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Epilepsy is a condition that seems to encompass many possible things:

A neurological disorder marked by sudden recurrent episodes of sensory disturbance, loss of consciousness, or convulsions, associated with abnormal electrical activity in the brain.

I've read that people with photosensitive epilepsy can react badly with exposure to flashing light, or irregular light patterns (such as sunlight peeking through trees in a forest).

Is it also true that tiredness can trigger seizures/passing out (for people with epilepsy), or, in particular, exposure to too much heat? Some people have told me this, but none of them are medical professionals and I'm not either. This website gives examples such as lack of oxygen/infections/stroke, but it's not clear from anywhere I've looked if epileptic attacks could ever be caused by excessive exposure to heat.

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Something that is worth mentioning right out of the gate is a febrile seizure (see also The Epilepsy Foundation). These are sudden seizures, generally experienced by young children, resulting from an especially high fever. The generally accepted (although not fully confirmed) mechanism is via something called respiratory alkalosis (see e.g. Mazarati (2007)). Hyperthermia, among other processes1, triggers an imbalance in blood pH and pCO2, which in turn can increase brain activity, leading to a seizure. Children (generally below the age of six) are especially vulnerable.

With that in mind, it would make sense that heat could be a possible seizure trigger. That said, there are some things to bear in mind. First, children who experience a febrile seizure are likely to never have a seizure again; these seizures are generally not linked to epilepsy, although a family history of seizures may contribute to this. Therefore, there are two questions to ask: Can heat stroke cause seizures, and is this more prevalent for people with epilepsy?

We also need to consider that instances of heat stroke can be quite different. Often, it is convenient to divide cases into two categories: Exertional heat stroke (EHS), and non-exertional/classical heat stroke (NEHS/CHS). Ferri's Clinical Advisor 2017 gives a rather nice comparison on page 551. There are a few differences that pertain to our case, some of which I've copied here:

Patient Characteristics |     Classic     | Exertional 
--------------------------------------------------------
Age                     | Young children/ |  15-55 yr
                        |   the elderly   |
--------------------------------------------------------
Fever                   |     Unusual     |   Common
--------------------------------------------------------
Activity                |    Sedentary    |  Strenuous
                        |                 |   exercise
--------------------------------------------------------
Acid-base disturbances  |   Respiratory   |   Lactic
                        |    alkalosis    |  acidosis

Notice that respiratory alkalosis is described as more common classic heat stroke, which afflicts children, whereas lactic acidosis is more prevalent in exertional heat stroke. Interestingly enough, I haven't been able to find any information on the latter causing seizures, although I have seen references indicating it can arise as a byproduct of seizures, especially tonic-clonic ("grand mal") seizures.

It is, of course, well-known that heat stroke can trigger once-off seizures in people, often including small children (which should be apparent from some of the above). This has been well-studied in, of all creatures, dogs, which have unfortunately high mortality relates, often from being left trapped in a car or other area which rapidly becomes very hot. One book states that an astonishing 35% of dogs who suffer from heat stroke experience seizures in the process; this may arise from cerebral edema (Wikipedia) or something else. I don't have comparable statistics for humans (I assume the mortality rates are better than the 50%+ I've seen quote for dogs), but it's clear that heat stroke can, and often will, cause seizures.

Again, though, these seizures - like febrile seizures - generally do not recur. They are distinct and separate from epileptic seizures. Therefore, our second question is still unanswered.

Countless forums (why people use all caps is beyond me) are full of stories of people with epilepsy suffering seizures purportedly due to heat stroke, heat exhaustion, or something similar. I'm assuming these are cases of exertional heat stroke. However, these seizures are not necessarily caused by epilepsy. They may be entirely coincidental. A person with epilepsy may have a seizure brought on by something unrelated to the primary cause of their epilepsy.

There are some exceptions - but they occur predominantly in children. I stated before that children who experience a febrile seizure are likely to never have another. This is true, but there are underlying conditions which may produce febrile seizures repeatedly, or, rather, make it more likely for heat-related seizures to occur. Two important ones are Generalized epilepsy with febrile seizures plus (GEFS+) and Dravet syndrome, also known as Severe Myoclonic Epilepsy of Infancy (SMEI). The latter may be regarded as a special case of the former. Both are genetic, caused by mutations in the SCN1A gene; they may or may not be inheritable.

Essentially, mutations in SCN1A (or perhaps another gene) occasionally inhibits sodium channels from working properly, which in term can lead to a seizure. The links between high temperatures and the channels are more tenuous, though Sun et al. (2012) have suggested that in cases of GEFS+, heat may impact the voltage across the channels, further impacting their ability to properly function (see also Oakley et al. (2008), studying mice with Dravet Syndrome).

In conclusion, then, it appears that

  1. Heat stroke can indeed cause seizures, especially in children and small animals. Fevers can have the same effects, and may be more severe. However, all of these seizures are normally one-time events.
  2. There are underlying diseases which result in epilepsy with heat sensitivity. Repeated febrile seizures may arise as a result. However, these are usually - though not always - limited to young children. Most people with epilepsy likely do not have to worry about being "triggered" by heat much more than most people do.

I do think that an appropriate analogy to draw here is with photosensitive epilepsy, as you did. While rapidly flashing lights can lead to a seizure, this is generally limited to a subset of those with epilepsy.


1 Hyperventilation is another possible seizure trigger, acting via a similar mechanism. I'm guessing you're more familiar with it.

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