| Arrow
9 |
|
Other
vasoconstrictors such as excessive release of catecholamines may elevate
blood pressure by increasing systemic vascular resistance. The excessive
catecholamines originate in a tumor of the adrenal medulla (less frequently
extra adrenal tumors may be the source, but 98% of tumors are intra abdominal)
termed pheochromocytoma.
clinical
features
laboratory
investigation
management |
|
|
|
clinical
features |
|
Such
tumors are uncommon, but should be considered in patients with orthostatic
blood pressure fall, present in about 60% of patients (vasculature is already
maximally vasoconstricted, and volume status is low due to pressure natriuresis
[see Note
1]), headaches, about 60% of patients, sweating, about 60% of patients,
and palpitations, about 60% of patients. Less common findings are pallor
and anxiety or nervousness, about 40% of patients. Known associations exist
with neurofibromatosis, and multiple endocrine neoplasms. Because over
80% of patients lose weight, it has been said that obese patients with
hypertension are unlikely to have pheochromocytoma.
There
are a number of conditions which may simulate pheochromocytoma. Such conditions
should be considered and excluded before embarking on an investigation
for this condition (e.g. rebound hypertension after abrupt cessation of
clonidine, panic attacks, thyrotoxicosis, ingestion of sympathomimetics,
drug abuse, etc).
top |
|
|
|
laboratory
investigation |
|
Measurements
of urinary metanephrine and normetanephrine (in patients with only
paroxysms of hypertension, collect urine after the BP elevation) are the
best screening tests providing the patient is not taking propranolol (
levels) nor labetolol ( levels). There are many factors which interfere
with both urine and plasma tests for catecholamines; all such factors should
be eliminated prior to testing. A 24-hr urine collection may be used to
confirm an abnormal result. If the result is borderline, clonidine suppression
(see Note
2 ) may be used. To localize the pheochromocytoma, CT or MRI
is required.
top |
|
|
|
management
|
|
|
| pheochromocytoma |
Patients
should receive medical therapy for 2 - 3 weeks prior to surgery.
top |
|
|
|
| medical
treatment |
|
Acutely,
phentolamine intravenously is given until BP is controlled (2 - 5 mg at
5 minute intervals). Propranolol may also be required for tachycardia (1
-2 mg over 5 - 10 minutes). More chronically (1 - 2 wks), phenoxy-benzamine
in an oral dose of 10 mg daily is given and increased until control of
BP is obtained (as above, -blockers may be needed for tachycardia). Another
approach is the use of metyrosine (alpha-methyl-tyrosine, a blocker of
catechol-amine synthesis) 1 - 2 g/day with prazosin (selective 1 blockers
leave the pre-synaptic 2 receptors on the neuronal surface free to turn
off release of nor-epinephrine, thereby preventing tachycardia and the
need for -blockade) 4 - 28 mg/day over 3 wks (see Note
3 for rationale of pre-operative medical treatment). |
| |
|
|
| Note
1 |
|
In
patients with unilateral renal artery stenosis, the kidney with the stenosed
renal artery will excrete less sodium than the contralateral kidney with
the normal renal artery. The sodium retention in the affected kidney is
in part a consequence of the increase in filtration fraction caused by
the stenosis. FF = GFR/RPF; since GFR is better autoregulated than RPF,
and RPF is diminished by the stenosis, filtration fraction is increased.
An increased filtration fraction will alter peritubular physical factors
such as hydrostatic pressure (decreased) and oncotic pressure (increased)
leading to enhanced proximal tubular reabsorption of filtrate. This proximal
increase coupled with the tubular sodium reabsorption enhancement caused
by systemic increase in renin-angiotensin-aldosterone will diminish sodium
excretion by the stenosed kidney (aldosterone will affect the sodium channels
in the collecting tubules, angiotensin II will affect the Na+/H+ antiporter
in proximal tubules).
If
the contralateral kidney is normal, the systemic hypertension will cause
"pressure natriuresis" in the unaffected kidney, thus compensating for
the sodium retension in the affected kidney, and maintaining the volume
status of the patient close to normal (or minimally expanded). However,
if the second kidney also has renal artery stenosis or the second kidney
is missing, natriuresis does not occur, and renal sodium retention either
in both kidneys or in the single existing kidney leads to volume expansion
and evential suppression of renin and angiotensin II. Hypertension becomes
secondary to the increased cardiac output being autoregulated back toward
normal by increased SVR.
"Pressure
natriuresis" is a well recognized occurrence. When blood pressure increases,
the systemic pressure increase may become transmitted eventually to the
renal medullary interstitium via the vasa recta. This interstitial increase
in pressure may prevent osmotic gradient mediated medicated translocation
of tubular water from the thin descending limb of Henles loop into the
medullary interstitium. Without such water exit, sodium concentration at
the hairpin turn of the loop of Henle does not increase thus limiting passive
sodium transport out of the thin ascending limb of the loop of Henle. A
second possibility is that the increased capillary pressure diminishes
entry of interstitial fluid into the capillaries and subsequently back
into the systemic circulation inevitably urinary sodium excretion increases.
return
to main text |
| |
|
|
| Note
2 |
|
Clonidine
is a centrally acting sympathetic inhibitor. For this test, norepinephrine
and epinephrine levels are measured 2 and 3 hours after a single oral 0.3
mg dose of clonidine. Norepinephrine and epinephrine levels fall to below
normal range in patients without a pheochromocytoma, but remain high in
those with the lesion.
return
to main text |
|
|
|
| Note
3 |
|
The
catecholamine excess causes hypertension which produces pressure natriuresis
and contracted plasma volume. At surgery, the sudden removal of catecholamines
which follows adrenalectomy produces vasodilatation and decreased cardiac
output. The combination of decreased cardiac output, vasodilatation, and
contracted plasma volume coupled with operative blood losses and down-regulation
of -receptors, combine to produce severe hypotension. The pre-operative
medical treatment prevents these events from occurring.
return
to main text |
| |
|
|
| |
|
|
| |
|
|
| |
|
|
| |
|
|
|
|
|
| |
|
|
| |
|
|
| |
|
|
| |
|
|
|
|
|
|
