user471: Health anxiety is defined in the quote below. A person suffering from it may mistakenly believe certain fluctuations in bodily sensations to be dangerous or a sign of impending critical illness, and this may cause them to become anxious. When they become anxious, they may get symptoms of anxiety (like rapid heart rate and chest discomfort) which may lead to a positive feedback loop, resulting in a panic attack, where they may think that they have a heart attack and go to the ER. That's the worst case. The best case is frequent unnecessary visits to the doctor.
Based on Buddhist techniques, what could a health anxiety sufferer do to help himself or herself?
On the other hand, if he really did have some critical illness, but convinces himself that it's not really the case, then it's not good for his health. So, how could Buddhist techniques help him find the balance?
According to this article on health anxiety:

The False Alarm
Health anxiety is the misinterpretation of normal bodily sensations as
  dangerous. Healthy bodies produce all sorts of physical symptoms that
  might be uncomfortable, painful, unexpected, and otherwise unwanted —
  but not dangerous. Picture a car with an alarm system. It’s useful if
  your car alarm goes off when a criminal is breaking in, but it’s
  problematic if it goes off every time someone walks by. Your car alarm
  would be misinterpreting innocent pedestrians as dangerous criminals.
Normal physical symptoms that often produce fear and worry include
  changes in visual acuity, heart rate and blood pressure, saliva
  levels, depth of breathing, balance, and muscle tone, to name a few.
  These are normal and harmless. But when a person misinterprets them as
  symptoms of some terrible disease, it creates undue worry. This
  explains why medical tests come out negative: The physical sensations
  are real, but they are not symptoms of a disease.
Overestimating Danger
Misinterpretation may be due to assumptions about health and illness,
  such as, “My cousin died of cancer, so it’s only a matter of time for
  me.” Or, “Viruses spread quickly. Since people in Africa are dying of
  Ebola, it could easily spread to the United States.” People with
  health anxiety might hold rigid definitions of good health, perhaps
  believing that any discomfort means bad health.
If they hear a news story about a few cases of a serious virus, people
  with health anxiety might start scanning their own bodies for symptoms
  of the virus. Looking for symptoms makes you notice subtle sensations
  that you might otherwise ignore. With uncertainty, the imagination has
  room to create stories. And that’s when your body’s alarm sounds off
  as you imagine the worst.
It Gets Tricky
Symptoms of anxiety produce very real physical symptoms: Dizziness,
  stomachaches, rapid heartbeat, tingling in the hands and feet, muscle
  tension, jitteriness, chest pressure, and the list goes on. These
  symptoms add fuel to the fire. Now you have real evidence that
  something is seriously wrong. Or do you? Perhaps it’s anxiety. So how
  do you know if these symptoms are serious? You go to the doctor… and
  then to a therapist.
Health anxiety persists despite reassurance from the doctor. Seeking
  reassurance from doctors, insisting on repeated medical tests, and
  visits to the ER and urgent care are common if you have health
  anxiety. This habit leads you to rely on such reassurance to obtain
  relief from health worries. A vicious cycle develops of noticing a
  sensation or learning of an illness in the world, misinterpreting it
  as threatening, then becoming anxious, and finally going to the doctor
  for reassurance. Reassurance from the doctor reduces the anxiety and
  brings relief temporarily. Soon the cycle starts again.


user-1: Reassurance from doctors or knowledge doesn't really work. I know it experientially.
Doctor's reassurance results in finding another doubt where the former one has been patched. It is temporary ease in the cycle of aversion and desire, typical samsaric, vicious cycle. Aversion from feeling anxiety and feeling ill that results in the desire of reassurance is the main agent that spawns endless knots, knots that have even aversions towards knots. Mind is quite crafty and will always try to find a hole in the new logic-driven defence line. It is, therefore, not through intellectual reassurance that one gets released from such trapping. Wisdom should arise naturally, through a mood change, when body and mind are calm.
Now, for being trapped in anxiety best you might do is to focus on breathing and note that you are feeling anxiety, if you are feeling pain, then note that you are feeling pain, all with kindness if possible. Then, thoroughly check your body reaction(s). Try to do mindfulness of scanning the body down and notice these tensions. For instance, typically you will have tense abdomen muscles and tense jaw area, mindfulness of the body will let you see it as its a fight or flight response. Muscles should then let loose after acknowledged, furthermore, combined with breath - worry will wane.
Another thing is not to cling or have aversion to anything and importantly, remember three characteristics, especially impermanence - everything will eventually die or dissolve. We should not feel aversion toward chronic, or terminal condition, illness.
We have to navigate through confusion to calm abiding, illuminating clarity and make peace with the bodily condition.

user43: Looking at Pali Canon... here is what I see. (It would take too much space to quote every idea, so I'm just summarizing it here.)

Look at your condition and check whether the medical situation is objectively improving or getting worse. If it's not getting worse - that's a reason to calm down. If it's improving - that's a reason to be glad. This should create a positive feedback cycle that will improve the situation.
If situation is getting worse, contact a doctor if you can. Otherwise, ask yourself if you have done anything bad that you regret, or whether you have done anything good that you can be proud of. If there are no major regrets and something to be proud of - that's a reason to be glad. This should create a positive feedback cycle that will improve the situation. If there is something to regret - you should repent and promise to yourself to learn from that mistake and not repeat again. A firm decision will make you feel inspired and create a positive feedback cycle that will improve the situation.
If you still feel bad, you should review the elements the person is made from and let go of any identification with them. Starting from physical, and then the information of the mind, and finally subjectivity (representation). Review how this works based on objective laws understood by science and let go of identification with self. Look at things philosophically and impersonally until you enter a detached state of mind. 

In general, try to switch attention to objectively valid reasons to be happy and detached about things. There is always something you can find if you try.

user111: When the patient feels sensation, he can practice Kayanupassana(mindfulness of the body) and Vedananupassana(mindfulness of feelings). When the thoughts of worry come to the mind the patient can practice Cittanupassana(mindfulness of mental activities) and Dhammanupassana(mindfulness of realities).
This technique will reduce the worry that leads to anxiety.

471: <p>Health anxiety is defined in the quote below. A person suffering from it may mistakenly believe certain fluctuations in bodily sensations to be dangerous or a sign of impending critical illness, and this may cause them to become anxious. When they become anxious, they may get symptoms of anxiety (like rapid heart rate and chest discomfort) which may lead to a positive feedback loop, resulting in a panic attack, where they may think that they have a heart attack and go to the ER. That's the worst case. The best case is frequent unnecessary visits to the doctor.</p>

<p>Based on Buddhist techniques, what could a health anxiety sufferer do to help himself or herself?</p>

<p>On the other hand, if he really did have some critical illness, but convinces himself that it's not really the case, then it's not good for his health. So, how could Buddhist techniques help him find the balance?</p>

<p>According to <a href="https://adaa.org/understanding-anxiety/related-illnesses/health-anxiety" rel="nofollow noreferrer">this article</a> on health anxiety:</p>

<blockquote>
  <p><strong>The False Alarm</strong></p>
  
  <p>Health anxiety is the misinterpretation of normal bodily sensations as
  dangerous. Healthy bodies produce all sorts of physical symptoms that
  might be uncomfortable, painful, unexpected, and otherwise unwanted —
  but not dangerous. Picture a car with an alarm system. It’s useful if
  your car alarm goes off when a criminal is breaking in, but it’s
  problematic if it goes off every time someone walks by. Your car alarm
  would be misinterpreting innocent pedestrians as dangerous criminals.</p>
  
  <p>Normal physical symptoms that often produce fear and worry include
  changes in visual acuity, heart rate and blood pressure, saliva
  levels, depth of breathing, balance, and muscle tone, to name a few.
  These are normal and harmless. But when a person misinterprets them as
  symptoms of some terrible disease, it creates undue worry. This
  explains why medical tests come out negative: The physical sensations
  are real, but they are not symptoms of a disease.</p>
  
  <p><strong>Overestimating Danger</strong></p>
  
  <p>Misinterpretation may be due to assumptions about health and illness,
  such as, “My cousin died of cancer, so it’s only a matter of time for
  me.” Or, “Viruses spread quickly. Since people in Africa are dying of
  Ebola, it could easily spread to the United States.” People with
  health anxiety might hold rigid definitions of good health, perhaps
  believing that any discomfort means bad health.</p>
  
  <p>If they hear a news story about a few cases of a serious virus, people
  with health anxiety might start scanning their own bodies for symptoms
  of the virus. Looking for symptoms makes you notice subtle sensations
  that you might otherwise ignore. With uncertainty, the imagination has
  room to create stories. And that’s when your body’s alarm sounds off
  as you imagine the worst.</p>
  
  <p><strong>It Gets Tricky</strong></p>
  
  <p>Symptoms of anxiety produce very real physical symptoms: Dizziness,
  stomachaches, rapid heartbeat, tingling in the hands and feet, muscle
  tension, jitteriness, chest pressure, and the list goes on. These
  symptoms add fuel to the fire. Now you have real evidence that
  something is seriously wrong. Or do you? Perhaps it’s anxiety. So how
  do you know if these symptoms are serious? You go to the doctor… and
  then to a therapist.</p>
  
  <p>Health anxiety persists despite reassurance from the doctor. Seeking
  reassurance from doctors, insisting on repeated medical tests, and
  visits to the ER and urgent care are common if you have health
  anxiety. This habit leads you to rely on such reassurance to obtain
  relief from health worries. A vicious cycle develops of noticing a
  sensation or learning of an illness in the world, misinterpreting it
  as threatening, then becoming anxious, and finally going to the doctor
  for reassurance. Reassurance from the doctor reduces the anxiety and
  brings relief temporarily. Soon the cycle starts again.</p>
</blockquote>
 -1: <p>Reassurance from doctors or knowledge doesn't really work. I know it experientially.</p>

<p>Doctor's reassurance results in finding another doubt where the former one has been patched. It is temporary ease in the cycle of aversion and desire, typical samsaric, vicious cycle. Aversion from feeling anxiety and feeling ill that results in the desire of reassurance is the main agent that spawns endless knots, knots that have even aversions towards knots. Mind is quite crafty and will always try to find a hole in the new logic-driven defence line. It is, therefore, not through intellectual reassurance that one gets released from such trapping. Wisdom should arise naturally, through a mood change, when body and mind are calm.</p>

<p>Now, for being trapped in anxiety best you might do is to focus on breathing and note that you are feeling anxiety, if you are feeling pain, then note that you are feeling pain, <strong>all with kindness if possible</strong>. Then, thoroughly check your body reaction(s). Try to do mindfulness of scanning the body down and notice these tensions. For instance, typically you will have tense abdomen muscles and tense jaw area, mindfulness of the body will let you see it as <strong>its a fight or flight response</strong>. Muscles should then let loose after acknowledged, furthermore, combined with breath - worry will wane.</p>

<p>Another thing is not to cling or have aversion to anything and importantly, remember three characteristics, especially impermanence - everything will eventually die or dissolve. We should not feel aversion toward chronic, or terminal condition, illness.</p>

<p>We have to navigate through confusion to calm abiding, illuminating clarity and make peace with the bodily condition.</p>
 43: <p>Looking at Pali Canon... here is what I see. (It would take too much space to quote every idea, so I'm just summarizing it here.)</p>

<ol>
<li>Look at your condition and check whether the medical situation is objectively improving or getting worse. If it's not getting worse - that's a reason to calm down. If it's improving - that's a reason to be glad. This should create a positive feedback cycle that will improve the situation.</li>
<li>If situation is getting worse, contact a doctor if you can. Otherwise, ask yourself if you have done anything bad that you regret, or whether you have done anything good that you can be proud of. If there are no major regrets and something to be proud of - that's a reason to be glad. This should create a positive feedback cycle that will improve the situation. If there is something to regret - you should repent and promise to yourself to learn from that mistake and not repeat again. A firm decision will make you feel inspired and create a positive feedback cycle that will improve the situation.</li>
<li>If you still feel bad, you should review the elements the person is made from and let go of any identification with them. Starting from physical, and then the information of the mind, and finally subjectivity (representation). Review how this works based on objective laws understood by science and let go of identification with self. Look at things philosophically and impersonally until you enter a detached state of mind. </li>
</ol>

<p>In general, try to switch attention to objectively valid reasons to be happy and detached about things. There is always something you can find if you try.</p>
 111: <p>When the patient feels sensation, he can practice Kayanupassana(mindfulness of the body) and Vedananupassana(mindfulness of feelings). When the thoughts of worry come to the mind the patient can practice Cittanupassana(mindfulness of mental activities) and Dhammanupassana(mindfulness of realities).</p>

<p>This technique will reduce the worry that leads to anxiety.</p>
