Transcranial Doppler (TCD) Test: What It Shows in POTS

What Is a Transcranial Doppler Test?

TCD stands for transcranial Doppler: transcranial means through the skull, and Doppler is the ultrasound method that measures speed. It's sometimes called a brain Doppler. It's an ultrasound of the arteries inside your head. Small probes rest on both temples, held in place by a light harness, and aim through the temple, just above the cheekbone, at the middle cerebral arteries, which supply much of each side of the brain. The method was first described in 1982 (Aaslid, 1982). A standard TCD uses no needles and no radiation, and it's painless (Tegeler, 2013).

Neurologists have used TCD for decades. A panel of ultrasound experts lists its established uses in stroke and narrowed or blocked arteries in the head, stroke risk in sickle cell disease, artery spasm after bleeding around the brain, brain death exams, monitoring during surgery and finding right-to-left shunts, abnormal connections that let blood skip the lungs, which are checked by injecting tiny bubbles (Alexandrov, 2012). Researchers have long used it to study how the brain keeps its own blood flow steady as blood pressure changes (Willie, 2011). MRI and CT show the brain's structure at 1 moment while you lie still, and TCD follows blood flow continuously while you change position. Our team brought it into the clinic because we kept seeing people fall apart on the tilt table while their heart rate and blood pressure looked fine, and we needed a way to see what was happening upstream.

What Does a Transcranial Doppler Measure?

TCD measures blood flow velocity: how fast blood is moving through the artery, in centimeters per second. It doesn't measure the volume of blood directly. TCD assumes the artery keeps the same width. In a small study, the artery narrowed slightly when volunteers breathed off carbon dioxide, so velocity understated how much blood flow fell (Coverdale, 2014). Carbon dioxide also changes brain blood flow on its own, because breathing it off narrows the brain's blood vessels (Ainslie, 2009). That's why our team records the carbon dioxide you breathe out during the test. Velocity is the number this test reports, and our team reads a drop in velocity as a drop in blood flow.

The numbers on a TCD report include:

• Mean velocity on each side. In healthy adults the average in the middle cerebral artery is about 62 cm/s with standard TCD, with wide variation between people (Aaslid, 1982), and it falls with age (Krejza, 1999). In healthy people the 2 sides are about equal (Tegeler, 2013).
• Change from your own lying baseline. On a tilt test this matters more than the raw number, because normal values vary so much from person to person.
• Pulsatility index, how much the velocity swings with each heartbeat compared with its average. It's often read as resistance further downstream, though it depends on several factors at once (de Riva, 2012).

How Is a Transcranial Doppler Tilt Test Done?

You lie on the table with a probe at each temple held by a light harness, a heart monitor, a finger cuff for blood pressure and a capnography sensor that measures the carbon dioxide at the end of each breath (end-tidal CO2). After a period of lying still to set your baseline, the table tilts you to 70 degrees, which is like standing and lying at the same time. A published autonomic lab protocol follows the same pattern, with 10 minutes lying down and 10 minutes or more tilted (Novak, 2016b).

Everything is recorded continuously, and our team reviews it minute by minute while asking what you're feeling. When fast heart rate is the main problem, the answer usually shows up within the first 10 minutes. When someone has a history of fainting or of blood pressure drops, our team runs the test longer to find the breaking point. If symptoms get too strong, the table comes back down. In our team's experience, medicines like midodrine are often paused before a tilt test, and that's a decision for your prescriber.

Our team also records TCD while people do mental math, move their eyes, turn and tip their head, or have their body turned under a still head. Changes during those tasks help show which pathways are involved.

Why Add a Transcranial Doppler to a Tilt Table Test?

A standard tilt test diagnoses POTS in someone with symptoms on standing when heart rate rises at least 30 beats per minute (40 at ages 12 to 19) within 10 minutes of being upright, without orthostatic hypotension (Sheldon, 2015; Vernino, 2021). It doesn't show what reaches the brain, and our team's view is that symptoms like lightheadedness and brain fog start there.

Tilt studies with TCD show blood flow velocity to the brain falling further in POTS than in healthy people: 19.5% against 10.3% in 1 small study, where the POTS patients had normal carbon dioxide (Ocon, 2009), and 19% against 3% in another, where the drop came with low exhaled carbon dioxide (Novak, 2018), a pattern called hypocapnic cerebral hypoperfusion. The drop can also happen with a normal heart rate. In 1 autonomic lab, 102 of 1,279 people tested for unexplained dizziness on standing had an abnormal drop in brain blood flow velocity without orthostatic hypotension. In those 102, it fell 24% on average during the tilt, against 4% in matched controls, while heart rate and blood pressure stayed normal (Novak, 2016b). Novak named this orthostatic cerebral hypoperfusion syndrome (OCHOS). In another study, 183 people whose standard tilt test came back normal despite symptoms still had a bigger drop in blood flow velocity at the peak of each heartbeat than 50 healthy people (Shin, 2016).

Our team's view is that the high heart rate in POTS is often a compensation: the body speeds up the heart to push more blood toward the brain. In 1 study of POTS with breathlessness on standing, brain blood flow velocity fell first, about 17 seconds before breathing sped up, followed by the nerve activity that raises heart rate (Del Pozzi, 2014). Recording TCD continuously alongside heart rate helps our team see that order and avoid treating the compensation as if it were the cause.

What Does a TCD Tilt Test Look Like in a Real Case?

A college student with POTS came to the clinic lightheaded and short of breath, with skipped heartbeats (premature ventricular contractions) and a lot of anxiety around them. Lying down, her blood flow velocities were 71 and 74 cm/s on the 2 sides. In the first minute of the tilt, the left side held at about 94% of its baseline, but the right side fell to 46%, with a high pulsatility index on that side. The right side recovered much of the way in the next minute and then hovered around 85 to 86%, which met our team's target, though it sat just under the 89% a published lab uses at minute 5 (Novak, 2016b).

Her exhaled carbon dioxide ran low, but in our team's reading not low enough to explain a drop that size. Her blood pressure rose the way it should. Her heart rate went from 77 to 143, then came down to 123, 97, 115 and 99. At minute 8, when she was asked to count backward by 7s, she couldn't continue, and the test was stopped.

Next, seated with the TCD still on, she turned, bent and tipped her head. Turning right cut blood flow velocity on the left side by at least 20%, turning left cut the right side, and bending the head forward, back or to the right cut the left side. Our team then held her head still and turned her body underneath it, which takes the inner ear and eyes out of the movement and mainly strains the neck. The same pattern showed up, which pointed to the neck as a contributor. This head-position testing is our team's own method, and it can't fully separate a structural neck problem from the brain's sense of where the head is. Head turns also brought on her skipped beats.

Her 10 days of care focused on control and stability of her neck, the posture underneath it, and the way her brain processed head position, since our team also found nystagmus (jumpy eye movements) that it attributes to the neck. On the retest 10 days later she finished all 10 minutes of the tilt. Her blood flow readings stayed in the 90s and 100s as a percentage of that day's baseline. Her absolute velocities were lower than on the first test, still in a healthy range for her age in our team's reading, and her exhaled carbon dioxide was trending up, which raises velocity on its own. Her heart rate spike was about 20 beats lower, and every head movement test came back clean. She still had work ahead of her, and that's typical: some people improve quickly and some slowly. This is 1 case, and other people's results will differ.

How Do TCD Results Change Treatment?

For our team, TCD earns its place in 4 ways:

• Finding the location. When 1 side of the brain drops more than the other, as it did for the student above, our team looks for causes that fit a side difference, such as a head injury, a neck injury or a blocked vessel.
• Separating the problem from the compensation. If brain blood flow drops first and the heart rate climbs to make up for it, slowing the heart down takes away the body's best fix. Heart rate medicines help many people feel better, and changing one is a decision for your prescriber. Our team wants to know which came first before deciding what to treat.
• Checking whether a treatment works. Midodrine tightens blood vessels to raise blood pressure, and it can help: in a small crossover trial of 20 people aged 12 to 20, it tightened leg blood vessels and lowered the upright heart rate in neuropathic POTS (weak nerve signals to the blood vessels), though in hyperadrenergic POTS (high norepinephrine) the heart rate fell on placebo too, so midodrine added nothing (Ross, 2014). A woman our team tested had low brain blood flow on the tilt while off her midodrine. When she was retested on it about 1 to 2 weeks later, at the same time of day, her brain blood flow was worse. Seeing that changed her plan, and she later held normal brain blood flow, blood pressure and heart rate without it. That's 1 woman's result: midodrine helps many people, and medication changes belong with your prescriber.
• Dosing rehab and seeing progress. Watching blood flow in real time lets our team give enough of an exercise to drive change without pushing someone into a crash, and in our team's experience, rising numbers can show a system starting to work again before symptoms fully catch up, though they don't mean you're healed yet.

A normal TCD is useful too: it tells our team your brain's supply held up, so the search moves to other causes.

Why Isn't TCD Part of Standard POTS Testing?

POTS is defined by symptoms on standing with a heart rate rise and no orthostatic hypotension, so the standard workup centers on heart rate and blood pressure (Sheldon, 2015; Vernino, 2021). TCD isn't part of POTS guidelines, the published tilt studies with it are small, and it hasn't found its way into most clinics. In 1 series of 624 people aged 2 to 86, no signal could be found through the temple in about 8%, more often in women and older adults (Marinoni, 1997). In our team's experience, the bigger reason is that a TCD result doesn't change the label: many doctors assume brain blood flow is probably reduced in POTS and treat it with medication either way.

Our team uses TCD alongside a careful neurological exam, because the numbers show that blood flow dropped and the exam of your eyes, balance, reflexes and neck helps explain why.

Transcranial Doppler Test: FAQ

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