Thin basement membrane disease

Thin basement membrane disease (TBMD, also known as benign familial hematuria and thin basement membrane nephropathy) is, along with IgA nephropathy, the most common cause of asymptomatic hematuria. The only abnormal finding in this disease is a thinning of the basement membrane of the glomeruli in the kidneys. Its importance lies in the fact that it has a benign prognosis, with patients maintaining a normal kidney function throughout their lives.

Signs and symptoms
Most patients with thin basement membrane disease are incidentally discovered to have microscopic hematuria on urinalysis. The blood pressure, kidney function and the urinary protein excretion are usually normal. Mild proteinuria (less than 1.5 g/day) and hypertension are seen in a small minority of patients. Frank hematuria and loin pain should prompt a search for another cause, such as kidney stones or loin pain-hematuria syndrome. Also, there are no systemic manifestations, so presence of hearing impairment or visual impairment should prompt a search for hereditary nephritis such as Alport syndrome.

Diagnosis
Thin basement membrane disease has to be differentiated from the other two common causes of isolated glomerular hematuria, IgA nephropathy and Alport syndrome. The history and presentation are helpful in this regard:
 * There is usually a family history of kidney failure, which may be associated with hearing impairment in Alport syndrome. Also, more males tend to be affected since it is more often X-linked.
 * In IgA nephropathy, episodes of frank hematuria are more common, and a family history is rare.

A kidney biopsy is the only way to diagnose thin basement membrane disease. It reveals thinning of the glomerular basement membrane from the normal 300 to 400 nanometers (nm) to 150 to 250 nm. However, a biopsy is rarely done in cases where the patient has isolated microscopic hematuria, normal kidney function and no proteinuria. The prognosis is excellent in this setting unless the clinical manifestations progress, as occurs in all males and some females with Alport syndrome and many patients with IgA nephropathy.

Genetics
The molecular basis for thin basement membrane disease has yet to be elucidated fully; however, defects in the gene encoding the a4 chain of type IV collagen have been reported in some families.

Some individuals with TBMD are thought to be carries for genes that cause Alport syndrome.

Treatment
Most patients with thin basement membrane disease need just reassurance. Angiotensin converting enzyme inhibitors have been suggested to reduce the episodes of hematuria, though controlled studies are lacking. Treating co-existing hypercalciuria and hyperuricosuria will also be helpful in reducing hematuria. The molecular basis for thin basement membrane disease has yet to be elucidated fully; however, defects in the gene encoding the a4 chain of type IV collagen have been reported in some families.

Prognosis
Overall, most people with thin basement membrane disease have an excellent prognosis. Some reports, however, suggest that a minority might develop hypertension. The high incidence of thin basement disease also means that it may be co-existing with other kidney diseases, such as diabetic nephropathy, which may have a not-so-benign prognosis.