Alport syndrome and thin basement membrane nephropathy
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A teenage boy with progressive sensorineural hearing loss and anterior lenticonus on eye exam has microscopic hematuria and proteinuria. His maternal uncle is on dialysis. Mother has lifelong asymptomatic microscopic hematuria, a classic carrier presentation.
Alport syndrome (AS) and thin basement membrane nephropathy (TBMN) are now understood as a type IV collagen disease spectrum, with TBMN as the mildest end and X-linked AS as the most severe.
| Form | Inheritance | Genes | Severity |
|---|---|---|---|
| X-linked Alport | XLD | COL4A5 | Severe in males (ESRD by 20s-30s); female carriers variable |
| Autosomal recessive Alport | AR | COL4A3, COL4A4 (biallelic) | Severe; both sexes equally |
| Autosomal dominant Alport | AD | COL4A3, COL4A4 (heterozygous, severe variants) | Variable, often progressive |
| Thin basement membrane nephropathy (TBMN) | AD | COL4A3, COL4A4 (heterozygous, milder variants) | Mild: isolated hematuria, generally benign |
The same heterozygous COL4A3/A4 variants can present as either AD Alport or TBMN depending on variant severity, modifiers, and chance; they are increasingly grouped under "autosomal dominant Alport spectrum."
- Type IV collagen defect affecting basement membranes
- Renal: persistent microscopic hematuria, proteinuria, progressive renal failure
- Males with XL form → ESRD typically by 20s-30s
- Females with XL form → variable; ~30% develop ESRD by age 60
- Biallelic AR → severe early disease both sexes
- Hearing: progressive sensorineural hearing loss (~70% of XL males, less in carriers)
- Ocular: anterior lenticonus (pathognomonic), macular flecks, corneal dystrophy
Historically called "benign familial hematuria." Now recognized as the mild end of the COL4A3/A4 spectrum:
- Persistent microscopic hematuria, often present from childhood
- Family history of microscopic hematuria (autosomal dominant pattern)
- No or minimal proteinuria
- Normal hearing, normal eye exam
- Renal function usually preserved, but a subset progress to CKD especially with cofactors (hypertension, obesity, additional kidney insults)
Why it matters: TBMN was historically considered benign, but ~30% of COL4A3/A4 heterozygotes develop CKD by age 60. Today, isolated hematuria with a COL4A3/A4 variant is treated as a kidney disease that warrants surveillance, not a no-touch diagnosis.
- Urinalysis: persistent dysmorphic RBCs, proteinuria
- Audiology baseline and serial
- Eye exam for lenticonus and macular flecks
- Kidney biopsy: electron microscopy shows characteristic GBM thinning, splitting, and lamellation in Alport; uniform thinning in TBMN
- Genetic testing: multigene panel covering COL4A3, COL4A4, COL4A5; preferred over biopsy when available because it discriminates the inheritance pattern and informs family testing
- IgA nephropathy: recurrent gross hematuria with URIs; mesangial IgA on biopsy
- Post-infectious glomerulonephritis: acute nephritic syndrome after strep
- Thin basement membrane nephropathy (within-spectrum): addressed above
- Fabry disease: GLA, X-linked, lysosomal; renal disease + skin angiokeratomas + cardiac
- Polycystic kidney disease: cysts on ultrasound, no hematuria pattern
- ACE inhibitors / ARBs: first-line, slow progression of proteinuria; start early
- SGLT2 inhibitors: emerging evidence for slowing progression
- Avoid nephrotoxins (NSAIDs, aminoglycosides where possible)
- Audiology + eye surveillance in Alport
- Transplantation for ESRD; ~3% develop anti-GBM disease against the new healthy kidney's α5(IV) collagen, so counsel before transplant
- Carrier counseling for relatives; even mild COL4 variants warrant kidney follow-up
Alport affects all the "ports": Eyes, ears, kidneys.
ALport = Anterior Lenticonus, the pathognomonic eye finding.
Alfourt involves type four collagen.
TBMN = "Thin BM, Mild Nephritis": the mild end of the COL4 spectrum. Isolated hematuria + family history without hearing or eye findings, but worth surveillance because ~30% progress.