Primary Aldosteronism Workup

Individuals with primary aldosteronism may present with hypokalemic metabolic alkalosis; however, as many as 38% of patients with primary aldosteronism may be normokalemic at presentation. ^{[17, 23]}

Routine laboratory studies can show hypernatremia, hypokalemia, and metabolic alkalosis resulting from the action of aldosterone on the renal distal convoluted tubule (DCT) (ie, enhancing sodium reabsorption and potassium and hydrogen ion excretion). ^[24]

Historical tests

Adrenal phlebography

This procedure had been attempted in the 1980s and aimed to invasively visualize the venous patterns encircling adrenocortical adenomas. The procedure has fallen into disrepute because of the risk of adrenal infarction and is no longer used.

Therapeutic trial of spironolactone (Aldactone)

This procedure is also no longer used as a diagnostic test for primary aldosteronism, because easier and more rapid alternatives exist; hence, it is currently of historic value.

For reasons of completeness, the spironolactone therapeutic trial involved the administration of spironolactone orally at a dose of 100 mg 4 times daily for 5 weeks. A positive test would be characterized by a decrease in diastolic blood pressure of at least 20 mm Hg.

Serum potassium and bicarbonate levels

Hypokalemia and metabolic alkalosis have low sensitivities and specificities for primary aldosteronism when these levels are tested by themselves. Hypokalemia (serum potassium level < 3.6 mEq/L) has a sensitivity of 75-80% while the patient is on a normal sodium diet. ^[5] Typically, it is associated with mild metabolic alkalosis (serum bicarbonate level >31 mEq/L) and inappropriate kaliuresis (urinary potassium excretion >30 mmol/day).

Sodium and magnesium levels

Mild serum hypernatremia in the 143-147 mEq/L range and mild hypomagnesemia from renal magnesium wasting are other associated biochemical findings in established primary aldosteronism. ^[25] (See Medscape Reference Laboratory Medicine articles Serum Sodium and Magnesium.)

Plasma aldosterone/plasma renin activity ratio

Because the random plasma aldosterone/plasma renin activity (PRA) ratio is fairly constant over many physiologic conditions, it can be used for screening. Normal values are less than 270 when aldosterone concentration is expressed in pmol/L, or are less than 10 when aldosterone concentration is expressed in ng/dL. (See the chart below.) ^[26]

When aldosterone is measured in ng/dL and PRA is measured in ng/mL/h, a plasma aldosterone/PRA ratio of greater than 20-25 has 95% sensitivity and 75% specificity for primary aldosteronism. When aldosterone is measured in pmol/L, a ratio greater than 900 is consistent with primary aldosteronism. ^{[27, 28]}

Limitations

Limitations in the usefulness of the plasma aldosterone/PRA ration include the following:

• A major limitation of these tests is the inherent variability of aldosterone secretion due to an intrinsic circadian rhythm

• Most recommendations suggest performing the test while all antihypertensives that can affect the renin-angiotensin system (RAS) are withheld; this can be difficult to accomplish when severe disease dictates the continuation of some medications to control hypertension (HTN) and hypokalemia during testing.

• Obtaining the plasma aldosterone/PRA ratio in the setting of chronic angiotensin-converting enzyme (ACE) inhibitor use (ie, >4 wk of use) increases the specificity of the ratio test but reduces the sensitivity

• Although the usefulness of plasma aldosterone/PRA ratio testing has been well validated in whites and Asians, it has not been validated in other major racial groups.

Medication interference

The plasma aldosterone/PRA ratio should not be calculated when the patient is taking medications that can interfere with this measurement. Spironolactone, an aldosterone receptor antagonist, should be stopped for 6 weeks prior to testing. Eplerenone, another aldosterone receptor antagonist, can also interfere with testing and should be stopped for at least 2 weeks before testing.

Alpha-blockers, such as doxazosin, do not interfere with the PA/PRA ratio. Beta-blockers and calcium channel blockers do not affect the diagnostic accuracy of the ratio in most cases.

PRA after salt and water depletion and/or upright posture

In primary aldosteronism, PRA is less than 1 ng/mL/h and fails to rise above 2 ng/mL/h following salt and water depletion, furosemide administration, or 4 hours of erect posture. This test, along with the captopril suppression tests, has been used either as a screening test or as a confirmatory (second-tier) test for primary aldosteronism, depending on personal preferences of various groups involved in primary aldosteronism research.

Confirmatory tests are based on the concept that aldosterone is secreted in an unregulated fashion in primary aldosteronism and therefore cannot be suppressed by usual physiologic regulatory inputs. In a similar fashion, the PRA is chronically and tonically suppressed and cannot be stimulated

Captopril and losartan suppression tests

This involves the oral administration of a single dose of captopril (25-50 mg), an ACE inhibitor. In healthy individuals, aldosterone levels will be suppressed to less than 15 ng/dL. The test has a sensitivity of 90-100% but a specificity of only 50-80%.

In a study of 135 patients who underwent captopril and losartan (an angiotensin-II receptor blocker [ARB]) tests, Wu et al concluded that the values for the PRA ratio and aldosterone concentration derived using the losartan test were more accurate than those obtained through the captopril test for the diagnosis of primary aldosteronism. ^[29]

The authors found that when a PRA ratio (ng/dL per ng/mL/h) of greater than 35 and an aldosterone concentration of more than 10 ng/dL were used, the diagnostic specificity values for captopril and losartan were 89.1% and 93.8%, respectively, and the respective diagnostic sensitivity values were 66.2% and 84.5%. The authors recommend the preferential performance of the losartan suppression test.

Serum aldosterone level

After 3 days of an unrestricted sodium diet and 1 hour of full recumbency, healthy individuals have aldosterone levels of less than 15 ng/dL. When serum aldosterone is elevated above 22 ng/dL and renin is suppressed, the serum aldosterone (S-Aldo) test virtually confirms the diagnosis of primary aldosteronism. However, because aldosterone secretion is variable, the negative and positive predictive value of a single random aldosterone level is limited.

As many as 40% of patients with primary aldosteronism have serum aldosterone levels that remain within the reference range on repeated testing, as is typically the case in essential hypertension (HTN). (See the chart below.)

24-Hour urinary aldosterone excretion test

The 24-hour urinary aldosterone (U-Aldo) excretion test is one of the most useful confirmatory diagnostic tools because it is an index for total daily aldosterone secretion (in a fashion similar to the 24-h urinary free cortisol [UFC], which is typically elevated in patients with Cushing syndrome).

In most patients with primary aldosteronism, the 24-hour U-Aldo is greater than 14 mcg/day (after 3 days of salt loading). Only about 7% of patients with primary aldosteronism have values of less than 14 mcg/day.

Salt-loading test

The salt-loading test can be done by using either an intravenous salt-loading protocol or an oral salt-loading protocol. The oral protocol calls for daily ingestion of at least 10-12 g of sodium chloride for at least 5 days before the test is performed. When the oral protocol has been met, 24-hour U-Aldo, sodium, potassium, and creatinine excretions are measured, and serum aldosterone and PRA should be determined. In normal individuals, the major U-Aldo metabolite, urinary aldosterone-18-glucuronide, should fall below a level of 17 mcg/day. Nonsuppressibility of U-Aldo-18G is highly suggestive of primary aldosteronism. Nonetheless, this test is cumbersome and rarely performed.

The 24-hour urinary creatinine measurement validates the adequacy of the urine sample collection, while a 24-hour urinary sodium value of at least 250 mEq/day confirms an adequate salt load during the days prior to the test and therefore validates the other measurements.

The alternate version of the salt loading test involves the intravenous the infusion of 500 mL/h of isotonic sodium chloride solution over 4 hours (total of 2 L of fluid volume). The serum aldosterone level and PRA are measured at baseline, 2 hours, and 4 hours. In healthy individuals, aldosterone levels are suppressed to less than 8.5 ng/dL, while the PRA is suppressed to less than 0.6 ng/mL/h. Again, S-Aldo is highly suggestive of primary aldosteronism. Rapid infusion of isotonic saline should be avoided in patients with frank volume overload due to renal or cardiac failure or other medical reasons.

Once the diagnosis of primary aldosteronism has been confirmed by a first- or second-tier test, the next step is to determine the subtype of primary aldosteronism and to identify surgically curable disease. For practical purposes, this means distinguishing between an adrenal adenoma and idiopathic adrenal hyperplasia (IAH). (See the chart below.) ^{[24, 30]}

In general, patients with adenomas are younger than patients with hyperplasia and have more severe hypertension (HTN) and hypokalemia, as well as higher urinary aldosterone (U-Aldo) levels, than do patients with IAH. However, these clinical parameters are not reliable enough to accurately distinguish adenoma from IAH.

Postural stimulation test

Aldosteronomas are associated with an anomalous decrease in the aldosterone level with upright posture, in contrast to patients with idiopathic adrenal hyperplasia (IAH), in whom a renin-angiotensin system (RAS)–mediated increase in aldosterone level occurs with upright posture.

Similarly, a serum aldosterone level surge occurs in patients with renin-responsive adenomas (RRAs), low-renin essential HTN, and very rare cases of unilateral adrenal hyperplasia (the latter presenting with features intermediate between idiopathic adrenal hyperplasia [IAH] and aldosterone-producing adrenal adenoma; occasionally designated as “intermediate aldosteronism”). ^[31]

When abdominal computed tomography (CT) and magnetic resonance imaging (MRI) scans are combined with postural stimulation, the positive predictive value (PPV) of an abnormal postural test in predicting surgically correctable primary aldosteronism due to a single adenoma is 98%.

The standard postural test protocol involves obtaining baseline values for serum aldosterone (S-Aldo) and plasma renin activity (PRA) levels, as well as these levels 2 hours after the patient has assumed an erect posture. S-Aldo levels typically rise in this setting at least 50% above baseline in healthy persons, in persons with essential HTN, and in the subgroup of patients with primary aldosteronism who have either idiopathic adrenal hyperplasia (IAH) or RRAs.

Among patients with aldosteronomas (aldosterone-producing adenomas; APAs), S-Aldo levels typically do not rise or paradoxically fall to this level. The sensitivity and specificity of this test in the differential diagnosis of the main causes of primary aldosteronism have been reported to be as high as 80-85%.

Furosemide (Lasix) stimulation test

This test is often combined with the upright posture test. The typical test involves the oral administration of 40 mg of furosemide the night before as well as the morning of the test. On the morning of the test, after the furosemide dose has been administered, the patient remains upright 2-3 hours; then, S-Aldo and PRA levels are assayed. The interpretation of the test results is similar to that described above for the postural stimulation test.

Diurnal rhythm of aldosterone

The circadian rhythm of aldosterone secretion in healthy individuals parallels that of cortisol and is corticotropin-dependent. The lowest values are observed around 11:30 pm to midnight, and the highest values occur early in the morning around 7:30-8:00 am (assuming a normal sleep-wake cycle). While this is preserved in patients with aldosteronomas, it is typically lost in patients with IAH.

Elevated levels of 18OH corticosterone and/or 18OH cortisol in plasma and urine may be found in some patients with aldosteronomas but are uncommon in IAH. ^{[32, 33]}

CT scanning

The initial radiologic investigation in the workup of primary aldosteronism is high-resolution, thin-sliced (2-2.5 mm) adrenal CT scanning with contrast.

Aldosteronomas tend to be small, in contrast to cortisol-producing adrenocortical adenomas; only aldosteronomas that are at least 1 cm in diameter can be detected reliably and consistently.

The overall sensitivity of high-resolution, thin-slice adrenal CT scanning is greater than 90%, but the picture is complicated by the many false-positive findings associated with incidentalomas, which are reported in some series to be found in up to 10-15% of the general population (their prevalence increases with age). ^[34]

Moreover, high-resolution CT studies can actually be detrimental, because these scans often detect the hyperplasia accompanying adenomas and may result in a tendency to overdiagnose idiopathic adrenal hyperplasia (IAH). Similarly, because long-term adrenal hyperplasia is associated with pseudonodule and nodule formation, this radiographic picture may often be confused with the diagnosis of autonomous adenomas.

Surgical indications

Some investigators suggest that when a solitary, unilateral macroadenoma (>1 cm) is detected on a CT scan in a young patient in the setting of unequivocal aldosteronism, unilateral adrenalectomy is indicated. However, because of the age-dependent risk that a solitary, unilateral adrenal macroadenoma may be a nonfunctioning adenoma, some experts believe that adrenal vein sampling ^[24] should be performed in patients older than 40 years.

MRI

It is generally accepted that MRI is not superior to contrast-enhanced CT scanning for adrenal visualization. High-resolution CT scans may actually have better adrenal definition. (See the image below.)

If the screening (CT scanning and/or MRI) of a patient with primary aldosteronism is completely normal, a 6- to 12-month treatment trial with aldosterone antagonists is generally recommended, after which the imaging studies should be repeated. Treatment with glucocorticoids may also be considered for glucocorticoid-remediable aldosteronism (GRA), if this condition is suspected.

Although fairly difficult to set up and not routinely available, this test can be useful in select cases for distinguishing between adenomas and hyperplasia. In large nuclear medicine referral centers, the discriminant value of the test approaches that of adrenal venous sampling (ie, close to 90%), especially with larger tumors of 1.5 cm in diameter or larger. (See the image below.) The typical administered activity is 37 MBq (1 milliCurie; mCi).

The test results are improved if there has been previous dexamethasone suppression of the adrenals using 0.5-1 mg of oral dexamethasone every 6 hours. In this setting, adenoma images remain visible, while hyperplastic gland images fade after 2-3 days of dexamethasone therapy.

Standard scanning may produce a false-negative result for small aldosteronomas; however, the diagnostic yield can be increased by coadministration of spironolactone. This is the major diagnostic alternative to adrenal venous sampling (AVS).

A retrospective study by Di Martino et al indicated that in select cases, primary aldosteronism can be preoperatively localized using the NP-59 test in place of adrenal venous sampling. Basing the test’s performance on pathologic results, the sensitivity and positive predictive value were 90.9% and 83.3%, respectively; basing the performance on postoperative blood pressure control resulted in a sensitivity and positive predictive value of 91.6% (for both). ^[35]

Because this procedure is highly dependent on the availability of technically proficient interventional radiologists, it cannot (and should not) be performed universally, despite the fact that it is the criterion standard for the confirmation of lateralizable aldosterone excess. ^{[36, 37, 38]} Indeed, adrenal venous sampling may be performed selectively only when preoperative imaging cannot definitively lateralize a presumed unilateral aldosteronoma. A report by Dekkers et al supports this line of argument and challenges the notion of performing adrenal venous sampling in all patients with primary aldosteronism. The study found that at 1-year follow-up, intensity of hypertensive medication and clinical benefits did not significantly differ between patients assessed by adrenal venous sampling and those evaluated by adrenal CT scan. ^[39]

Adrenal venous sampling probably has its greatest utility when adrenal imaging findings are completely normal despite biochemical evidence for primary aldosteronism and in settings in which bilateral adrenal pathology is present on imaging and the biochemistry suggests the presence of a functional aldosteronoma.

The test also has utility in resolving the exact etiology in cases of primary aldosteronism in which discordance exists between the biochemical findings and the radiologic findings with regard to whether the primary aldosteronism is due to idiopathic adrenal hyperplasia (IAH) or an aldosteronoma.

One series reported that 41% of patients with a normal adrenal CT scan who had biochemical evidence of primary aldosteronism actually had lateralizable disease, while 49% of patients with bilateral micronodules on CT scan also had lateralizable disease. ^[40] Even in cases in which only a single adrenal nodule was found on imaging, when adrenal venous sampling was performed, it confirmed lateralizable disease in just 51-66% of cases.

A retrospective study by Kline et al indicated that using the strictest adrenal venous sampling criteria in the lateralization of primary aldosteronism may cause a large number of patients to be incorrectly diagnosed as having a bilateral, rather than unilateral, condition and, consequently, to be excluded from surgical intervention. Looking at 80 primary aldosterone cases that were addressed with surgery, the investigators found that the use of lateralization indexes of 3:1 to 5:1 would have resulted in a missed surgical opportunity in 10-23%. ^[41]

Technique

The adrenal veins are catheterized via a percutaneous femoral venous approach. Right and left venous catheters should be placed in the ipsilateral adrenal veins to prevent errors in handling the samples. (Cannulation of the right adrenal vein is technically difficult because of the short length of this vessel. The left adrenal vein is longer, allowing for more stable catheter placement.)

At baseline and following corticotropin stimulation (preferably by continuous infusion at 50 mcg/h for the duration of the sampling study), blood samples are obtained simultaneously from both adrenal veins, as well as from the inferior vena cava, and the samples are assayed for aldosterone and cortisol.

In order to document the placement of the catheters within the adrenal veins, an adrenal-to–vena cava cortisol ratio (post corticotropin) is calculated; it should be greater than 5-10.

Diagnosis

The accuracy of the test exceeds 95% when the procedure is technically successful. If autonomous, unilateral secretion of aldosterone is present on either side, the ratio of aldosterone concentrations between the right and left adrenal veins generally exceeds 10:1. False-positive results can occur when renal artery stenosis is present; hence, renal artery stenosis needs to be thoroughly excluded, especially before a highly invasive test is performed.

Most patients with a unilateral source of aldosterone have adrenal-to-adrenal aldosterone-to-cortisol ratios of greater than 4. Ratios of less than 3 suggest hyperplasia, and values of 3-4 are considered indeterminate results.

Risks

Adrenal venous sampling is not without risks. Adrenal and iliac venous thrombosis, adrenal hemorrhage, adrenal insufficiency, or even major venous hemorrhage due to transmural tears and catheter dislocations are among the potential complications.

Because of the different adrenocortical zones involved in idiopathic hyperplasia, in comparison with adenomas, assays of plasma 18-hydroxycorticosterone (18-OHB) or plasma and/or urine 18-oxocortisol (18-oxo-F)/18-hydroxycortisol may be of diagnostic use. (See the image below.) ^{[32, 33]}

Aldosteronomas are typically associated with 18-OHB levels of greater than 100 ng/dL. Similarly, glucocorticoid-remediable aldosteronism (GRA; type 1 familial hyperaldosteronism), though a hyperplastic disease by definition, is also associated with an increased production of these 18-oxo/hydroxy derivatives. This distinct biochemistry is not present in renin-responsive adenomas (RRAs). At this point, few centers send out for these specialized biochemistry tests, and their current value is mainly historical.

This test works on the same principle that the sodium chloride intravenous infusion or the oral salt-loading tests are based on for confirming a diagnosis of primary aldosteronism. The fludrocortisone suppression test has lost much of its popularity, however, because it requires hospitalization of the patient and 4-5 days to complete, and it currently is mainly of historical interest.

For reasons of completeness, the description of this test is as follows: fludrocortisone is administered orally at a dose of 0.1-0.2 mg every 6 hours, along with supplemental sodium chloride and potassium. In the healthy individual, following this stimulation, the serum aldosterone (S-Aldo) level is typically suppressed to less than 8 ng/dL, with a corresponding urinary aldosterone (U-Aldo) excretion of less than 12 mcg/day.

In patients with primary aldosteronism, neither the urinary aldosterone level nor the plasma aldosterone level suppresses to the above-noted thresholds.

This test is relevant only in the setting of possible familial aldosteronism. Customarily, in patients with primary aldosteronism, dexamethasone is associated with a transient slight-to-moderate reduction of plasma and urinary aldosterone levels, although not into the normal reference range. ^[9]

In the subset of primary aldosteronism patients with glucocorticoid-remediable aldosteronism (GRA), small doses of dexamethasone (1-2 mg/day) induce full normalization in plasma and urinary aldosterone levels. This is invariably associated with improvement in hypertension (HTN) in these patients. Other reports suggest a cut-off level for plasma aldosterone of less than 4 ng/dL and/or a relative plasma aldosterone suppression of greater than 80% of the baseline for the diagnosis of GRA following the dexamethasone challenge.

Three variants of familial primary aldosteronism exist. Type 1 familial primary aldosteronism (also called GRA) is associated with improvement in HTN using low-dose dexamethasone. Types 2 and 3 familial primary aldosteronism are not dexamethasone suppressible.

This is a noninvasive test for distinguishing between aldosteronomas and idiopathic adrenal hyperplasia (IAH). It takes advantage of the differential expression of the dopamine receptors on the cell membrane of adrenocortical cells.

Under normal conditions, dopamine causes tonic inhibition of aldosterone, whereas serotonin (5-HT) causes increased aldosterone secretion in vivo. Metoclopramide is a D2 dopamine receptor antagonist, as well as a serotonin receptor-4 (5-HT4) partial agonist, and, hence, its administration leads in increased aldosterone levels. This normal pattern of response is retained in patients with aldosteronomas and low-renin hypertension (HTN), but not in patients with IAH. ^[42]

Following a 10-mg intravenous injection of metoclopramide, serum aldosterone levels increase significantly in patients with aldosteronomas, but they remain either unchanged or paradoxically reduced in patients with IAH. This test saw some increased usage in the mid 1990s, especially in Europe, but is rather rarely used in the United States today.

Corticotropin stimulation test

The corticotropin stimulation test uses the standard 250-mcg intravenous injection. In aldosteronomas, a robust aldosterone response is typically observed. In idiopathic adrenal hyperplasia (IAH), the aldosterone surge is considerably more feeble. The test is no longer used, because the discriminant value of the test is rather poor. ^[8]

This test involves evaluating the response of plasma renin activity (PRA) and serum aldosterone (S-Aldo) to a continuous angiotensin-II infusion. The response characteristics are similar to those observed in the posture tests, with an appropriate increase in aldosterone level observed in IAH but not in aldosteronomas.

The angiotensin-II infusion test is much less popular than other aldosteronism tests because it requires continuous infusion and close hemodynamic monitoring, and it is rarely used today.

Histologic findings vary according to the type of primary aldosteronism that a patient has. Typical aldosteronomas are characterized by adenomatous tissue, usually with zona fasciculata–type morphology. Most of these tumors are small (< 3 cm in diameter), and most of the cells are lipid-laden cells arranged in acini or cords. Associated focal and/or diffuse hyperplasia often occurs.

Renin-responsive adenomas (RRAs) are characterized by zona glomerulosa–type morphology. The only other distinctive features are predictable, unique biochemical features.

Idiopathic adrenal hyperplasia (IAH) is characterized by diffuse hyperplasia that may be micronodular, macronodular, or a mixture of both. The morphology of the cells is commonly akin to the zona glomerulosa.

In primary adrenal hyperplasia (PAH), diffuse hyperplasia, which is unilateral and has zona fasciculata morphology, is typically observed.

Primary aldosteronism syndrome rarely occurs in the setting of adrenal carcinoma, whereby the cause of the hypersecretion of aldosterone is a malignancy (rather than a benign adenoma). In such cases, histopathology reveals the typical features of adrenocortical carcinoma (ACC), including mitotic figures, local invasion, and locoregional lymph node metastases.