Diagnosis and Autonomic Testing

Diagnosing Pure Autonomic Failure (PAF) is a process of “connecting the dots” between your symptoms and the physical evidence of nerve failure. Because it is a rare condition, doctors use a series of specialized tests to confirm that your autonomic system is failing and to rule out other conditions like Parkinson’s disease or Multiple System Atrophy (MSA) ^[1][2].

A Diagnosis of Exclusion

It is important to know that PAF remains a clinical diagnosis. There is no single test that confirms the failure is completely “pure.” Instead, doctors must rule out many other causes of nerve failure. Your clinician will review your medications (as many blood pressure or prostate drugs cause orthostatic hypotension) and check for dehydration, anemia, diabetes, endocrine diseases, B12 deficiency, underlying heart conditions, and systemic diseases like amyloidosis or autoimmune autonomic ganglionopathy ^[1]. Only when these are ruled out and there are no central neurological signs (like tremors) does PAF become the likely diagnosis.

Confirming Neurogenic Orthostatic Hypotension (nOH)

The first step in diagnosing PAF is confirming that your blood pressure drops because of nerve failure, rather than simple dehydration or heart issues ^[3]. This is done by measuring your orthostatic vital signs—your blood pressure and heart rate while lying down and after standing for at least three minutes ^[4][5].

In PAF, the criteria for a diagnosis include:

• The Drop: A sustained fall in blood pressure of at least 20 mmHg (systolic) or 10 mmHg (diastolic) within 3 minutes of standing or being tilted upright ^[6][5].
• The Heart Rate: A hallmark of neurogenic (nerve-related) failure is that your heart rate does not rise enough to make up for the drop ^[7]. In a healthy person, the heart beats faster to push blood to the brain; in PAF, this response is blunted ^[8]. (Note that this response is only suggestive, as age, volume status, and medications like beta-blockers can alter it.)

Specialized Autonomic Testing

If basic vitals suggest a problem, you may undergo a formal autonomic reflex screen in a specialized lab ^[9]. These tests look at three different parts of your autonomic system:

1. Tilt-Table Test: You are secured to a table that tilted upright (usually to 60 or 70 degrees) while your heart rate and blood pressure are monitored continuously ^[6]. This is more sensitive than standing vitals and can detect “delayed” drops that happen after several minutes ^[10].
2. QSART (Quantitative Sudomotor Axon Reflex Test): Small capsules are placed on your skin (usually the arms and legs) to measure how your sweat glands respond to a mild electrical stimulation ^[9]. (“Sudomotor” refers to the nerves that control sweating). This helps determine if the nerves controlling your sweating are damaged.
3. Thermoregulatory Sweat Test (TST): In this test, you are coated in a powder that changes color when it gets wet and placed in a warm room ^[11]. This creates a “map” of where you can and cannot sweat, helping doctors see the extent of your nerve involvement ^[12].

The Role of Norepinephrine

Your body uses a chemical called norepinephrine as a messenger to maintain blood pressure. Measuring the levels of this chemical in your blood is one of the ways doctors distinguish PAF from other conditions, though it is supportive rather than purely diagnostic ^[13].

• In PAF: Because the peripheral (postganglionic, or outer) nerves are damaged, they release less norepinephrine. This often leads to very low supine plasma norepinephrine levels (measured while you are lying down) ^[13][14].
• In MSA: Because MSA primarily affects the brain rather than the peripheral nerves, norepinephrine levels in the blood are often normal or only slightly reduced ^[14][15].
  Important Caution: Results depend heavily on posture, collection conditions, age, and medications. MSA values can overlap with PAF values. Do not stop any medications for testing unless explicitly instructed by your clinician.

Alpha-Synuclein Skin Biopsies

A newer tool in the diagnostic toolkit is a skin biopsy to look for phosphorylated alpha-synuclein—the misfolded protein that causes PAF ^[16].

A small punch biopsy (about the size of a pencil eraser) is taken from three or four spots, such as the ankle, thigh, and neck ^[17][18]. If the biopsy shows this protein within your small nerve fibers, it supports that you have a “synucleinopathy” (a disease caused by this protein) ^[16].

While a positive biopsy is very helpful, its interpretation is still center- and method-dependent. It does not distinguish between PAF, Parkinson’s, or MSA on its own, as all three involve this protein ^[17][19]. Instead, your doctor will look at the distribution of the protein and your clinical symptoms ^[20][17]. Areas of nerve loss in the heart (detected by cardiac MIBG imaging) can also support a pattern of cardiac sympathetic denervation in PAF over MSA, but this imaging is affected by cardiac disease, diabetes, and medications, and requires clinical correlation ^[19][21].