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Monday, March 10, 2014
When is a basement membrane `thin?`
Does an avg. thickness of 278 nm in a 16-yr. old male definitively indicate TBM? This was the finding of my son`s renal biopsy, yet I`ve read in multiple journals that TBM is typically 225 nm or less. What ARE the numbers used to determine TBM? I don`t want him under the impression he has a kidney disease if he does not (and there seems to not even be agreement on whether TBM is a kidney "disease" or merely a fairly common condition). It`s not an inconsequential question. Each time we have to complete a health history, I`m not sure what to put, and soon he will be out on his own and I`m not sure what to tell him to put on his health history either.
He had the renal biopsy after developing hypertension and having had orthostatic proteinuria for years. I agreed with his nephrologist that there was enough concern to go forward with the biopsy. He had had every other conceivable kidney test - years of labs, ultrasound, CT angiogram - and all were completely normal. His labs remain completely normal. He has never once had hematuria despite 8 years of lab testing. So his only abnormal findings are the orthostatic proteinuria (confirmed by two separate 24-hr. urine tests) and now the hypertension.
Do I tell him to tell doctors that he has Thin Basement Membrane (Disease?) or not? I`m not convinced of it, but I also would not subject him to another renal biopsy just to try to either confirm or negate the TBM finding.
Is there a standard in nephrology for determining TBM?
TBM disease is sometimes a diagnosis of exclusion: a patient with suspected TBM may be biopsied just to prove that they don't have something else. The problem with your son's case is that he lacks the two main features of TBM: a basement membrane thickness of less than 225 nm, and microscopic hematuria. And on the other hand, he has two features that are not commonly seen in TBM: hypertension and proteinuria.
If there is a family history of hypertension, it is possible that he has a combination of early-onset "essential" hypertension and orthostatic proteinuria, which is sometimes seen in young people, especially tall, thin, teenaged boys. In order to qualify for the diagnosis of orthostatic proteinuria, however, he must have less than 1 gram of protein in an all-day urine collection (e.g., 8 am to 12 midnight) and NONE in an all-night sample obtained while he's lying down (e.g., 12 midnight to 8 am).
I would be concerned, however, that he has an underlying kidney disease that has not yet been diagnosed. Two diagnoses that come to mind as being associated with a thin basement membrane, hypertension, and proteinuria are 1) focal segmental glomerulosclerosis and 2) a mild form of Alport's syndrome. If his renal function begins to be abnormal (as indicated by an increased BUN and serum creatinine), or if the amount of protein in the urine increases, he may well require a second biopsy to make the diagnosis. So for now, I would list proteinuria and hypertension in his medical history. He does not seem to me to qualify for the diagnosis of TBM disease.
Hope this information helps, and please feel free to write again if you have more questions.
Mildred Lam, MD
Associate Professor of Medicine
School of Medicine
Case Western Reserve University