There are no significant light microscopic abnormalities early in the disease [1]. At later stages, glomerulosclerosis, interstitial fibrosis, and prominent interstitial foam cells, nonspecific and just indicative of proteinuria, are typical. Glomeruli show varying stages of matrix expansion and sclerosis.

Standard immunofluorescence (IF) may show nonspecific trapping of immunoglobulin M (IgM) in the mesangium. Special IF studies for subtypes of type IV collagen on either skin or renal biopsy may be helpful in distinguishing between causes of thin glomerular basement membrane (GBM), which may be the only lesion in early Alport, the carrier state for X-linked Alport, or the so-called benign familial hematuria (see below) [3, 5, 8–11].

When any one of the three alpha chains, α3, α4, or α5, is mutated, the normal collagen IV (α3, 4, 5) heterotrimer cannot form. In kidney biopsies, about 70–80 % of males with X-linked Alport, where α5 is mutated, thus lack staining of the GBM, distal tubular basement membrane, and Bowman’s capsule for α3, α4, and α5 (IV) chains [3, 10, 11]. In autosomal recessive Alport, where α3 or α4 is mutated, the GBMs usually show no immunostaining for α3, α4, or α5, because there is an inability to form the normal α3, α4, or α5 type IV collagen heterotrimer of the GBM. In these autosomal recessive cases, in contrast to X-linked cases, α5 remains normally expressed in Bowman’s capsule and distal tubular basement membrane, because the α(1,1,2)/(5,5,6) heterotrimers can still be assembled in these patients. Female heterozygotes for X-linked Alport syndrome frequently show mosaic staining of GBM and distal TBM for α3, α4, and α5 (IV) chains and skin mosaic staining for α5 (IV). Patients with autosomal dominant Alport have not been studied immunohistochemically. Skin biopsies are thus only useful in diagnosis of classic X-linked Alport. In these affected patients, skin biopsy often shows absent staining for α5 type IV collagen. In contrast, α3 and α4 are not normally expressed in the skin, so this approach is not useful for diagnosis of recessive Alport [10, 12].

Of note, some cases with Alport syndrome clinically and by renal biopsy showed apparent normal α5 type IV immunostaining pattern. About 20 % of male classic Alport patients and affected homozygous autosomal recessive Alport patients show faint or even normal immunostaining of the skin or GBM for α5 [3]. This is postulated to reflect a mutation that results in protein that albeit abnormal still expresses the epitope recognized by the available antibodies. Thus, the absence or mosaic staining of α5 type IV in the biopsy is helpful in indicating a basement membrane abnormality, but an apparent normal staining pattern in either skin or kidney does not definitively rule out Alport syndrome [10, 11]. Further, many patients with apparent benign familial hematuria have thin basement membranes and are carriers of autosomal recessive Alport with normal immunostaining (see below).

The diagnostic lesion consists of irregular thinned and thickened areas of the GBM with splitting and irregular multilaminated appearance of the lamina densa, the so-called basket weaving (Fig. 7.1) [2]. In between these laminas, granular, mottled material is present. In boys with early-stage classic Alport syndrome, the GBM may show only thinning rather than thickening. Female carriers of the COL4A5 mutation also show only thin basement membranes, as do carriers of the autosomal recessive form of Alport. The GBM thickness normally increases with age [13–15]. Normal thickness in adults in one series was 373 ± 42 nm in men versus 326 ± 45 nm in women. Glomerular basement thickness <250 nm in adults has been used as a cutoff for diagnosis in many series. Of note, thinning of GBMs should be a diffuse lesion, affecting more than half the extent of the GBM, to consider a diagnosis of thin GBM and its differential. In children, the diagnosis of thin basement membranes must be made with caution, establishing normal age-matched controls within each laboratory. In our laboratory, we found a range of GBM thickness in normal children from approximately 110 nm at age 1 year to 222 ± 14 nm in 7-year-olds.