21. Nephritic syndrome (acute poststreptococcal glomerulonephritis). Rapidly progressive glomerulonephritis. Diabetic nephropathy.

I know this is actually topic 22 but I thought it made more sense for this topic to be before topic 21, so I switched them.

Nephritic syndrome

Glomerulonephritis causes acute glomerulonephritis syndrome, sometimes called just nephritic syndrome. According to the definition of glomerulonephritis syndrome, the following clinical features are present:

• Glomerular haematuria
• Proteinuria (usually mild)
• Decreased kidney function (GFR)
• Increased blood pressure
• Periorbital oedema
• Bilateral kidney enlargement

Not all features must necessarily be present.

Common for all conditions that cause nephritic syndrome is that there is a proliferation of cells within the glomeruli, often with inflammatory leukocytic infiltrate. The inflammatory reaction injures the capillary walls, which allows blood to pass into the filtrate. The inflammation also decreases the GFR, which is what causes the oliguria and azotaemia. Hypertension occurs due to fluid retention and increased renin release from the kidneys.

Etiology

Several disorders can cause nephritic syndrome:

• Secondary to infectious diseases
  • Acute poststreptococcal glomerulonephritis
  • Endocarditis
  • GN associated with other infections (staph, pneumococcus, viruses, etc.)
• Secondary to systemic autoimmune diseases
  • ANCA-associated vasculitis
  • SLE
  • Cryoglobulinaemia
• Primary glomerulonephritis (rare)
  • IgA nephropathy
  • Membranoproliferative glomerulonephritis

Poststreptococcal glomerulonephritis

Introduction

Also called PSGN, this condition refers to acute glomerular inflammation that occurs after a streptococcal infection. It’s a disease of children. PSGN mostly occurs in children between 6 and 10. PSGN usually occurs 1 – 4 weeks after streptococcal infection like streptococcal pharyngitis or impetigo.

Acute poststreptococcal GN is more and more rare as we treat all streptococcal diseases with antibiotics nowadays before PSGN develops. On the other hand, GN associated with other infections are becoming more common. PSGN can be caused by other infective agents as well (which is why it should be called postinfectious) like staphylococcus endocarditis, pneumococcus pneumonia, HBV, HCV, HIV and malaria.

Pathology

PSGN can only be caused by “nephritogenic” group A β-haemolytic streptococci. The bacterium is considered nephritogenic if it carries the virulence factor “M protein”.

It’s an immune complex-mediated disease. Antibodies bind streptococcal antigens in the circulation (or in situ). The resulting immune complexes deposit in the glomerulus and cause tissue injury, mostly by activating the complement system.

Under light microscopy the glomeruli are enlarged and hypercellular. Electron microscopy shows subepithelial deposits in the glomerular basement membrane. These deposits produce “humps”, a morphology that is characteristic for PSGN. Under immunofluorescence there are granular deposits of IgG and complement in the capillary walls.

Clinical features

It usually presents in children with abrupt development of fever, nausea, oliguria and haematuria. The latter is usually seen as cola-coloured urine. Lab tests show increased levels of antibodies against streptococcus, and decreased level of complement factors.

Prognosis

PSGN is usually mild. 95% of children recover completely, while the remaining 5% develop renal failure. Adults with PSGN have worse prognosis. They have a higher risk of developing renal failure. PSGN may develop into rapidly progressing GN or into chronic glomerulonephritis.

Rapidly progressive glomerulonephritis

Introduction

Rapidly progressive glomerulonephritis (RPGN) is a clinical syndrome and not a specific form of GN, as it can occur as a complication of any GN. It’s characterised by rapid, progressive loss of renal function, nephritic syndrome, and severe oliguria. There is usually a rapid decline GFR over weeks or months, which is why it’s “rapidly progressive”. If untreated will it lead to acute renal failure within weeks or months.

RPGN is sometimes called crescentic glomerulonephritis because of the characteristic half-moon shapes which are present on histology.

Pathology

When examined histologically after a biopsy, crescents (half-moon shapes) are present in the glomeruli and are characteristic for RPGN. These crescents are formed from proliferating cells which displace the glomerulus.

In untreated cases, these cells continue to proliferate, causing the crescents to continuously grow. They eventually compress and reduce the circulation, causing irreversible necrosis. Eventually the whole glomerulus will be destroyed. Treatment must be initiated before the process becomes irreversible.

While normal histology is enough to diagnose RPGN, immunohistology (immune fluorescence) is required to separate the three types of RPGN.

The kidneys of these patients are enlarged, pale and have petechial haemorrhages on the surface. The patients present with nephritic syndrome with severe oliguria and azotaemia. If they have Goodpasture syndrome they will have haemoptysis as well.

Types and etiology

Three types of RPGN exist, each with their own possible causes:

• Type I – Anti-GBM-associated RPGN
  • Anti-GBM glomerulonephritis
  • Goodpasture syndrome
• Type II – Immune complex-mediated RPGN
  • Infection
  • SLE
  • Henoch-Schönlein purpura (IgA vasculitis)
  • Cryoglobulinaemia
• Type III – Pauci-immune RPGN or ANCA vasculitis-associated RPGN
  • Microscopic polyangiitis (MPA)
  • Granulomatosis with polyangiitis (GPA)
  • Eosinophilic granulomatosis with polyangiitis (EGPA)

Of these, type III is the most common (60 – 70%), and of these, MPA is the most common.

Type I

Type I RPGN is associated with anti-glomerular basement membrane antibodies. On immunohistology there is linear deposition of IgG along the glomerular basement membrane.

Anti-glomerular basement membrane antibody disease (anti-GBM antibody disease) is caused by the presence of antibodies against the type IV collagen in the GBM. These antibodies bind to the GBM and initiate an immune response. This type shows a linear pattern on immunofluorescence.

Anti-GBM antibodies can also bind and damage alveolar basement membranes, causing pulmonary haemorrhage. Goodpasture syndrome is the condition where a patient has both RPGN and pulmonary haemorrhage due to anti-GBM antibodies.

Type II

Type II RPGN is associated with deposition of immune complexes in the glomeruli. On immunohistology there is granular deposition of IgG along the glomerular basement membrane.

Any condition that causes immune complex deposition in the glomeruli can develop into RPGN. This includes poststreptococcal GN, SLE, Henoch-Schönlein purpura. This type shows a granular pattern on immunofluorescence.

Type III

Type III RPGN is associated with ANCA-associated vasculitis, but there is no deposition of antibodies in the glomerular basement membrane. As such, immunohistology is negative or almost negative. Type III is also known as pauci-immune RPGN. Pauci-immune means “few-immune”.

Pauci-immune RPGN has no anti-GBM antibodies and show no immune complexes in the glomeruli. ANCAs are typically found in the serum.

Management

All three types are treated with steroids and cytotoxic drugs. Plasmapheresis is effective only in type I. Dialysis or kidney transplantation may be necessary.

Diabetic nephropathy

Introduction

Due to the high prevalence of diabetes is diabetic-related kidney damage the leading causing of end-stage renal disease in the developed world. It can cause nephrotic syndrome or chronic kidney disease. Chronic kidney disease is a common outcome in diabetes.

Pathomechanism

Chronic hyperglycaemia causes glucose to bind to proteins all over the body, including the basement membrane, to form advanced glycation end-products (AGEs), a process called non-enzymatic glycation. This causes the basement membrane to thicken, which narrows blood vessels and causes them to become stiffer. This process is called hyaline arteriosclerosis, and in diabetes it especially occurs in the efferent arteriole. As this arteriole becomes narrower will the pressure inside the glomeruli increase, which increases the GFR. This causes hyperfiltration.

The hyperfiltration triggers mesangial cells to undergo mesangial expansion, where the cells produce more mesangial matrix. The matrix may become fibrotic, a process called diffuse glomerulosclerosis. As the matrix continues to expand it will form nodules called Kimmelstiel-Wilson nodules, which are PAS positive. This process is called nodular glomerulosclerosis.

The mesangial expansion causes the glomerulus to expand. This increases the size of the slit pores between the feet processes of the podocytes, which allows proteins to be filtered out. These changes damage the glomerulus in the long run, eventually causing it to become so damaged that GFR decreases.

Pathology

The three aforementioned changes, GBM thickening, glomerulosclerosis and Kimmelstiel-Wilson nodules, are the three major histological changes in diabetic nephropathy. The more of these findings a patient has, the more progressed the diabetic nephropathy is.

Clinical features

The earliest manifestation of diabetic nephropathy is microalbuminuria (30 – 300 mg/day), which is a reliable sign that diabetic nephropathy is developing. As the disease progresses will there be macroalbuminuria, with more than 300 mg/day.

Management

Strict long-term control of the blood glucose slows down the progression of diabetic nephropathy. ACE inhibitors and angiotensin receptor blockers may be useful as they treat hypertension and dilate the efferent arteriole.