The symptoms are abating but I’m very, very unsure when it would be safe or wise to resume my meditation practice, as I’m now leaning toward a psychological/energetic explanation over a physical abnormality.
Based on your descriptions, I'd suggest that it's far more likely to be meditation-related rather than a physical abnormality. If you're still concerned about this as a possibility though, obviously seeking more medical advice is the way to go.
Originally I did not think my meditations were long or intense enough to qualify as an explanation, but now I am not so sure.
Insight meditation done properly, and even without consciously knowing that you're actually doing it, is incredibly powerful. If you've been practicing regularly since March then it's likely that these issues are related to your practice.
1. Approximately three weeks ago I started having attacks of groundless fear, confusion, and a sense that I might be losing my grip on reality that would last several hours.
Can you recall how you felt prior to this? Do you remember there being a notable 'spiritual' experience prior to this fear arising?
2. These attacks occurred about every 5 days.
Is this an approximation or have you recorded a 5-day cyclical pattern to these feelings?
3. There is no precedent in my psychology for these attacks and they mystify and frighten me.
Don't worry about it, this sort of thing is actually par for the course when doing vipassana properly. Regardless of how intense it may get, always remember that none of these feelings are permanent and that they will change without your input. To fight with these feelings or try to change them is to invite more stress and tension, learn to simply experience them as they occur and look directly at their impermanence, their emptiness and inherently unsatisfactoriness.
4. I have been doing samatha and vipassana meditation for 1 – 1.5 hours daily since March, following the books and podcasts of Alan Wallace. The emphasis was on samatha. Prior to this formal meditation interval, my primary practice was self-enquiry in the Advaita tradition of Ramana Marharshi, although in an informal sense.
The self-enquiry method is essentially a form of insight practice; bare attention paid to the sensations implying an "I". It's worth knowing that, in the Suttas, there is no distinction between samatha and vipassana so it's not unlikely that you've been inadvertently doing insight practice alongside concentration.
5. I have also been doing lucid dreaming practice and dream yoga, regularly comparing waking life to a dreamlike state.
This process of comparing waking life to the dream state leads one to look more closely at the phenomenology of your experience, the way that sensations are perceived or the emotions arising and how they differ from everyday life. Tenzin Wangyal Rinpoche has an excellent book on this which I recommend highly, if you're not already aware of it.
7. I have been experiencing pressure in the back of my neck and various places in my head.
These sorts of physical sensations make me think that, in terms of the Progress of Insight model, you're either cycling up to 10th ñana, Knowledge of Re-Observation, or you're hitting 3rd ñana, Knowledge of the Three Characteristics. The intensity of the experiences you describe suggest 10th ñana as I always experience this sort of discomfort in the upper chest and head area, whereas 3rd ñana tends to be more of a whole-body thing. Just a suggestion, I may be totally wrong.
8. My qigong and yoga practice seem to help the situation.
I'd be quite confident in saying that you're in the dukkha ñanas, a.k.a. Dark Night. A lot of people find that these sorts of energetic practices are very helpful in negotiating these stages so it's good that you've found a way to manage it somewhat.
1. One week before the attacks began I had extreme vertigo every time I rolled over in bed. I tentatively believe this to be unrelated to the fear attacks and to be a common condition called “Benign Paroxysmal Positional Vertigo”. This condition only lasted about 24 hours.
I honestly can't say whether or not there is actually any connection between the two, although I have experienced similar things while cycling through Dark Night, particularly in late 5th ñana and early 6th, Knowledge of Dissolution moving into Knowledge of Fear.
2. The fear attacks are almost always accompanied by gastrointestinal distress. I’ll spare you the details. Is there perhaps a connection with the enteric nervous system?
It
might be related to going through 8th ñana, Knowledge of Disgust, but I don't know for certain. I used to experience a lot of nausea and discomfort, but never to the point of gastrointestinal distress. I have no idea about the connection to the enteric nervous system, to be honest.
3. The first attack was preceded (6 hours) by a feeling of vague discomfort after taking 8mg of Galantamine during the afternoon as a supplement to provoke a lucid dream during a nap.
I think Tom. A. Vitale has answered this better than I possibly could.
4. Two months before the first attack I participated in a shamanistic ceremony in the Peruvian Amazon.
I'm very interested in how entheogenic experiences might work to bring one to similar, albeit considerably amplified, experiences to those encountered via insight practice. I know of several people who crossed 4th ñana, Knowledge of the Arising & Passing Away (A&P), while under the influence of LSD and so I wonder whether or not it's possible to utilize entheogens in a similar way. With this in mind, I wonder whether or not you've maybe encountered what the Progress of Insight model describes as the A&P during your shamanic experience. It's just an idea I thought I'd throw out there.
Hope that helps in some way but feel free to ask whatever you like.