Renal hypoplasia pertains to the reduction in the number of
renal lobes. The classification of renal hypoplasia establishes four types, termed simple, oligomeganephronic, segmental, and cortical.
Simple hypoplasia Simple hypoplasia is characterized by either one
kidney weighing 50% or less of that of the normal, or the combined weight of both kidneys being less than 33% of that of the normal. Often accompanied by
hypertension. The number of
lobules and
calyces is reduced to five or fewer, compared to the usual ten or more. Enlargement of the
nephrons is not present in this type, considering it is associated with
oligomeganephronia. The
histology is normal.
Cysts or
dysplasia are absent and apart from its decreased size, every other facet of the kidneys is unaffected. If unilateral, the contralateral kidney can undergo
hypertrophy to compensate for the affected kidney, and renal function stays normal. If bilateral, progressive
renal insufficiency is expected.
Oligomeganephronia Oligomeganephronia (or oligomeganephronic hypoplasia) is a rare
pediatric renal disease where the number of
nephrons is reduced but is distinctly enlarged.
Tubules are also enlarged. This type of
hypoplasia is more prevalent in
males with a ratio of three-to-one and is caused by either
renal-coloboma syndrome,
branchiootorenal syndrome, acro renal syndrome, or
wolf-hirschhorn syndrome. Both
kidneys are affected symmetrically unless paired with
unilateral agenesis, which is uncommon.
Morphology is normal. The combined length of both kidneys is 80% or lower than a single normal kidney and the
glomerular filtration rate is reduced to 30% of the normal. The number of
renal lobes is reduced to five to six or occasionally as few as one or two. The number of nephrons per lobe is reduced as well.
Malformations of the urinary tract and
nephrosclerosis are absent and
vesicoureteral reflux is insignificant. Unlike
segmental hypoplasia,
hypertension is rarely present in this type. The affected are generally born
premature or small for their
gestational age. Most cases are
sporadic but some are
inherited. It has been reported that the affected person can have a
twin or
sibling with the same condition. In the first few years of the affected individual's life,
dehydration,
polyuria and
polydipsia are present, followed by
hyperfiltration alongside
proteinuria, with progressive
renal failure to come after, its onset determined by the combined renal mass. The
end-stage renal disease occurs between six months and seventeen years.
Segmental hypoplasia Segmental hypoplasia or Ask-Upmark kidney is a rare
renal disease where a part of the
kidney has undergone
hypoplasia. The number of
renal lobes is reduced, and the kidney size is less than two
standard deviations from the average, with the weight often being over 50g in adults and 12–25g in children. This type of hypoplasia can be either unilateral or bilateral and is more prevalent in
females by 72%. The
enzyme renin is overly
secreted, causing
hypertension and is often severe.
Calyces are enlarged,
glomeruli are absent in the
cortex, and the
medulla is either non-existent or underdeveloped.
Vesicoureteral reflux and
urinary tract infections are commonly present, while
cysts,
dysplasia, and
proteinuria are absent. The surface of the kidney has a pelvic recess and one or more transverse cortical grooves, presumed to be a failed formation of a renal lobe. The
renal parenchyma next to the hypoplastic part of the kidney is seemingly normal; however, it can undergo
hypertrophy, making the grooves more noticeable.
Cortical hypoplasia Cortical hypoplasia is a not widely known type of renal
hypoplasia where the number of
nephron generations is reduced, causing the
renal medulla to reduce in size and the
renal cortex to decrease in thickness throughout the person's life. This type of hypoplasia is frequently paired with other forms of renal hypoplasia and seems to portray an arrest of
nephrogenesis, leading to fewer nephron generations, resulting in smaller renal size in general. The nephrogenesis before the arrest is normal. On the other hand, the
medullary rays and
renal pyramids have decreased in size with the radial glomerular count reduced to eight or less. ==Signs and symptoms==