In the vast majority of patients suffering from hypertension, their condition is primary – i.e. it has no identifiable cause. However, when secondary hypertension is suspected, an underlying cause is sought.
Three main groups of patients referred to the hypertension clinic are considered prime candidates for secondary, rather than primary, hypertension:
- Those with ‘young onset’ hypertension, i.e. under the age of 40
- Those already suspected of having a secondary cause for their hypertension
- Those who have resistant hypertension, i.e. despite taking 3 or more medicines, their blood pressure still isn’t controlled to target.
There are three initial screening tests available for diagnostic purposes:
- A blood test, checking for the blood pressure hormones, aldosterone and renin, and also kidney function. By far the most common cause of secondary hypertension is hyperaldosteronism, i.e. too much aldosterone in the blood. Cholesterol is also measured in the blood. This is because, although high cholesterol does not affect blood pressure, it does increase the risk of a heart attack or stroke, which is the broader perspective of treating high blood pressure.
- A 24-hour urine test, using a bottle containing acid, designed to measure the breakdown products of adrenaline, called metanephrines, to see whether adrenaline is being overproduced.
- Another type of 24 hour urine test, using a plain bottle to check the level of urinary salt excretion. This finding will closely mirror the actual amount of salt in the food consumed during this period. Cortisol is also measured, as in excess it can cause Cushing syndrome, which may be the underlying cause of hypertension in some patients.
Further tests may be conducted, in the form of an MRI or CT scan, looking for any narrowings in the arteries supplying the kidneys or any structural kidney disorder. The adrenal glands are also imaged, as they produce the blood pressure hormones aldosterone, cortisol and adrenaline. Finally, the main blood vessel, the thoracic aorta, is checked for any narrowing (called coarctation).
Some patients, such as those with a metallic implant, pacemaker, hearing aid or anxiety may not be candidates for an MRI (the preferred scan) and they will be offered a CT. Pregnant or breastfeeding women will be investigated with an ultrasound scan of the kidneys.
Once the primary cause is identified, doctors will advise on any further investigations and treatment options.
Blood pressure readings are taken with the patient seated and then on standing, to check for postural hypotension (where BP falls significantly on standing). Also home or ambulatory BP monitoring over 24 hours may be offered, to rule out the ‘white coat effect’ (BP higher in a clinical environment than at home).
If secondary hypertension has been excluded, then treatment of primary hypertension is advised with a combination of addressing relevant lifestyle factors and anti-hypertensive medication. Where patients are already taking more than one medication then this may be reviewed to find the most effective medication combination.