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What is a "Differential Block?"

The pelvic region is a extensive and complex system of seemingly endless circuits and wiring – because of this, it can be virtually impossible to identify what specific nerve is causing pain in the genitalia, rectum, perineum, groin, pelvic region or lower abdomen.  It is truly like “looking for a needle in a haystack” given the sheer complexity of neural pathways layered on top of one and other and makes it virtually impossible to locate the culprit.  MRI’s and Ultrasound are of little help when it comes to this sort of pain due to the fact that these images will not tell you which nerve is sending the pain signals.  This is because conventional medical imaging can only tell you if a nerve looks different from what a “normal” nerve is expected to look like – it doesn’t tell you what it is saying to your brain.  Remember, a painful nerve will usually look completely normal with an MRI or Ultrasound.  So chances are, if you are a patient who developed pelvic pain suddenly and without any known cause, an MRI is going to be essentially worthless.

So how does one find out where the pain is actually coming from?

The answer: A diagnostic test to determine what part of the nervous system is causing the pain.

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What’s the CNS?

The central nervous system (CNS) consists of the spinal cord and the brain. Different nerves enter the spine and access the spinal cord at different levels

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Whose it For?

A Differential Block is a valuable diagnostic tool for evaluating patients who do not know what caused their pelvic pain or how to treat it.

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How Does it Work?

A local anesthetic is delivered to the CNS just like the “epidural” during childbirth – based on how your pain responds, your doctor can narrow down its location.

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pelvic pain, pudendal neuralgia, levator ani syndrome, impar block, pudendal nerve blockHow Does a Differential Block Work?

Pelvic pain is one of, if not THE, most complex pain condition known to modern medicine.  The circuitry of the pelvic floor is far more intricate than any other area of the body by far with 26 major nerves (13 on each side) and not one, BUT TWO sympathetic hubs (aka plexuses – Superior Hypogastric Plexus & Ganglion of Impar).  This makes is extremely difficult to determine which pathways the pain signals are using to travel around and even more difficult to pinpoint where the pain is actually coming from.  It is quite literally like looking for a needle in a haystack.  Imaging (i.e. MRI, CT scans, Ultrasounds) is useless in most cases unless the pain is coming from something obvious like an infection or mass.  Ultimately, diagnostic nerve blocks and process of elimination are the effective means for diagnosing, and ultimately treating, pelvic pain

The first and most important step in this process is to ascertain if the pain is using the nociceptive versus sympathetic (aka conscious vs. unconscious) nervous system communicate pain signals to the brain.  In other words, is the pain traveling through the pudendal nerve or through a web of postganglionic sympathetic fibers via the ganglion of impar.  This is absolutely crucial as the treatment for these 2 pathways is grossly different and can mean the difference between something you can fix like a compressed pudendal nerve versus something that you cannot like CRPS which can get worse and worse if left untreated.

Figuring this out is easier said than done considering the number of neural structures sitting stacked upon each other – layers upon layers.  When performed correctly and under the care of a board-certified interventional pain doctor, diagnostic nerve blocks are fairly effective at ruling a particular nerve in or out as the culprit.  In certain situations, the blocks may all be negative leaving the patient no closer to getting an answer.  What if there was a way to shut off both sets of nerves all at once and then let them turn back on at different times.

Imagine you have a leak in house – the first step is to turn off the water going to the leak.  You could try to turn off one pipe at a time, going one by one until you find the right one – that would take a lot time and a lot of trial and error.  Or, could shut off the water to sections of the house at a time to narrow it down first, then trying to isolate which pipe is the culprit in that particular section.

That’s where a differential block comes into play – shut of all sensation to the pelvic floor at once and then letting the nociceptive and sympathetic pathways wake up at different times.  Depending on when the pain returns, your doctor will be able to narrow down if the pain is coming from a nociceptive nerve in the pelvic floor that woke up “X” number of hours after the procedure versus an obscure web of neural fibers within the sympathetic nervous system that took “Y” number of hours to come back on line.  The procedure is quite simple to perform: similar to the form of anesthesia used during childbirth to numb a woman from the waist down, your doctor will deliver a volume of particular local anesthetic called chloroprocaine to rapidly block all signals from the pelvic region at once, and then let your body metabolize it. Chloroprocaine takes about 5-7min to start working and about 15-30min to wear off.

Depending on how long it takes for the pain disappear and then how long it takes for it to return, your doctor will be able to determine with certainty if the pain is nociceptive, sympathetic, or centralized.  Based on that, your doctor will know which nerves to treat.

pelvic pain, pudendal neuralgia, levator ani syndrome, impar block, pudendal nerve blockWhat is the Sympathetic Nervous System?

autonomic nervous sympatheticThe Sympathetic Nervous System (SNS) is a network of circuits inside the body but outside the central nervous system that allows us to subconsciously control essential functions like digestion, reproduction, blood flow, urinary system, etc. Along with its counterpart, the Parasympathetic Nervous System (PNS), the SNS is aids in control of the most of the body’s internal organs. The SNS and PNS could be compared to two sides of a coin, as each tends to counteract the other and provide balance to the body. The SNS is known for allowing the body to function under stress in what is known as the fight or flight response, whereas the PNS seems to prefer a calmer state for the body. Research has shown that the SNS plays a major role in chronic pain, particularly neuropathic pain.[1]

The SNS can either play a role in making chronic pain worse but increasing blood flow to painful and inflammed areas, or it can function as an escape route for pain signals to find their way around the normal set of checks and balances. In either event, once the SNS becomes in involved in the picture, the pain becomes unresponsive to typical nerve blocks and seems to spread to involve other areas (i.e. left sided pain spreads to involve the right side as well, pain that started in the rectum or genitalia spreads to involve the perineum or the feet, etc).

Some of its functions in the pelvic region include:

  • Filling and emptying the bladder by closing and opening sphincter
  • Ejaculation
  • Regulating blood flow to the pelvic organs and intestines
  • Involuntary contraction of the pelvic floor

Pudendal Neuralgia, Chronic Pelvic Pain, Pudendal neuralgia examination, Pelvic Pain New York CityNeurons in the sympathetic nervous system are located in small structures called “ganglia.” These ganglia are located in strategic areas throughout the body to regulate specific regions:

As all signals in the SNS travel through these ganglion at some point or other, they provide appropriate targets for pain management to aid in the treatment of “sympathetically-mediated” pain.[2] Blocking the right ganglion or plexus can effectively block pain signals from an entire at once. Unfortunately, the block will wear off as the medication is metabolized. That is where amniotic fluid comes into play. Read more on this part below…

pelvic pain, pudendal neuralgia, levator ani syndrome, impar block, pudendal nerve blockIndications

A Differential Block is a useful diagnostic method for determining where pelvic pain is coming from.  So how do you know if this is a procedure you should consider?

No known cause for the pain?
Did the pain come out of nowhere?
Imaging and testing is normal for the most part?
Have other injections and treatment options failed?

If you answered yes to any one of these questions, then you should consider a Differential Block before trying any further treatment options.

Sympathetically driven pain is usually progressive and gets worse over time which means time is extremely important.  To make matters worse, sympathetically driven pain does not respond to conventional treatments or nerve blocks that target nociceptive pain.  Which means will one might move quickly to get treatment, but waste valuable time by trying treatments that have no affect on the SNS or sympathetically driven pain.  As time goes on, even the right treatments will be less effective so make sure not to waste time on the treatments that do not target the SNS. 

Know which pathway is involved so you can make sure you only choose treatments that suit your particular condition.  Some conditions that are capable of using either pathway include

Pudendal Neuralgia
Levator Ani Syndrome
Post surgical pain

Like any treatment, do your research and ask lots of questions. Do not rush into any procedure because it “sounds right” for you. While there is no way to truly and consistently diagnose what is causing your pelvic pain, the most reliable means has always been a diagnostic block – as it pertains to complex pelvic pain, a Differential Block is the only way to know which major pathway is involved in your pain.

Why Won’t “Normal” Treatments Work For Me?

Pelvic pain is one of THE most difficult to treat conditions in modern medicine. The difficulty for finding the right treatment is due in large part to the extreme complexity of the nervous system in the area and the overwhelming number different of nerves and organs in the area – this makes it virtually impossible to pinpoint the exact source of the pain. In some cases, the pain signals maybe coming from several areas at once which is why blocking one nerve at a time does not result in any relief.

  • No matter what treatment you try, you just can’t seem to get lasting relief…
  • You have found treatments that seem to work, but only for a few days or weeks at best, and then the pain just comes right back…
  • You doctors just kept testing you for a UTI and put you on one antibiotic after another…
  • Every test you get seems to come back normal and no one seems to be able to tell you what the problem is…
  • Your pain just seems to be getting worse and worse, and now it’s got to the point that you can’t even sit down…

If any one of these seem to describe you and your pain, you are not alone. It can take years before patients with pelvic pain find their way to a pain doctor, much less one that specializes in their condition. By that time, the pain will have spread to a different location because the pain signals are now being generated from a different part of the nervous system, closer to the spinal cord, making it much harder to treat. Trigger point injections and blind nerve blocks (aka “hydrodissection”) will sometimes offer slight relief, but only for a few days or a few weeks at best. These injections aren’t really doing anything except flooding the area with local anesthetic which occasionally runs over onto part of the SNS to temporarily block the pain.

pelvic pain, pudendal neuralgia, levator ani syndrome, impar block, pudendal nerve block

While nerve blocks are great treatments options for pain…perhaps their most important use lies in their ability to diagnose pain as well.  Whether the injection is being performed as a treatment or for diagnostic purposes, the procedure will ultimately be performed the same way – the only difference will be what is actually getting injected onto the nerve.  Therapeutic injections will typically have some type of corticosteroid to eliminate pain and inflammation along with a local anesthetic to numb the area of pain.  Diagnostic injections will usually consist only of local anesthetic, and without any corticosteroid so that way if there is any change in pain, one knows the reason was due to the local anesthetic alone and it turning the neural target off.

There are a several different types of local anesthetics, each have a different speed at which they start working, and how long they last.  The most common one is lidocaine which has a fast onset of about 45-60 sec and only lasts an hour which is just enough time to get a cavity filled in or have a mole removed on your skin.  There are other varieties that work just as fast as lidocaine but wear off quicker or take longer to kick in, but last 6-8 hours.  Once the medication get metabolized the nerve wakes up, goes right back to the way it was before the injection.  The most important thing to pay attention to is “how did the pain change during that time that the local anesthetic was working” and how long did it take for the pain to return.

Nociceptive pain and sympathetically driven pain take different lengths times to return after being blocked so noting how long your pain took to reappear is just as important as simply noting whether or not it changed during the injection, itself.

So the purpose of this procedure is two-fold:

  1. Did the pain change during the nerve block?
  2. How long did it take for the pain to go back to its usual level?

How Its Done

This is a very old fashioned procedure as it pertains to the “how” it is done, what is cutting edge is “why” its being used — to see which pathway pelvic pain is coming from:

  1. Nociceptive Pain
  2. Sympathetically Driven Pain

Depending on where your pain is will dictate what treatments or medications it the pain will respond to.

The procedure is performed like just a Lumbar Epidural Steroid Injection; the only difference is instead of injecting medication, your doctor will place a small catheter through the needle (exactly the way an anesthesiologist would to prepare woman in the hospital to deliver a baby).  The needle is removed and the catheter is secured to the skin using a sterile dressing.  Once the catheter is secured, you will be transferred to a stretcher and taken to the recovery area to begin the Differential Block, itself.

Next, the doctor begin injecting the medication to see if and when the pain starts to disappear.  The local anesthetic being used is chloroprocaine which starts working in a matter of 5-7minutes and wears off after about 30 minutes. To ensure the test is performed correctly, you will be injected with enough anesthetic that your legs will feel heavy and weak as well – this will wear off rather quickly and sensation will begin to return a few minutes after that.

The doctor will record your pain score every few minutes and perform periodic physical exams to see if and where you are feeling numb and ascertain when the anesthetic has worn off.

Once the local anesthetic has worn off and normal feeling returns (about 15-30 minutes), the catheter will be safely removed with a sterile dressing placed over it.  You will be advised to keep the wound dry for 24 hours and not to submerge in water for a few days.  At this point, you will be discharged how and instructed to record the time when your pain starts to return as this will tell your doctor which pathway the pain is using the send signals to your brain.

Is a Differential Block Right for Me?

If you suffer from chronic pelvic pain condition that has failed to resolve with medications, physical therapy conservative therapies, nerve blocks or even the so-called “hydrodisscection,” and you still have no idea where your pain is coming from, this procedure may be an option for you. A Differential Block should be performed under the strict supervision of a board-certified pain management specialist.

Contact the Ainsworth Institute to set up an initial evaluation to find out if you are a candidate.


[1] Lanz S, Maihöfner C: Symptoms and pathophysiological mechanisms of neuropathic pain syndromes. Nervenarzt. 2009; 80:430-444
[2] Roberts WJ: A hypothesis on the physiological basis for causalgia and related pains. Pain. 1986; 24:297-311
[3] Chaturvedi A1, Dash HH., Sympathetic blockade for the relief of chronic pain, J Indian Med Assoc. 2001;99(12):698-703.
[4] Image from Gray’s Anatomy


Suggested Reading

[1] Benzon, Honorio. Essentials of Pain Medicine. Philadelphia: Saunders Elsevier, 2011. Print.
[2] Plancarte R, Amescua C, Patt RB, et al.: Superior hypogastric plexus block for pelvic cancer pain. Anesthesiology. 1990; 73:236-239.
[3] Kanazi GE, Perkins FM, Thakur R, Dotson E. New technique for superior hypogastric plexus block. Reg Anesth Pain Med. 1999; 24:473-6.
[4] Gamal G, Helaly M, Labib YM: Superior hypogastric block. transdiscal versus classic posterior approach in pelvic cancer pain Clin J Pain. 2006; 22:544-547.
[5] Cariati M, De Martini G, Pretolesi F, Roy MT. CT-guided superior hypogastric plexus block. J Comput Assist Tomogr. 2002; 26:428-31.
[6] Wechsler RJ, Maurer PM, Halpern EJ, Frank ED. Superior hypogastric plexus block for chronic pelvic pain in the presence of endometriosis: CT techniques and results. Radiology, 1995; 196, 103-106.
[7] Mishra S, Bhatnagar S, Gupta D, Thulkar S. Anterior ultrasound-guided superior hypogastric plexus neurolysis in pelvic cancer pain. Anaesth Intensive Care. 2008; 36:732-5.
[8] Plancarte R, de Leon-Casasola OA, El-Helealy M, et al.: Neurolytic superior hypogastric plexus block for chronic pelvic pain associated with cancer. Reg Anesth. 1997; 22:562-568.
[9] deLeon-Casasola OA, Kent E, Lema MJ: Neurolytic superior hypogastric plexus block for chronic pelvic pain associated with cancer. Pain. 1993; 54:145-151.
[10] Plancarte R, de Leon-Casasola OA, El-Helealy M, et al.: Neurolytic superior hypogastric plexus block for chronic pelvic pain associated with cancer. Reg Anesth. 1997; 22:562-568.
[11] Rosenberg SK, Tewari R, Boswell MV, et al.: Superior hypogastric plexus block successfully treats severe penile pain after transurethral resection of the prostate. Reg Anesth Pain Med. 1998; 23:618-620.