First, you need to understand about receptors. Think lock and key... the lock is something called a receptor - itís found on the surface of a cell and the key is the chemical (drug, hormone, neurotransmitter, etc) that fits into that particular lock. Specific chemicals have specific receptors that are activated by that chemical. This is basically the way all the cells in our body communicate with each other. Any chemical, be it from within (like insulin or oxytocin) or from without (like morphine or pitocin) has its effect only because our cells have receptors that those chemicals fit into. When a chemical attaches to a receptor, the inside of the cell is signaled to "do something" - it might make it secrete another chemical, or contract (in the case of muscles) or make a specific protein or stop doing something or whatever- but that's the way cells get "told" to "do something".
Some of our cells have receptors for substances called "opiates"- these are chemicals that are either derived from or modeled after opium- a natural substance from opium poppies (that's why you can have a positive drug test after eating poppy seeds, btw)- morphine, heroin, demoral, fentanyl, duramorph, stadol, vicodin, codeine- all are opiates (there's lots of others, too). There are actually 2 types of "opioid" receptors (mu and kappa- impress your anesthesiologist with that bit of info) that are of importance here- and they are found in the central nervous system (i.e. your spinal cord and brain). The reason all those different types of opiates have different affects, one from another, is because they each bind to the receptors differently: some bind for a short time, and so don't last as long, others bind longer and last longer. Some are easier to "pop" off and others harder- some bind more to the mu receptor, some only to the kappa receptor, some to both... all of this relates to how "strong" the drug is and what exactly its total effects are (obviously there's a difference between how you feel after getting codeine vs. shooting up heroin).
Here's where I don't remember the specifics, but suffice it to say, the action of the mu and kappa receptors interrupts your brain's interpretation of sensory stimuli (including pain). They also have the effect of depressing your respiratory function and making your cognitive function less than it should be (in other words, you get sleepy, woozy or high). An epidural or spinal is supposed to avoid those problems because the opiate doesn't reach the brain- just the receptors on the spinal cord, interrupting sensory function (making you unable to feel painful stimulus but hopefully not woozy, etc). Now, again, I don't recall the specifics but either it's part of what the mu or kappa receptor does or itís another opioid receptor but there is a receptor that in some people causes an itchy sensation when an opiate binds to it. It's pretty common but not universal and varies widely in severity from person to person (I get very mildly itchy but nothing that bothersome. I know women who itch so badly they have to be sedated).
It isn't a true allergy because an allergy is actually a response by your immune system to a substance that your body views as dangerously foreign (called an allergen) - and allergies to specific chemicals (like penicillin or bee venom) can be deadly because they often get worse and worse to the point of anaphylactic shock (which can quickly kill you) each time you are exposed to the allergen. The itching problem with opiates is more an "adverse reaction" or "side effect", not an allergy. It shouldn't get worse each time, and it should be dose dependent (itch more the more you get) but you also probably won't "get over it". It is worth telling your anesthetist about because there might be something they can premedicate you with to make it less dramatic. However, if you say that you are "allergic" to morphine, most people will at least understand what you mean, even if you are being inaccurate.
This illustrates the difficulty of anesthesia and why physicians are pretty vague and mysterious about it: your receptors are unique to you. Yes, we all have mu and kappa receptors but there can be subtle variations in them, and different people can have different numbers and proportions of them- and that's why it's foolish to think that a standard dose of a drug will have exactly the same effect on every person that gets it. Some women have lousy pain control with opiates that wear off quickly, some are overdosed and canít feel breathing motions and are drugged for a long time. There's a big variation in how people respond to ANY drug and this includes all the drugs used in birth (includes things like pitocin, too): what might be safe for your neighbor could just be deadly for you or your baby (unusual, but it does happen). We call those "anesthetic accidents" and basically, you don't know its going to happen until it does.
It's why there's a lot of uninformed consent when it comes to drugs in childbirth - see how long it's taken me to explain this and what I'm telling you is that probably these drugs are basically safe but we really don't know for each individual until we do it. If women really understood that, would they sign up for their epidural at their first prenatal exam? I'm not opposed 100% to the use of some drugs in some circumstances. I am opposed to the routine use of any drugs because I believe that the risks easily outweigh the benefits when there is a lack of any specific indication for their use, including situations where there are non-drug alternatives that have not been used first (i.e. doula or Jacuzzi tub instead of an epidural for pain management).
The last little tidbit I'll throw out is an interesting, if still theoretical (I think, maybe someone has actually done a study but I haven't seen it) suspicion about epidurals and their effects on the baby. A question that was immediately raised when it was discovered that people had receptors for opiates was, "Why? Why would we have receptors for a substance that is derived from a plant?" Scientists very correctly assumed that there must be some substance that we make internally that binds to those receptors, or else they wouldn't exist. Turns out, there is- endorphins. These are our own "natural opiates". Our brain secretes endorphins in response to painful stimuli- that's the "runner's high" that people report after strenuous exercise. Some people wonder about the role that endorphins might play in birth- not so much for the mother, that's pretty obvious but for the baby- because endorphins do cross the placenta and may well provide a natural and normal "buffer" if you will for the baby during the labor.
But, if you give an epidural, you interrupt the perception of pain in the mother's brain, and she stops producing endorphins. Does this leave the baby abnormally sensitive to the process of labor? Are these babies having a more negative/painful/frightening experience than they ought to? We don't know, and since the drugs in the epidural do reach the baby (in spite of what your physician tells you, that's been documented) maybe they are just affected by the unnatural opiates as opposed to the natural opioids.
It is probably safe to say that they aren't receiving the benefit of the very fine-tuned natural chemical dance that would normally occur. Again, not a blanket condemnation of all epidurals, but certainly a call to consider all the risks and benefits before choosing one, and hopefully, a realization that there can be risks that you wouldn't at first think about. And hopefully, an understanding that it is NEVER as simple and straightforward as most physicians would like you to believe- but they really don't want the burden of explaining it to you in enough detail that you do have a grasp of how complicated and not guaranteed it is.
For the full story on epidurals, please check out this website. It's particularly good for those who can't believe anyone other than a "doctor" would be competent to discuss something like epidural safety.
Revised July 19, 2006