Chapter 9: Pharmacotherapy of Pain Management

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Welcome to the Deep Dive.

Today, we are really undertaking a critical mission.

We're going to be talking about a countered clinical situation you're going to face, and honestly, probably the most challenging to manage effectively.

That is absolutely right.

And for you, the advanced practice students listening, think of this as your, well, your crash course.

We're pulling the essential clinical pharmacology and the decision -making framework straight from our core source material.

Right.

And before we even talk about a single drug, you've got to internalize this core definition.

Pain, according to the IASP, is an unpleasant sensory and emotional experience.

It's emotional too.

That's crucial.

It's inherently subjective.

And that subjectivity, that's what makes management so complex.

And that complexity has, well, it's just shot up, hasn't it?

We're in this post -opioid crisis world now, guided by things like the CDC 2016 guidelines.

Exactly.

The whole prescribing philosophy has shifted really dramatically away from just opioid monotherapy.

Now it's about a mandated multimodal approach.

You have to.

Which means you need a solid understanding of all the analgesic options.

So this deep dive is designed to give you exactly that, that necessary nuanced understanding.

Let's get into it.

Okay.

So first step in any clinical decision -making here is classifying the pain.

This really dictates the entire treatment pathway.

Right.

Where do we start?

We categorize based on the source.

Is it not susceptible?

Is it neuropathic or is it a mix of both?

Okay.

Not susceptible.

That one seems maybe the most straightforward.

Like the body's alarm system, right?

It's purposeful.

Purposeful is the perfect word.

Yeah.

It results when there's actual stimulation of nerve receptors from, say, mechanical, thermal, or chemical insults.

Something is actually happening to the tissues.

And that breaks down further.

It does.

We subdivide not -susceptive into somatic pain that's well localized, coming from tissues like skin, muscle, bone.

Think of a sprained ankle.

Okay.

And visceral pain that originates from internal organs, often more diffuse, harder to pinpoint.

And we also include inflammatory pain under this umbrella.

It's a subtype, really, driven by the release of those pro -inflammatory cytokines.

Think rheumatoid arthritis, that kind of thing.

So not -susceptive means there's a clear sort of damage signal.

How do we contrast that clinically with pain that's,

well, that's gone rogue basically.

Pain that isn't protective anymore.

That brings us right to neuropathic pain.

And this is fundamentally different.

It's caused by abnormal signal processing within the nervous system itself, central or peripheral.

The system itself is malfunctioning.

Precisely.

It's no longer protective.

So if your patient uses descriptors like

electric -like, burning, tingling, shooting, stabbing, those words should be a huge red flag.

Oh, okay.

Like in diabetic neuropathy.

Classic example.

That immediately points you towards And sometimes it's not one or the other, right?

It can be a mix.

Very often.

Think about cancer pain.

That frequently involves both direct tumor invasion, which is no -susceptive, and nerve damage from the tumor or treatment, which is neuropathic, or something like CRPS, chronic regional pain syndrome.

Really complex, difficult condition.

Definitely a mix of both no -susceptive and those

maladaptive neuropathic signals.

So we have the source.

What about the timeline?

Acute versus chronic.

Right.

The chronicity is the other major classification axis.

Acute pain, sudden onset, usually resolves pretty quickly.

You can often identify the cause.

You see those physical signs too, right?

Like increased heart rate, sweating.

Yeah, that sympathetic activity, tachycardia, diaphoresis, increased blood pressure.

The goal with acute pain is clear.

Treated effectively and promptly.

We want to prevent that transition.

The transition to chronic pain, which is the real challenge.

Exactly.

Chronic pain persists beyond that normal healing time.

It serves no biological purpose anymore.

And it's not just about the duration, is it?

There are actual changes happening.

Absolutely.

It's often driven by neuroplastic changes in the nervous system.

We call it sensitization.

And it's almost always tangled up with psychological comorbidities like anxiety and depression.

Which changes the whole goal of treatment.

Completely.

For chronic pain, the goal isn't usually total pain elimination.

It's about decreasing the pain to a tolerable level.

And crucially, improving function.

And that requires that multimodal approach we mentioned.

So to treat pain effectively, you need to understand the pathway.

Think of it as four distinct steps.

And each one is a potential pharmacologic target.

Four steps.

Let's break them down.

Where does it start?

It starts with transduction.

This is right at the site of injury.

The noxious stimulus heat, pressure, whatever activates the nociceptors, those specialized nerve endings, the A delta and C fibers.

And chemicals are involved there too?

Yes.

Local chemical mediators really facilitate this.

Things like prostaglandins, PG's and briti -brinconin.

They make the nociceptors more sensitive.

Ah, okay.

So PG's, that sounds like a target.

Exactly.

That's our first major drug intervention point.

If you block the production of those prostaglandins using NSAIDs, nonsteroidal anti -inflammatory drugs,

you inhibit that initial activation.

Right at the source.

Right at the site of injury.

You're stopping the signal, or at least dampening it, before it really gets going.

Makes sense.

So after transduction, where does the signal go?

It moves into transmission.

The signal travels along the nerve fibers up to the dorsal horn of the spinal cord.

That's the relay station.

Okay, step two.

Step three is perception.

This is when the signal reaches the brain and you become consciously aware of the pain.

And this part is heavily influenced by psychosocial factors, fear, anxiety, past experiences.

They all shape how you perceive that signal.

Right, the emotional component again.

Definitely.

And finally, step four is modulation.

This is actually the body's own pain control system.

It's an inhibitory pathway.

The body fights back.

In a way, yes.

The brain sends signals back down the spinal cord, releasing endogenous opioids or natural painkillers plus neurotransmitters like norepinephrine and serotonin.

These dampen the pain signals in that dorsal horn relay station.

So like turning down the volume?

Exactly.

And here's the fascinating link.

Pharmaceutical opioid analgesics.

They work primarily by hijacking this natural modulation system.

They bind manly to the mu opioid receptor, mimicking those endogenous opioids and strengthening that inhibitory pathway.

Okay, that clarifies how opioids fit in.

But what about when pain becomes chronic?

You mentioned sensitization.

Right.

Sensitization is the key process that makes chronic pain so different and difficult.

There are two main types.

First, peripheral sensitization.

At the site of injury?

Yes.

Continuous irritation or inflammation in the periphery makes those nerve endings hypersensitive.

Their firing threshold lowers, so even mild stimuli can trigger pain signals.

Think of a sunburn.

Even light touch hurts.

Got it.

And the second type?

The second and often more problematic is central sensitization.

The source material defines this as an amplification of neural signaling within the CNS.

Basically, the spinal cord and brain become hyper excitable.

Amplification, like the volume knob gets stuck on us.

That's a great analogy.

Repeated barrages of pain signals lead to changes like NMDA receptor sensitization and even neuroinflammation within the central nervous system.

And what does that look like clinically?

It results in things like aledinia, where a normally non -painful stimulus like light touch becomes painful,

and hyperalgesia, an exaggerated pain response to a mildly painful stimulus.

Wow, okay.

And this central sensitization is precisely why just giving more and more opioids often fails in chronic pain.

The problem isn't just the incoming signal anymore.

The central processing is altered.

Which is why you need other types of drugs, the coanalgesics.

Exactly.

They target these maladaptive central changes.

Okay, so we understand the types and the mechanisms.

Now, how do we approach the patient?

Assessment has to be step one, right?

Absolutely fundamental.

And the NIH standard is crystal clear here.

The patient's self -report is the single most reliable indicator of their pain.

We have to believe them.

So how do we get that report in a structured way?

We need a systematic approach.

The PQRSTU mnemonic is a really useful tool for routine clinical assessment.

You should integrate this into your practice.

Okay, let's walk through it.

P.

P is for presenting factors, precipitating factors, and palliating factors.

What brought the pain on?

What makes it worse?

What makes it better?

Q.

Q is for quality.

Give the descriptors.

Is it sharp, dull, aching, burning, stabbing, electric?

This helps differentiate non -susceptive from neuropathic.

R is region and radiation.

Where exactly is the pain?

Does it travel anywhere else?

S.

S is severity.

This is usually the 0 to 10 numeric rating scale.

We generally categorize 1 -3 as mild, 4 -6 as moderate, and 7 -10 as severe.

T is for temporal pattern.

Is it constant?

Does it come and go?

Intermittent.

Worse at certain times of day.

And finally, U.

You mentioned this was crucial for chronic pain.

Absolutely crucial.

U stands for how does it affect you?

This gets at the functional impact.

How does the pain interfere with daily activities, work, sleep, mood, quality of life?

So the assessment goals are actually different depending on if it's acute or chronic pain.

Yes, definitely.

For acute pain, success is often measured by that intensity score dropping.

You want to see the number go down.

Makes sense.

But for chronic pain, while we want the intensity score to decrease, the primary goal shifts to improvement.

Is the patient able to do more?

Is their quality of life better?

So just relying on the 0 -10 scale isn't enough for chronic pain.

It's not.

We really should be using multi -dimensional scales.

The source mentions one, the Indiana Polyclinic Combined Pain Scale as an example.

These tools capture not just the pain number, but also function, anxiety, depression, the whole picture.

Okay, so after assessment, what are the general treatment principles?

The textbook probably has a table for this.

It does.

Table 9 .1 basically lays it out.

For mild to moderate pain, you start with non -opioids like acetaminophen, NSAIs, maybe topicals.

Pretty standard.

For moderate to severe pain, you might move to opioid agonists.

But, and this is key, ideally combined with non -opioids, that multimodal approach again.

Maximize efficacy, minimize opioid dose.

And for severe pain?

Severe pain often requires pure, mopey, great opioid agonists.

Think morphine, hydromorphone.

And for persistent pain, especially cancer pain,

ATC dosing around the clock is recommended.

ATC, so scheduled doses, not just PRN.

Exactly.

To maintain a steady level of analgesia and prevent that pain from breaking through constantly.

The source also mentioned adjunctive options, right?

Box 9 .3.

Yes, and this is critical.

Don't forget, box 9 .3, adjunctive pain control options.

We're talking about non -pharmacologic interventions.

Things like cognitive behavioral therapy, acupuncture, physical therapy, heat -cold applications.

These aren't just optional extras.

Not for chronic non -malignant pain, no.

They should be considered core components of the treatment plan right alongside medications.

All right, let's dive into the specifics of the drugs.

Starting with the non -opioids, acetaminophen, APP, Tylenol.

What's the key takeaway here?

Well, its exact mechanism isn't fully nailed down, but is thought to inhibit prostaglandin synthesis, mainly in the CNS, maybe via COX3.

So central action explains why it's not a potent anti -inflammatory peripherally.

Exactly.

It has weak anti -inflammatory effects.

The good news is it's generally well tolerated.

Minimal GI issues doesn't really affect platelets like NSAIDs do.

But there's a big but, isn't there?

The liver toxicity.

Huge but.

This is critical.

Acetaminophen metabolism produces a toxic metabolite.

Normally, our body neutralizes it with glutathione, but in an overdose or even sometimes with chronic high doses, especially with alcohol use, you deplete those glutathione stores.

The toxic metabolite builds up and causes potentially fatal liver necrosis.

So the dosing limit is non -negotiable.

Absolutely non -negotiable.

Maximum 4 ,000 milligrams per day for healthy adults.

And you need to lower that, think 3 ,000 milligrams or even less for older adults or regular alcohol users.

And watch out for hidden APAP in combination products.

Good point.

Okay, moving to NSAIDs, ibuprofen naproxen.

Right.

These are your workhorses for inflammation.

They are anti -inflammatory, analgesic, and antipyretic.

Triple threat.

And their mechanism is inhibiting COX enzymes.

Correct.

They inhibit cyclooxygenase.

But there are two main types,

COX1 and QOX2.

QOX1 is generally protective.

It helps maintain the stomach lining, kidney function, platelet aggregation.

And QOX2.

QOX2 is primarily induced during inflammation and drives the pain and swelling.

So the older first generation NSAIDs like ibuprofen, they hit both.

They do.

They inhibit both QOX1 and QOX2.

That's why they work well for pain and inflammation.

But it's also why they carry that risk of GI bleeding because they knock out the protective QOX1 in the gut.

Which led to the development of QOX2 selective inhibitors like salicoxib, salabrax.

Exactly.

The idea was to get the anti -inflammatory benefit without the GI risk.

And they do generally have less GI toxicity.

But another but, cardiovascular risks emerged.

Big but,

yes.

QOX2 inhibitors carry a black box warning for increased risk of cardiovascular thrombotic events, heart attack, stroke.

Seems QOX2 might have some protective cardiovascular roles too.

So the bottom line on NSAIDs.

Use the lowest effective dose for the shortest duration possible.

Long -term high dose use of any NSAID carries risks GI, cardiovascular, renal.

We generally try to avoid them or use them very cautiously in older adults due to these cumulative risks.

Okay, let's shift gears to the opioids.

The pure moo agonists.

Morphine, hydromorphone.

Right.

These are the heavy hitters.

Morphine is still considered the gold standard against which others are compared.

Hydromorphone, Dilaudid, Oxycodone, Fentanyl are others in this class.

What about Meparidine, Demerol?

Is that still used?

You know, it really shouldn't be for pain management.

The textbook is clear and clinical guidelines agree.

Meparidine has a major drawback.

Which is?

It has an active metabolite Normaparidine which is neurotoxic.

It can build up especially in patients with any kidney impairment and cause tremors, muscle twitches, confusion, and even seizures.

So best to avoid it.

Definitely best to avoid for analgesia.

There are much safer options.

Let's talk side effects for the whole class.

Respiratory depression is the big scary one, right?

It is the most feared, yes.

But sedation, nausea, vomiting, itching, prudence are also common.

However, there's something interesting about tolerance.

Tolerance develops.

To most of these side effects, yes.

With consistent use, patients often become less sedated, less nauseous.

Tolerance develops to the respiratory depression too, which is important.

But not to everything.

Not to constipation.

This is a huge clinical pearl.

Tolerance generally does not develop to opioid -induced constipation.

Meaning?

Meaning, if you start someone on regular opioids, you must start a prophylactic bowel regiment at the same time.

Don't wait for them to get constipated.

What kind of regiment?

Usually a stimulant laxative like Senna, often combined with an osmotic agent like polyethylene glycol peg.

Maybe a stool softener like Doc you say, though its evidence is weaker.

The point is be proactive from day one.

Okay, critical point.

Now what about choosing an opioid if the patient has kidney or liver problems?

This seems like advanced practice territory.

Absolutely crucial.

Organ function dictates choice.

Morphine and hydromorphone are tricky in renal failure.

Why is that?

They both produce active metabolites that are cleared by the kidneys.

If kidney function is poor, these metabolites can accumulate and cause neurotoxicity confusion, myoclonus, muscle jerking, even paradoxical hyperalgesia where the opioid increases pain sensitivity.

Use with extreme caution or avoid insignificant renal impairment.

So what are better choices in renal failure?

Oxycodone is often considered safer because its metabolites are largely inactive.

But the real go -to drugs in moderate to severe renal failure are generally fentanyl and methadone.

Why them?

Because they don't have significant active metabolites that rely on renal clearance.

Fentanyl is also considered the drug of choice and significant liver failure.

Okay, fentanyl.

Let's talk about the patch.

Right, the transdermal fentanyl patch.

Key point here.

It is only for opioid -tolerant patients with chronic stable pain.

What defines opioid tolerant?

The generally accepted definition is someone taking the equivalent of at least 60 milligrams of oral morphine per day for a week or more.

You cannot start a patch on an opioid -naive patient.

Very dangerous risk of overdose due to the slow absorption and long duration.

So patch for chronic tolerant pain.

What about the other forms of fentanyl?

The transbicosal forms, lozenges, films, sprays, they have a much faster onset.

They are specifically indicated for breakthrough pain in opioid -tolerant cancer patients.

Got it.

Different forms, different uses.

You mentioned codeine earlier.

Pharmacogenomics.

Yes, box 9 .5 highlights this.

Codeine itself is actually a weak opioid.

It's a pro -drug.

It needs the liver enzyme CYP2D6 to convert it into morphine to provide significant analgesia.

And people vary in how well that enzyme works.

Hugely.

Some people are poor metabolizers.

They don't convert much codeine to morphine so they get a little pain relief.

Others are ultra -rapid metabolizers.

They convert it very quickly and efficiently leading to high morphine levels which can cause toxicity or even overdoses, especially in children.

So codeine is kind of unpredictable.

Very unpredictable and carries safety risks because of this genetic variability.

It's generally not recommended for routine pain management anymore, especially in pediatric populations.

Okay, let's move to multiple mechanism analgesics.

Trimidol.

Trimidol is interesting.

It's a weak meo -opioid agonist so it has some opioid effect but it also inhibits the reuptake of norepinephrine and serotonin.

Like some antidepressants.

Exactly.

That dual mechanism gives it some potential benefit for neuropathic pain alongside nociceptive pain.

It's typically used for moderate pain.

Any downsides?

Yes.

It can lower the seizure threshold so caution is needed in patients with epilepsy or risk factors for seizures.

And because of the serotonin effect, there's a risk of serotonin syndrome if combined with other serotonergic drugs like SSRIs.

Okay.

And the other multiple mechanism drug,

methadone.

This one sounds complex.

Methadone is very effective but also very tricky.

It's a potent meo -opioid agonist like morphine but it's also an NMDA receptor antagonist.

NMDA.

That relates back to the central sensitization, right?

Precisely.

That NMDA antagonism makes methadone particularly useful for difficult to treat neuropathic pain and potentially helpful in reversing some aspects of central sensitization or opioid tolerance.

So very useful but what's the catch?

Several catches.

First, dosing is complex.

It has a very long and variable half -life like 24 to 36 hours or even longer.

This means it takes days to reach steady state and the risk of accumulation and overdose is significant if doses are increased too quickly.

Slow and steady wins the race with methadone dosing.

Absolutely.

Second, a major concern is cardiac toxicity, specifically QTC interval prolongation on the EKG which can lead to dangerous arrhythmias like torsades de pointes.

So you need baseline and follow -up EKGs.

Yes.

EKG monitoring is essential.

Given the complex pharmacokinetics and the QTC risk, initiating and managing methadone often requires specialized expertise.

Referral to a pain specialist is highly recommended.

That QTC risk really stands out.

You mentioned two common mistakes generalists might make.

Can you reiterate those?

Sure.

Mistake one, titrating too fast.

Because the pain relief might kick in before the drug fully accumulates due to that long half -life, there's a temptation to increase the dose every day or two.

That's dangerous.

You have to wait several days, maybe five, seven days between dose increases.

Okay, patience is key.

Mistake two.

Mistake two, forgetting or underestimating the QTC prolongation

Not getting that baseline EKG, not monitoring it during titration, or not screening for other risk factors like electrolyte imbalances or interacting medications.

That NMDA antagonism is great, but the cardiac safety requires vigilance.

Crucial context.

Okay, finally, the co -analgesics, especially for neuropathic pain.

Antidepressants.

Yes, certain antidepressants are first -line agents for neuropathic pain, specifically the tricyclics, TCA's like tryptaline and nortryptaline, and the SNRIs, serotonin, norepinephrine, reuptake inhibitors like diloxetine, cymbalta, and venlafaxine effects.

How do they help with pain?

It's not just mood, right?

No, it's a direct analgesic effect.

They primarily work by blocking the reuptake of norepinephrine and serotonin in those descending modulatory pathways we talked about earlier, boosting those natural pain inhibitors.

TCA's versus SNRIs.

TCA's are often very effective, but they come with a lot of anticholinergic side effects.

Dry mouth, constipation, sedation, confusion, which can be particularly problematic in older adults.

SNRIs generally have a better side effect profile and are often preferred, especially diloxetine, which has specific approvals for neuropathic pain and fibromyalgia.

Makes sense.

What about anticonvulsants?

The other major class for neuropathic pain.

We're talking mainly about the gabapentinoids, gabapentin, neurontin, and pregabalinlyrica.

How do they work?

Their mechanism involves binding to a specific subunit, the alpha -2 delta subunit of voltage -gated calcium channels in the CNS.

This reduces the influx of calcium into neurons, which in turn decreases the release of excitatory neurotransmitters like glutamate.

Kind of dampens down that hyper -excitability.

And they're used for things like?

Classic indications are diabetic neuropathy and posterpetic neuralgia.

They're often first or second line agents for these conditions.

One last category I mentioned was NMDA antagonists, like ketamine.

Yes, ketamine is gaining interest.

Used as a subanaesthetic, low doses, it acts as an NMDA receptor antagonist.

Remember how NMDA receptors get sensitized in chronic pain?

Right, central sensitization.

Ketamine can help block that excessive NMDA firing, potentially reducing central sensitization and improving pain control, especially in really refractory acute or chronic pain states like CRPS.

But it's not a simple drug to use.

Definitely not.

Even at analgesic doses, IV ketamine can cause side effects like dissociation, hallucinations, vivid dreams, and sedation.

Its administration typically requires specialized settings and monitoring by personnel trained in advanced life support.

It's not a first line agent for most situations.

Okay, that's a lot of pharmacology.

Let's try to apply it with some quick scenarios, maybe like the case studies in the text.

Good idea.

Let's solidify these distinctions.

Okay, scenario one.

You have a patient with chronic pain, maybe poorly controlled.

They keep calling the office, asking for early refills, seem really anxious about running out of medication.

Staff might label them drug seeking.

What does the source tell us to consider first?

The source strongly suggests considering pseudo addiction.

This isn't true addiction or substance use disorder, SUD.

It's often maladaptive behavior driven by inadequate pain relief.

The patient is desperate because their pain isn't controlled.

So the first therapeutic response isn't suspicion, it's reassessment.

Exactly.

Reassess their pain.

Reassess the adequacy of their current regimen.

Are they on the right dose?

Right drug?

Is there an untreated neuropathic component?

Improving their pain control often makes those drug seeking behaviors disappear.

Address the undertreated pain first.

Critical distinction.

Okay, scenario two.

Same patient, maybe they were stable on morphine for their chronic pain, but now they develop acute renal failure maybe from something else and suddenly they're confused, twitching, maybe even hallucinating.

Red flags for neurotoxicity.

And based on what we discussed?

We immediately suspect accumulation of those active morphine metabolites because of the renal failure.

The action is clear.

Discontinue the morphine and switch to an opioid without active metabolites that rely on renal clearance.

The best options would likely be fentanyl, probably as a patch if their pain was chronic and stable, or potentially methadone.

Manage carefully.

Oxycodone might be an option if the renal failure isn't severe, but fentanyl or methadone are generally safer bets in significant impairment.

Makes sense.

Scenario three.

A patient has chronic low back pain, but also clear signs of radiculopathy shooting pain down the leg, maybe some numbness or tingling.

They're a stable dose of, say, oxycodone, but their function isn't improving.

Pain is still interfering with life.

This sounds like a mixed pain picture, not deceptive low back pain, plus neuropathic radiculopathy.

The oxycodone is hitting the new receptors, but it's likely failing to adequately address that neuropathic component driven by nerve irritation or damage.

So just increasing the opioid might not be the best answer?

Probably not, and it increases risks.

The textbook approach here, the multimodal approach, would be to add a coanalgesic specifically targeting neuropathic pain.

Like what?

Like starting an SNRI, such as deloxetine,

or initiating a gabapentinoid like gabapentin or pregabalin, or potentially considering switching to methadone, leveraging its NMDA antagonism alongside its muagonism.

The key is adding a different mechanism to target that central sensitization or neuropathic signaling.

Great examples.

So let's wrap up with the main therapeutic takeaways from this deep dive.

Okay.

Takeaway number one, pain management absolutely requires an individualized plan.

You have to base it on the type of pain, the severity, and critical patient factors, especially age and organ function.

No good book approach.

Right.

And takeaway two.

Multimodal therapy is king, especially for chronic pain.

Combining agents with different mechanisms of action, maybe an opioid plus an NSAID plus a gabapentinoid, along with those pharmacologic strategies almost always provides better outcomes than relying on a single drug class, especially opioids alone.

And the goalposts are different for chronic pain.

That's takeaway three.

Remember the goal.

For chronic pain, we're focusing on improving function and quality of life.

Reducing pain to a tolerable level is part of that, but it's not just about chasing a zero on the pain scale.

It's about helping the patient live better despite the pain.

Excellent summary.

So building on all that complexity,

especially the roles of neuroplasticity, central sensitization, and all those psychological factors tangled up in chronic pain,

here's something to think about.

Okay.

How should our future pain assessment tools evolve?

We need to move beyond just simple numeric scores, right?

How can we develop and routinely use tools that truly capture and measure the success of our multimodal therapies and actually improving a patient's functional capacity and their overall quality of life?

That really is the challenge, isn't it?

Measuring what matters most.

A perfect place to wrap up.

Thank you for joining us for this deep dive into the clinical and pharmacologic essentials of pain management.

Hopefully we've helped extract the key information you need to master this critical area.