Biogenic Amines

Question 1

Basic physiology: examples and sites of biosynthesis, significance

Answer 1

Naturally occurring, biologically active amines - many are neurotransmitters and hormones

Examples: catecholamines, serotonin, histamine

Sites of biosynthesis:
- catecholamines –> dopaminergic and noradrenergic neurons, sympathetic nerves, chromaffin cells of adrenal medulla, paraganglia

• serotonin –> CNS serotonergic neurons, pineal gland, some peripheral endocrine tissues

Clinical significance:
- hormones and metabolites measured in blood and urine –> useful for NE tumour Dx e.g. pheochromocytoma, paragangliomas, neuroblastoma, carcinoids

Question 2

Catecholamine metabolism

Answer 2

Tyrosine --> DOPA --> Dopamine
then
----> Homovanillic acid (HVA)
or
--> NA --> Adrenaline by PNMT which is specific to adrenal glands (phenylethanolamine-N-methyltransferase)

NA –> Normetanephrine –> VMA (vanillylmandelic acid)
A –> Metanephrine –> VMA

Question 3

Serotonin metabolism

Answer 3

Tryptophan –> 5-hydroxytryptophan –> Serotonin —-> 5-HIAA (5-hydroxyindoleacetic acid)

Question 4

Chromaffin cell tumours: Phaeochromocytoma vs Paraganglioma

Answer 4

MAIN DIFFERENCE IS THE SITE

Phaeochromocytoma

• arising from adrenomedullary chromaffin cells
• commonly produces one or more catecholamines
• most are biochemically active
• 80-85% chromaffin cell tumours

Paraganglioma

• arising from extra-adrenal chromaffin cells of sympathetic or parasympathetic NS
• those from parasympathetic NS don’t secrete catecholamines
• 15-20% chromaffin cell tumours

Question 5

Phaeochromocytoma prevalence, aetiology

Answer 5

0.8 per 100000 person-years
Peak occurrence in 40-50s

(along with paragangliomas) are in 0.2-0.6% of patients with HT

1.7% of children that have HT
5% of patients with adrenal incidentalomas

Aetiology

• most are sporadic
• 10% familial (PPGL likely bilateral; younger onset) –> associated syndromes e.g. VHL, MENII, NF1

Secretion of excessive NA, A, NM, M and VMA

Question 6

Phaeochromocytoma clinical features

Answer 6

10% extra-adrenal (i.e. paraganglioma)
10% bilateral
10% malignant
10% secrete adrenaline/dopamine (90% NA)

Symptoms and signs:

• PAROXYSMAL when present
• classic triad: episodic headache, sweating, tachycardia/palpitations
• paroxysmal HT (5-15%) – atypical –> young, resistant to antihypertensives, complications (nephropathy, retinopathy, encephalopathy)
• atypical DM
• phaeochromocytoma crisis (hyper/hypotension, hyperthermia, mental status changes, other organ dysfunction)

Question 7

When to investigate for phaeochromocytoma and paraganglioma

Answer 7

• signs and symptoms or young onset HT
• symptoms provoked by drugs e.g. D2 receptor antagonists, beta blockers, sympathomimetics
• incidentaloma
• hereditary predisposition or syndromic features e.g. MTC + primary hyperPTH or muocutaneous neuromas (MENII), hemangioblastoma/retinal angioma/clear cell RCC/pancreatic NE tumours (VHL), neurofibromas/multiple cafe au lait spots (NF1)
• previous hx of PPGL

Question 8

First line lab tests for phaeochromocytoma: available tests, sensitivity/specificity, false positives

Answer 8

1. Plasma free or urine total metanephrines

a) 24hr urine:
- can measure Cr (ensure complete collection), NA, A, **fractionated NM, M, VMA

–> NA/A >2x, NM/M >2x, VMA >3x = +ve

Sensitivity and Specificity:

• -> most sensitive if measure at onset of “attack”
• -> fractionated NM/M highest specificity
• -> r/o false positives
• stop medications for 2 wks e.g. labetalol (decrease reuptake), TCA, MAOi (increase release/ decrease metabolism), clozapine (decrease reuptake), levodopa (metabolised to catecholamines)
• major physical stress e.g. hypoBG, stroke, raised ICP
• OSA (low PaO2 stimulates SNS)

b) Plasma catecholamines
- fasting, supine (posture-dependent), draw blood from indwelling catheter in place >30 min
- false positives same as urine + diuretics (change in plasma volume), smoking
- epinephrine suitable for CKD/ ESRD

c) Plasma free metanephrines
- highest sensitivity but relatively new and not widely available
- not posture dependent
- produced by tumour continuously
- levels remain relatively unaffected during SNS stimulation
- urine VMA/M/NM are from catecholamines produced other than tumour
- false positive: paracetamol, ESRD

• high plasma metanephrine:catecholamines also makes tumour probable

2. Locate tumour with imaging if highly probable
   - CT, MRI, MIBG scan

Question 9

Suppression Test for Phaeochromocytoma

Answer 9

Clonidine suppression test
- overnight fasting, empty bladder and take 0.3 mg PO at 2100, collect urine from 2100-0700

• indication: strong clinical suspicion of phaeochromocytoma with borderline increase in catecholamines (after eliminating medications/causes of false +ve)
• precaution: stop anti-HT drugs 1wk prior and during
• principle: normal physiological catecholamine is suppressed by clonidine but autonomous tumour isn’t
• risk: profound drop in BP (but safer than glucagon stimulation)

Interpretation
–> normal: NA <60 nmol/mmol Cr and A <20 nmol/mmol Cr
==> anything above = not suppressible = phaeochromocytoma

Question 10

Stimulation Test for Phaeochromocytoma

Answer 10

Glucagon stimulation test
- overnight fasting, take blood –> 1mcg glucagon IV bolus injection –> take blood 2 min after and check BP at 1 min intervals from 10-20 min after injection

• indication: strong clinical suspicion of phaeochromocytoma with borderline increase or normal catecholamines
• precaution: refrain anti-HT drugs 48hrs prior to and during
• principle: stimulate catecholamine secretion from tumour (stimulate symptoms)
• risks: extreme rise in BP (need phentolamine IV 10 mg standby)

Interpretation:
- NA peak>2000 pg/ml or >3x increase over baseline

Question 11

Neuroblastoma origin, prevalence, sites, presentation, screening/Dx

Answer 11

Malignant tumour from neuroblasts that has capacity to synthesise and secrete catecholamines

• > almost exclusively in children
• MC malignancy in 1st yr of life

Most are sporadic (1-2% familial)

65% intra-abdominal (adrenal gland or upper abdomen)
60% extra-adrenal

Presentation:

• tumour mass
• compression effects
• haematological abnormalities from BM involvement
• HT is rare

Screening and Dx:
- 24hr urine VMA and HVA (high specificity; plasma concentration too low)

Prognosis poor if VMA:HVA <1, high neuron-specific enolase/LDH/ferritin

Question 12

Carcinoid Tumour

Answer 12

Derived from enterochromaffin cells
Most frequently found in GI (64%) and respiratory tract (28%)

Develops in all age groups, mean age 63

Most patients asymptomatic until liver metastasis (carcinoid syndrome)
==> vasoactive substances not adequately metabolised by the liver –> enters systemic circulation
- bowel obstruction, abdominal pain, flushing, diarrhoea, wheezing, dyspepsia, hypotension, RHF

Question 13

Classification of Carcinoid

Answer 13

Foregut

• bronchus, lung, stomach, duodenum, pancreas
• -> secrete 5-hydroxytrytophan and histamine

Midgut

• ileum, jejunum, appendix, proximal colon
• -> large quantities of serotonin

Hindgut

• rectum, distal colon
• -> no serotonin or 5-hydroxytryptophan secretion

Question 14

Lab investigation of carcinoid tumours

Answer 14

24hr urine 5-HIAA (plasma concentration too low to analyse)
- preferably start collection when patient symptomatic

• midgut carcinoid usually >10x increase in 5-HIAA

False positive: - avoid 3 days prior and during

• serotonin-rich foods e.g. bananas, pineapples, chocolate, kiwi, cough medications with guaifenesin
• smoking may cause slight elevation due to nicotine stimulated release

False negative:
- suppressed 5-HIAA: alcohol, aspirin
———————————
Elevated ALP (cholestatic picture) due to SOL in liver = metastasis

Question 15

(extra) plasma vs urine sampling

Answer 15

Plasma –> low conc can make analysis difficult (blood collections require time, more convenient for patients, better regulated sampling, influences of diet easily controlled, applicable to ESRD)

Urine –> well established and widely available, analysis easy, easy to implement, inconvenient for patients, unreliable collections, difficult to control diet and daily life influences on sympathoadrenal function, not suitable for ESRD