Hyporeninemic Hypoaldosteronism

In the United States, patients’ dietary potassium intake may exceed 120 mEq/day, and elsewhere, it may be even higher. Patients excrete 90% of this intake renally. Even with CKD, the kidneys usually can compensate and maintain potassium homeostasis, albeit at the cost of reduced ability to handle a surge of potassium intake. Potassium is filtered at the glomerulus and then reabsorbed in the proximal nephron.

The main site of potassium excretion is located in the distal tubule, or, more precisely, the principal cells of the cortical collecting tubule (CCT). To achieve adequate potassium excretion, sodium delivery to that site must be adequate, aldosterone must be present to facilitate the sodium-potassium (Na-K) exchange, the principal cells must respond to aldosterone, and urine flow must be brisk enough to wash out the excreted potassium. ^{[2, 3]}

The degree of acidosis varies and may be related to the underlying CKD. Whereas in type I (ie, distal) RTA, the defect is in proton secretion with resulting high urine pH (>5.3), in type IV RTA, the primary defect is in ammoniagenesis. This defect, albeit significant, still permits elaboration of acidic (pH < 5.3) urine. Hyperkalemia inhibits renal ammoniagenesis in several ways. Furthermore, it may produce acidosis by shifting protons from cells out to the extracellular space as homeostatic mechanisms attempt to buffer potassium by intracellular uptake.

The first step in the renin release cascade involves the juxtaglomerular apparatus of the nephron. Here, renin is released, allowing angiotensin I to be cleaved from angiotensinogen; this is the rate-limiting step in the cascade. Angiotensin I, in turn, is broken down into angiotensin II by angiotensin-converting enzyme (ACE). Angiotensin II is a cofactor, along with potassium, in aldosterone synthesis by the adrenal gland.

Renal tubular damage may cause inadequate renin production and release; adrenal dysfunction may lead to inadequate aldosterone production; and the principal cells of the CCT may not respond normally to aldosterone. In true hyporeninemic hypoaldosteronism, atrophy of the juxtaglomerular apparatus may be present; this may be more prevalent in diabetics. Any combination of these factors may cause hyporeninemic hypoaldosteronism or RTA type IV. Indeed, as shown by Schambelan et al, all 3 factors may be present in some patients. ^[4]

As a rule, renal interstitial disorders are more likely to produce a picture of type IV RTA than glomerular diseases are. Interstitial diseases produce more tubular damage, cause more renin production impairment (eg, in the juxtaglomerular apparatus), and are more likely to compromise tubular potassium secretion in the distal nephron.

The tubulointerstitial diseases commonly associated with RTA type IV include the following:

• Sickle cell disease

• Analgesic nephropathy

• Lead nephropathy

• Chronic pyelonephritis

• Obstructive nephropathy

Diabetic nephropathy, though primarily a glomerular disease, is an exception because it is associated with decreased renin production. Furthermore, patients with diabetes may have impaired extrarenal potassium homeostasis, caused by a lack of insulin, and autonomic neuropathy with resulting impaired beta₂ -mediated influx of potassium into cells. ^[5]

Patients with HIV disease are at risk for adrenal insufficiency, which may present as hyperkalemia. At times, the adrenal defect may be selective for mineralocorticoid production. Furthermore, trimethoprim, a component of chemoprophylaxis regimens for patients with AIDS, may impair tubular potassium excretion.

Kulkarni reported on a diabetic patient aged 66 years who was found to have type IV RTA following a kidney transplant. ^[6]  A study by Lin et al noted the development of type IV RTA in association with the use of calcineurin inhibitors (cyclosporine A and tacrolimus) in patients who underwent solid organ transplantation. Reduction of the drug dosage, the addition of low-dose fludrocortisone, or the temporary replacement of calcineurin inhibitor with sirolimus was reported to improve the RTA-related prognosis. ^[7]  In another study, Schmoyer et al reported that a patient who underwent orthotopic liver transplantation developed tacrolimus-induced type IV renal tubular acidosis (RTA). ^[8]

Type IV RTA can in rare cases be seen in patients with systemic lupus erythematosus (SLE) as a result of SLE-associated renal disease. ^{[9, 10]}

Many commonly used drugs affect renin release, aldosterone production, or tubular potassium excretory capacity. In these cases, some confusion exists in the literature regarding nomenclature. For example, if beta blockade reduces renin release and leads to hyperkalemia in a given patient who is usually normokalemic, some authors would declare such a patient to have hyporeninemic hypoaldosteronism, whereas others would limit that diagnosis to cases in which drug effects have been excluded.

In addition, some drugs either contain potassium or impair extrarenal potassium homeostasis. The following are some of the commonly used drugs that affect potassium excretion and homeostasis ^{[11, 12]} :

• Inhibitors of renin release – These include beta blockers, including beta₁ selective blockers, and nonsteroidal anti-inflammatory agents (NSAIDs), including cyclooxygenase-2 (COX-2) inhibitors

• Renin inhibitors (eg, aliskiren) ^[13]

• Inhibitors of aldosterone production – ACE inhibitors block formation of angiotensin II (a cofactor in aldosterone production), an effect similar to that of angiotensin II receptor blockers (ARBs); heparin interferes with adrenal gland aldosterone biosynthesis

• Inhibitors of tubular potassium excretion – Spironolactone and eplerenone are direct competitive inhibitors of aldosterone; triamterene and amiloride inhibit the sodium channel necessary for potassium excretion, with triamterene exerting a mild effect on this channel; calcineurin inhibitors, including cyclosporine A and tacrolimus, may interfere with the aldosterone receptor

• Potassium-containing drugs (eg, oral or intravenous [IV] penicillin)

• Drugs that impair potassium homeostasis – Nonselective beta blockers (and selective ones at higher doses) block beta₂ -mediated potassium influx into cells, which is part of moment-to-moment potassium regulation; acute osmotic loads (eg, from mannitol or radiocontrast material) impair potassium homeostasis by causing osmotic efflux of water from cells, with convective drag of potassium—an effect mostly seen in diabetics, who lack the homeostatic protections of insulin release and an intact autonomic system

• Certain herbal products – Some herbal formulations are rich in potassium themselves or contain digitalislike substances that may inhibit tubular potassium excretion

A study by Tseng et al suggested that in infants who, despite having no identifiable risk factors, develop type IV RTA as a complication of urinary tract infection, mutation of the NR3C2 gene may be a contributing factor. ^[14]

A study by Kömhoff et al indicated that mutation of the RMND1 gene, which is required for proper mitochondrial function, leads to hyporeninemic hypoaldosteronism. Moreover, the report found that hyporeninemic hypoaldosteronism, not, as previously thought, aldosterone insensitivity, causes electrolyte disturbances associated with the mutation. Hyponatremia, hyperkalemia, and acidosis are frequently reported in patients with RMND1 mutation. Looking at three individuals with the mutation, the investigators found that, while hyporeninemic hypoaldosteronism was present, aldosterone sensitivity was intact. Consequently, according to the report, electrolyte imbalances in patients with RMND1 mutation are treatable. ^[15]

Specifying the incidence or prevalence of RTA type IV is difficult for the following reasons:

• The condition is often undetected

• It may manifest only when the patient is challenged by dietary potassium excess

• It is often iatrogenic (in the sense that an underlying proclivity is exposed by certain medications)

• It improves with the removal of exacerbating agents

RTA type IV involves a broad spectrum of symptom severity, and only the more severe cases provoke attention and therapy. In an aging population with a high prevalence of diabetes and polypharmacy, the clinical picture of RTA type IV is not uncommon. Research suggests that there has been an underdiagnosis of hyporeninemic hypoaldosteronism in diabetes mellitus. ^[16]

A retrospective report ^[17] from Germany showed an incidence of type IV RTA of 3.8% of hospital admissions in a single center.

Age-, sex-, and race-related demographics

RTA type IV generally develops in middle-aged or older patients but can occur in younger patients with such disorders as diabetes type I or sickle cell anemia. True RTA type IV and its drug-induced counterpart are increasing problems among elderly patients and are aggravated by polypharmacy.

No sexual predilection exists; however, sex-related differences in frequency have been documented for the underlying renal diseases (eg, more systemic lupus erythematosus [SLE] occurs in women, and more lead nephropathy occurs in men).

In the United States, renal disease is more common in blacks, Native Americans, and Hispanics; therefore, RTA type IV would be expected to show a higher prevalence in those groups. Diabetes also is more common in these groups, further compounding the problem of hyperkalemia.

RTA type IV can almost always be treated through some combination of addition or elimination of eliminating medications and implementation of dietary restraint. The underlying renal disease, however, often progresses towards eventual end-stage renal disease (ESRD). Note that the 2 classes of agents with proven benefit in delaying progression of renal disease (ie, ACE inhibitors and ARBs) also are common causes of hyperkalemia, which may limit their utility in delaying the progression of CKD in some patients.

Occasionally, a patient presents with hyperkalemia-induced cardiac arrhythmias, which may be fatal. Muscle weakness and dyspnea may also be presenting symptoms. More typically, the patient presents with hyperkalemia on routine chemistry testing. If untreated, the risk of a fatal arrhythmia exists, but this risk is not quantified. Sublethal hyperkalemia, per se, is usually asymptomatic, but chronic acidosis contributes to bone demineralization over the long term.