypokalemic metabolic alkalosis with normal BP 2. Check serum magnesium (expect marked hypomagnesemia in Gitelman) 3. Check urinary calcium (low in Gitelman; normal-high in Bartter) 4. Genetic testing: SLC12A3 gene sequencing 5. Exclude secondary causes (diuretic abuse, vomiting)

Management:

Potassium Replacement: - Similar to Bartter; however, repletion often difficult due to magnesium depletion limiting K+ correction - Goal: Serum K+ 3.5-4.5 mEq/L - Forms: Potassium chloride (combined with magnesium replacement often necessary)

Magnesium Replacement: - Critical difference from Bartter: Magnesium depletion severe and must be corrected for K+ repletion to be effective - Oral replacement: Magnesium oxide, magnesium gluconate - Dose: 0.3-0.6 g elemental Mg daily (divided doses; often 2-4 g MgO daily) - Target serum Mg²⁺: >1.7 mg/dL (ideally >2.0) - GI side effect: Diarrhea common (may limit dose); switch formulations if needed - Monitoring: Serum Mg²⁺ every 4-8 weeks initially; monthly once stable - Duration: Lifelong supplementation typically required

NSAIDs: - Similar rationale and dosing as Bartter - Caution: NSAIDs may cause GI symptoms; magnesium-containing NSAIDs (magnesium salicylate) may provide additional benefit - Less dramatically effective in Gitelman than Bartter; often used adjunctively

Potassium-Sparing Diuretics: - Amiloride: 0.3-0.6 mg/kg/day - Mechanism: Blocks ENaC in collecting duct; reduces K+ and H+ secretion - Efficacy: May be very effective in Gitelman (better than Bartter response) - Caution: Risk of hyperkalemia; must monitor serum K+ - Role: Alternative or additive to NSAIDs if response inadequate

Thiazide Diuretics (Paradoxically): - Hydrochlorothiazide at very low doses - Rationale: TSHould worsen hypokalemia but paradoxically, in Gitelman, may improve symptoms - Proposed mechanism: Increases urinary sodium losses → improved volume depletion compensation - Use: Reserved for severe refractory cases; not first-line

Dietary: - High sodium diet (similar to Bartter; 3-4 g daily) - Adequate magnesium intake (difficult to achieve orally; supplementation necessary) - Adequate fluid intake (address polyuria)

Long-Term Outcome: - Much better prognosis than Bartter (renal function typically preserved throughout life) - Most patients achieve normal adult height and normal development with adequate therapy - ESRD very rare in Gitelman (unlike Bartter with higher progression rate) - Complications: Chondrocalcinosis (calcium pyrophosphate deposition) common but often asymptomatic - Early diagnosis and treatment crucial for normal growth and development

A. Overview of RTA

Definition: Metabolic acidosis resulting from dysfunction of renal tubular acid secretion or bicarbonate reabsorption, WITHOUT significant reduction in GFR.

Types: - Type 1 (Distal) RTA: Impaired acid secretion in collecting duct - Type 2 (Proximal) RTA: Impaired HCO₃⁻ reabsorption in proximal tubule - Type 4 (Hyperkalemic) RTA: Aldosterone deficiency or resistance in collecting duct

Note: Type 3 RTA (rare mixed proximal + distal) rarely seen in clinical practice

B. Type 1 (Distal) RTA

Genetics and Etiology:

Primary (Hereditary): - Autosomal recessive: Gene SLC4A1 (band 3 anion exchanger; basolateral membrane) or ATP6V0B (vacuolar H+-ATPase) - Autosomal dominant: Gene SLC4A1 (missense mutations) - Incidence: Rare; 1 in 10,000 children (variable by ethnicity)

Secondary (More common): - Amphotericin B - NSAIDs (chronic use) - Systemic lupus erythematosus - Sjogren syndrome - Chronic pyelonephritis - Medullary sponge kidney - Kidney transplant rejection

Pathophysiology: - Distal collecting duct cannot secrete H+ ions adequately - Result: Inability to lower urine pH below 5.5 (normal: <4.5) - → Hyperchloremic metabolic acidosis (normal anion gap) - → Increased urinary potassium and sodium losses (to maintain electroneutrality; Cl⁻ is reabsorbed instead) - → Hypokalemia (often severe) - → Hypocitraturia (citrate required for acid buffering; wasted in urine) - → Hypercalciuria (acidosis mobilizes bone calcium; reduced citrate promotes urinary calcium precipitation)

Clinical Presentation:

Infancy/Early Childhood: - Failure to thrive - Recurrent vomiting - Polyuria, polydipsia - Muscle weakness (from hypokalemia) - Developmental delay if untreated - Bone pain (osteomalacia from chronic acidosis)

Childhood/Adolescence: - Growth failure - Bone disease (rickets) - Recurrent nephrolithiasis (calcium phosphate stones despite low urine pH) - Chronic diarrhea (from loss of bicarbonate)

Laboratory Findings:

Diagnostic Confirmation: 1. Fludrocortisone stimulation test: Acute acid load (ammonium chloride 0.1 g/kg) → normal kidneys lower urine pH to <5.5; RTA Type 1 cannot 2. OR: Furosemide + fludrocortisone test (alternative)

Management:

Alkali Replacement: - Goal: Normalize serum HCO₃⁻ (>20 mEq/L); achieve urine pH <5.5 if possible - Agents: Sodium bicarbonate, potassium citrate (preferred; replaces both HCO₃⁻ and K+) - Dose: 1-3 mEq/kg/day (divided in 2-4 doses) - Target: Serum HCO₃⁻ 20-24 mEq/L - Titration: Adjust based on serum HCO₃⁻ checks every 4-8 weeks

Potassium Supplementation: - Often necessary despite bicarbonate therapy - Potassium citrate preferred (addresses both K+ and HCO₃⁻ deficiency) - Avoid potassium chloride (increases Cl⁻ load, worsening metabolic acidosis)

Prevention of Nephrolithiasis: - Aggressive alkali therapy (maintains higher urine pH, reducing calcium phosphate precipitation) - Thiazide diuretics: Hydrochlorothiazide 0.5-1 mg/kg/day (reduces urinary calcium excretion) - Citrate supplementation: Potassium citrate (increases urinary citrate, inhibitor of stone formation) - Adequate hydration

Bone Management: - Vitamin D supplementation (1,25-dihydroxyvitamin D3, calcitriol, 0.02-0.08 mcg/kg/day) - Calcium supplementation if deficient - Address acidosis (improves bone metabolism)

Long-Term Outcome: - With adequate alkali and K+ replacement, growth and development normal - Adult height usually normal with early diagnosis and therapy - Renal function preserved (GFR normal throughout life unless nephrolithiasis causes obstruction) - Hearing loss possible with mutations in vacuolar H+-ATPase genes (genetic hearing loss syndromic association)

C. Type 2 (Proximal) RTA

Genetics: - Autosomal recessive: Gene SLC4A4 (sodium-bicarbonate cotransporter in proximal tubule) - Rare in isolation; often part of syndromic presentation (Fanconi syndrome)

Pathophysiology: - Proximal tubule cannot reabsorb filtered HCO₃⁻ adequately - Result: Bicarbonate wasting in urine - Serum HCO₃⁻ drops until levels low enough to be filtered below reabsorption capacity - → “Equilibrium point” reached (serum HCO₃⁻ usually 15-18 mEq/L) - → Urine pH appropriately low (<5.5); kidneys can acidify normally (distinguishes from Type 1)

Clinical Presentation: - Similar to Type 1: growth failure, bone disease, weakness - Often associated with Fanconi syndrome (additional proximal tubular dysfunction: phosphate, glucose, amino acid, urate wasting)

Laboratory Findings:

Diagnostic Distinction from Type 1: - Type 1: Cannot lower urine pH (<5.5 normal); hyperchloremia + hypokalemia prominent - Type 2: CAN lower urine pH (<5.5 normally); serum HCO₃⁻ less dramatically low

Management: - Alkali replacement: sodium bicarbonate or potassium citrate (similar doses as Type 1) - Large doses often required (may need 5-10 mEq/kg/day) due to continued wasting - Thiazide diuretics: Paradoxically effective in Type 2 (reduce glomerular filtration → reduce HCO₃⁻ wasting) - If associated with Fanconi syndrome: additional management required (see Fanconi/Cystinosis section)

D. Type 4 (Hyperkalemic) RTA

Classification:

Type 4A: Hyporenin-Hypoaldosteronism (HHA) - Genetics: Gene mutations in renin, angiotensinogen, or aldosterone synthase - Pathophysiology: Reduced renin-aldosterone response to acid/K+ load - Etiology: Congenital (rare); acquired (diabetes, chronic kidney disease, NSAIDs, ACE inhibitors/ARBs)

Type 4B: Aldosterone Resistance (Pseudohypoaldosteronism) - Genetics: Gene ENaC (epithelial sodium channel); autosomal recessive - Pathophysiology: Receptor or post-receptor defect in collecting duct response to aldosterone - Clinical: Salt-wasting, hyperkalemia, metabolic acidosis despite normal/high aldosterone levels

Pathophysiology (Both Types): - Impaired potassium secretion in collecting duct (due to aldosterone deficiency or resistance) - → Hyperkalemia (serum K+ >5.5 mEq/L) - → Reduced ammoniagenesis (K+ accumulation suppresses ammonia production) - → Metabolic acidosis (reduced renal acid excretion)

Clinical Presentation: - Hyperkalemia (primary finding) - Metabolic acidosis (mild-to-moderate) - Cardiac arrhythmias (from hyperkalemia) - Type 4B (pseudohypoaldosteronism): Salt-wasting, hyperkalemia, hypokalemic features (muscle weakness despite high K+ due to transcellular shift) - Growth failure if untreated

Laboratory Findings:

Type 4A-Specific: - Plasma renin: Low (differentiates from Type 4B) - Aldosterone: Low

Type 4B-Specific: - Plasma renin: Elevated (high K+ should stimulate) - Aldosterone: Elevated (resistance to hormone effect)

Management:

Type 4A (Hypoaldosteronism): - Fludrocortisone: 0.05-0.2 mg daily (synthetic aldosterone; expands volume, increases K+ excretion) - Sodium supplementation: Enhance volume expansion effect - Diuretics: Furosemide (promotes K+ wasting; careful with volume status) - Avoid NSAIDs, ACE inhibitors/ARBs (reduce aldosterone)

Type 4B (Aldosterone Resistance): - Fludrocortisone: Often ineffective (resistance to aldosterone) - Diuretics: Loop and thiazide diuretics (promote K+ wasting independent of aldosterone) - Dietary potassium restriction: <2 g/day - Potassium binders: Sodium polystyrene sulfonate, patiromer (newer agent), sodium zirconium cyclosilicate - Alkali: Sodium bicarbonate or potassium citrate (carefully; K+ content)

Long-Term Management: - Regular monitoring of serum K+ and HCO₃⁻ - Dietary compliance (sodium/potassium balance) - EKG if K+ persistently >6.5 mEq/L (assess for arrhythmia risk)

A. Fanconi Syndrome

Definition: Generalized dysfunction of renal proximal tubule characterized by wasting of multiple solutes: - Phosphate - Glucose - Amino acids - Urate - Carnitine - Bicarbonate (Type 2 RTA features)

Etiology:

Primary (Idiopathic): - Rare; genetic basis incompletely characterized

Secondary (More common): - Cystinosis (most common genetic cause) - Tyrosinemia Type 1 - Lowe syndrome (oculocerebrorenal) - Wilson disease - Mitochondrial diseases - Dent disease - Heavy metals (mercury, lead, cadmium) - Drugs: Aminoglycosides, amphotericin B, ifosfamide, tenofovir, adefovir

Pathophysiology: - Mitochondrial dysfunction; impaired cellular energy production - Loss of intact proximal tubule cells (epithelial cell damage) - Impaired active reabsorption mechanisms for small solutes and proteins

Clinical Presentation:

Infantile Onset (Cystinosis): - Failure to thrive (first year of life) - Recurrent vomiting, anorexia - Polyuria, polydipsia - Rickets (phosphate wasting → hypophosphatemia) - Photophobia, eye pain (cystine crystal accumulation in cornea) - Growth failure

Delayed Presentations (Other etiologies): - Variable age of onset - Growth failure - Bone disease (rickets) - Renal insufficiency (progressive in some types)

Laboratory Findings:

B. Cystinosis

Genetics: - Gene: CTNS (cystine transporter 1; chromosome 17p13) - Inheritance: Autosomal recessive - Incidence: 1 in 100,000-200,000 live births

Pathophysiology: - Defect in cystine efflux transporter in lysosomal membrane - → Cystine accumulation in lysosomes (cystine cannot exit; accumulates in crystals) - → Cellular toxicity; apoptosis of renal tubular cells - → Proximal tubule dysfunction (Fanconi syndrome) - → Progressive accumulation in other organs (eye, bone, thyroid, pancreas, CNS)

Clinical Presentation:

Infantile-Onset Nephropathic Cystinosis (Most common, 95% of cases):

Infancy (0-1 year): - Presentation age 3-6 months typically - Failure to thrive, poor growth - Polyuria (2-4 liters/day; up to 10 in severe cases) - Recurrent vomiting, anorexia - Severe dehydration risk (large urinary losses) - Rickets (develops within first year)

Early Childhood (1-5 years): - Growth failure becomes apparent (significant growth deficit if untreated) - Photophobia and eye pain (cystine crystalline deposits in cornea; “cystine crystals”) - Ocular findings: Crystal deposits in cornea (anterior and posterior surface), conjunctiva, retina; can lead to reduced visual acuity - Fanconi syndrome fully apparent: phosphate wasting, aminoaciduria, glycosuria

Progression to ESRD: - Progressive renal insufficiency; most reach ESRD by age 10 years if untreated - With cysteamine therapy: ESRD delayed to age 20-30s (dramatic improvement in prognosis) - Without therapy: Renal failure by age 6-10 years

Ocular Manifestations (Distinctive): - Photophobia, eye pain (due to crystalline deposits) - Corneal crystals (bilateral, anterior and posterior) - Visual impairment (progressive) - Retinal crystals (uncommon but possible) - Cystine crystal deposition in eye is PATHOGNOMONIC for cystinosis

Other Organ Involvement (Late manifestations without treatment): - Pancreatic insufficiency (diabetes mellitus; pancreatic enzyme deficiency) - Thyroid dysfunction (hypothyroidism) - Neurodegeneration (tremor, myopathy, CNS involvement) without therapy - Bone disease (rickets from phosphate wasting, vitamin D deficiency)

Adolescent/Adult-Onset Cystinosis (Rare, <5% of cases): - Slower progression - May not reach ESRD until adulthood or remain stable - Ocular symptoms may be mild

Laboratory Findings: - Elevated urinary cystine (hallmark finding; >300 mg/day normal <100) - Elevated leukocyte cystine content (diagnostic; >2 nmol half-cystine per mg protein normal <0.2) - Fanconi syndrome features: Glycosuria, amino aciduria, hypophosphatemia, hyperparathyroidism - Progressive renal insufficiency (serum creatinine rises; GFR declines) - Elevated PTH, FGF23

Diagnosis: 1. Clinical suspicion: Fanconi syndrome + photophobia + ocular crystals (pathognomonic) 2. Elevated 24-hour urinary cystine (>300 mg/day) 3. Leukocyte cystine content (definitive; >2 nmol/mg protein diagnostic) 4. Genetic testing: CTNS gene sequencing (confirms; allows for family counseling) 5. Slit-lamp examination: Shows crystalline deposits in cornea/conjunctiva

Management:

Cysteamine Therapy (Cystine-Depleting Agent): - Mechanism: Converts cystine to cysteine (plus cysteine-cysteamine disulfide) via mixed disulfide formation; products can exit lysosome - Drug: Cysteamine hydrochloride (Cystagon) or cysteamine bitartrate (Procysbi, delayed-release) - Dosing: - Standard cysteamine: 1.3 g/m²/day (divided into 4 doses every 6 hours; OR can divide into 2 doses of delayed-release) - Delayed-release (Procysbi): 0.65 g/m²/dose twice daily (more convenient; lower GI side effect profile) - Target: Leukocyte cystine content <1 nmol/mg protein (goal is <0.5) - Dosing adjustment: May require 1.3-1.9 g/m²/day depending on response

Efficacy: - Renal function: Delays progression to ESRD by 10+ years; some patients never reach ESRD with early therapy - Growth: Normalizes with adequate cysteamine (if started <age 2 years) - Ocular protection: Reduces cystine crystal deposition; slows/prevents corneal damage - Pancreatic: May slow development of diabetes (unclear if preventive)

Side Effects: - GI: Nausea, vomiting, abdominal pain (particularly early; improved over time or with delayed-release formulation) - Odor: Characteristic rotten egg smell (due to cysteamine metabolism) - Rash (rare; discontinue if severe) - Neuropsychiatric: Depression (reported; monitor mood)

Monitoring: - Leukocyte cystine content every 3-6 months; adjust dose to maintain <1 nmol/mg protein - Serum creatinine and eGFR: Every 3-6 months - Urinalysis, serum phosphate, potassium, calcium every 3-6 months - Ophthalmology: Slit-lamp exam annually; assess visual acuity

Additional Management:

1. Renal Replacement Therapy:
   • Target GFR: Initiate dialysis/transplantation when GFR <15 mL/min/1.73 m²
   • Transplantation: Often recommended; cysteamine continued post-transplant
   • Recurrence: Cystinosis recurs in transplanted kidney (cystine accumulation begins again); cysteamine continues to work
2. Bone Management:
   • Phosphate binders: Calcium carbonate, sevelamer
   • Vitamin D supplementation: Calcitriol 0.02-0.08 mcg/kg/day (adjust for serum calcium)
   • Calcium supplementation
3. Metabolic Acidosis:
   • Sodium bicarbonate or potassium citrate (if RTA features present)
4. Growth Support:
   • Ensure adequate caloric and protein intake
   • Growth hormone therapy if marked growth failure despite medical management
5. Pancreatic Monitoring:
   • Screen for diabetes starting age 5-10 years (HbA1c, glucose tolerance test)
   • Pancreatic enzyme replacement if insufficiency develops
6. Ocular Care:
   • Protective sunglasses
   • Corneal lubricant eye drops
   • Ophthalmology follow-up
7. Neurological Monitoring:
   • Assess for tremor, weakness, encephalopathy (late manifestations of systemic cystinosis)

Long-Term Outcome with Early Cysteamine Therapy: - Renal: ESRD typically delayed to age 20-30s (vs. 6-10 years without therapy) - Growth: Normal height achievable with early therapy initiation (<age 2 years) - Visual: Slowed progression of ocular crystals; visual function better preserved - Neurological: Risk of late neurodegenerative complications reduced but not eliminated - Quality of life: Significantly improved with early diagnosis and cysteamine initiation

A. Overview

Definition: Inability of kidney collecting duct to respond to antidiuretic hormone (ADH/vasopressin) → inability to concentrate urine → massive polyuria.

Classification: 1. Congenital: Genetic mutations 2. Acquired: Medications, metabolic abnormalities, chronic kidney disease

B. Congenital Nephrogenic Diabetes Insipidus

Genetics:

X-Linked NDI (Most common, 90% of familial cases): - Gene: AVPR2 (vasopressin receptor 2 V2R; X chromosome) - Hemizygous males: Severe phenotype - Heterozygous females: Variable severity (X-inactivation affects manifestation)

Autosomal Recessive NDI (Less common): - Gene: AQP2 (aquaporin-2 water channel on chromosome 12) - Both sexes equally affected

Pathophysiology: - V2R mutation: Collecting duct principal cells cannot respond to ADH signaling → no aquaporin-2 insertion into apical membrane - AQP2 mutation: Water channel absent or non-functional; cannot transport water across cell membrane - Result: Unable to reabsorb water from collecting duct → massive urine output (5-15 liters/day)

Clinical Presentation:

Neonatal Period: - Hypernatremia (often severe; Na+ >150 mEq/L) - High osmolality (>300 mOsm/kg) - Polyuria (large output despite dehydration) - Failure to pass meconium (some cases; severe dehydration) - Fever, lethargy, seizures (from severe hypernatremia) - Diagnosis may be made emergently in NICU for hypernatremia

Infancy/Early Childhood: - Persistent polyuria (5-15 liters/day in untreated cases) - Severe polydipsia (if access to water; comatose if denied water) - Growth failure - Developmental delay (if recurrent hypernatremic episodes) - Nephrogenic polyuria refractory to desmopressin (DDAVP)

Later Childhood: - Chronic thirst; large fluid intake required - Massive polyuria (unresponsive to desmopressin) - Growth can be normal if water access adequate - Behavioral/emotional impact (frequent urination, bed-wetting)

Laboratory Findings:

Diagnostic Confirmation: 1. Desmopressin (DDAVP) trial: - Central DI responds with ↓ polyuria, ↑ urine osmolality - NDI shows no response to desmopressin (hallmark finding) 2. Genetic testing: AVPR2 or AQP2 gene sequencing

Management:

Hydration: - Free access to water at all times (critical; risk of severe dehydration/hypernatremia if access restricted) - Large, frequent water intake required (5-15 liters/day) - In infants: Breast/bottle feeding with water offered frequently between feeds - Education: Family must understand critical importance of unlimited water access

Thiazide Diuretics: - Hydrochlorothiazide: 1-2 mg/kg/day (divided) - Mechanism: Produces mild volume depletion → increased proximal tubule sodium + water reabsorption → ↓ urine output to collecting duct - Efficacy: Reduces polyuria by 30-50% (additive with NSAIDs) - Monitor: Electrolytes, renal function, volume status

NSAIDs: - Indomethacin: 0.5-1 mg/kg/day - Mechanism: Inhibits prostaglandin synthesis; reduces cAMP in collecting duct; ?direct effect on aquaporins - Efficacy: Modest reduction in polyuria (20-30%) - Caution: Monitor renal function; risk of chronic kidney disease with long-term use

Amiloride (Potassium-Sparing Diuretic): - Dose: 0.3-0.6 mg/kg/day - Mechanism: Blocks ENaC in collecting duct; reduces intracellular Na+, enhancing aquaporin-2 expression - Particularly useful if X-linked NDI (may be more effective than other agents) - Monitor: Serum potassium (risk of hyperkalemia)

Combined Therapy: - Often more effective: Thiazide + NSAID + amiloride - Goal: Reduce polyuria to 3-5 liters/day (more manageable)

Renal Complications: - Medullary nephrolithiasis (calcium phosphate stones from chronic dehydration) - Hydronephrosis (from recurrent dehydration episodes) - Chronic kidney disease (from repeated osmotic injury; progressive in some cases)

Long-Term Outcome: - Life-threatening complications if water access restricted - Normal growth possible with adequate management - Neurodevelopmental outcome depends on frequency/severity of hypernatremic episodes - Renal function usually preserved but at risk for late CKD - Social impact: Frequent urination, school disruption, psychological burden

C. Acquired Nephrogenic Diabetes Insipidus

Common Causes:

Management: - Address underlying cause (discontinue offending drug if possible) - Hydration, thiazide + NSAID therapy - More responsive to desmopressin than congenital NDI (partial response common)

A. Nephronophthisis

Definition: Progressive, bilaterally symmetric tubulointerstitial nephritis leading to renal insufficiency; most common genetic cause of ESRD in children (4-10% of pediatric ESRD).

Genetics: - Autosomal recessive inheritance (most common) - Gene: NPHP genes (NPHP1-13; mutations encode proteins involved in ciliary structure/function) - Most common: NPHP1 (del chromosome 2q13, homozygous in 85% of Northern European cases)

Pathophysiology: - Defect in primary cilium structure or function in renal tubular epithelial cells - Primary cilium: Sensory organelle; involved in fluid flow sensing, cell-cell signaling - Loss of ciliary function → abnormal cell signaling → tubulointerstitial inflammation - Characteristic histology: Corticomedullary cysts (medullary or inner cortical cysts); chronic tubule-interstitial fibrosis; glomeruli initially spared

Clinical Presentation:

Early Childhood (Age 1-10 years, typically): - Polyuria, polydipsia (earliest symptom; from distal tubule dysfunction) - Nocturnal enuresis (common) - Anemia (mild; from CKD) - Growth failure - Progressive renal insufficiency; serum creatinine rising

Progression: - Progressive loss of renal function; GFR declining - Median age at ESRD: 13 years (range 5-20+; depends on genotype) - NPHP1 deletions: Earlier ESRD (median age 13); NPHP mutations: Slower progression (median age 20+)

Extrarenal Manifestations (Syndromic Forms): - NPHP1: Pure nephronophthisis (no extrarenal features) — most common - NPHP4 (Senior-Loken syndrome): Nephronophthisis + ocular dystrophy (retinitis pigmentosa; progressive vision loss) - NPHP8-10 (Joubert syndrome - related): Nephronophthisis + cerebellar hypoplasia, developmental delay - NPHP13: Nephronophthisis + short-rib thoracic dysplasia - Other ciliopathy associations: Situs inversus totalis, heterotaxy, bronchiectasis, liver disease (congenital hepatic fibrosis)

Diagnosis:

Clinical Suspicion: - Polyuria + progressive renal insufficiency in child - Family history of ESRD in cousins/siblings (autosomal recessive)

Imaging: - Renal ultrasound: Small, echogenic kidneys; characteristic cysts at corticomedullary junction (may not be obvious early) - CT: Corticomedullary cysts; helps confirm diagnosis - MRI: Excellent visualization of cysts; shows interstitial fibrosis

Histology: - Kidney biopsy (if diagnosis uncertain): Tubulointerstitial fibrosis; corticomedullary cysts; relatively spared glomeruli - Electron microscopy: May show ciliary abnormalities (short, irregular cilia or complete absence)

Genetic Testing: - NPHP1 homozygous deletion testing (covers 85% of cases) - Full NPHP gene panel (NPHP1-13) if deletion testing negative - Ciliary function testing: Immunohistochemistry for ciliary marker proteins

Management: - Supportive care: ACEI/ARB (slow renal function decline; reduce proteinuria) - CKD management: Blood pressure control, anemia management, renal osteodystrophy prevention - Renal replacement therapy: Dialysis/transplantation when ESRD reached - Genetic counseling: Autosomal recessive; 25% risk to siblings - Screening relatives: Ultrasound for asymptomatic family members - Syndromic assessment: Ophthalmology (if Senior-Loken), neurology (if Joubert-related), cardiology (if situs anomalies)

Prognosis: - Inevitable progression to ESRD (median 13-20 years depending on genotype) - Transplantation: Good outcomes; disease does not recur in transplanted kidney

B. Ciliopathies Overview

Spectrum of Ciliary Disorders: Ciliopathies involve dysfunction of primary cilium (found on most cell types, including renal tubular epithelium).

Renal Manifestations: 1. Nephronophthisis (tubulointerstitial disease; progressive) 2. Cystic kidney disease (polycystic kidney, cystic dysplasia; linked to ciliary dysfunction in PKD genes) 3. Glomerulonephritis (rare; inverted architecture secondary to ciliary dysfunction)

Syndromic Ciliopathies with Renal Involvement:

Common Theme: Ciliary dysfunction → abnormal epithelial signaling → abnormal tubule development, inflammation, and cyst formation