This is quite a long post. I considered breaking it up into two parts but decided against it. The subject matter is a tad cerebral, and I think you will get more out of reading it in one session, if you can manage it. It will be well worth the effort, especially if you ever find yourself in the future having to consider whether or not to consent to a spinal anaesthetic. Or an epidural, which we’ll also be covering in this post. So, put the kettle on, grab yourself a few choccy biccies, put your phone on silent and wade right in.

Unless you’ve ever been on the sharp end of an anaesthetist’s spinal needle, the chances are you don’t know exactly what a spinal anaesthetic is. You might not even have ever heard of the term. You’ve probably heard of epidurals, though. Spinals and epidurals are the same but different. They are both ways to numb a part of the body, usually everything below the waist, for a limited period of time. Spinals and epidurals both involve the injection of local anaesthetic close to the nerves in your back. From a patient’s perspective, both can feel similar. However, spinals and epidurals differ in important ways.

I’ll cover both procedures in this post, explaining how and why they’re performed, what to expect as a patient if you have one or the other, their plus points, and potential side effects and complications.

First of all, the boring stuff – know your anatomy

Sensory nerves carry messages from sensors in the body to the spinal cord, where they connect with other nerves at junctions called synapses. Those second-order nerves then pass those messages up to the brain. These sensors detect touch, pressure, vibration, heat, cold, pain and body position. Meanwhile, motor nerves, running in parallel to the sensory nerves, convey commands from the brain to our skeletal muscles, heart, diaphragm and other organs. As we learnt in the last post, local anaesthetics block transmission in these nerves in both directions.

The spinal cord is a downward extension of the brainstem. On its way down, nerves branch off the spinal cord at regular intervals, one on each side for every vertebral bone in your spinal column. That’s 31 pairs of spinal nerves in total. In an adult, the spinal cord terminates higher than you might think. Not down by your tailbone (coccyx) but at the level of your upper lumbar spine (a couple of inches above your belly button). Since the spinal cord is shorter than the bony spinal column, the lower spinal nerve roots continue downward in the spinal canal as a bundle of fibres called the horse’s tail (cauda equina if you’re into Latin – who isn’t?). This seemingly useless piece of information matters if you’re performing or enduring a spinal anaesthetic, as we’ll soon see.

The brain floats in a bag of cerebrospinal fluid (CSF) contained within three different membranes called meninges. This arrangement acts as a shock absorber, stopping your brain from bashing against the inside of your skull every time you move your head suddenly. As its name suggests, CSF also surrounds the spinal cord and the horse’s tail.

How to perform a spinal anaesthetic

Armed with our newfound knowledge of the relevant anatomy, let’s go through the mechanics of how to perform spinal anaesthesia, step by step.

1.   In the anaesthetic room, you will be asked to sit up straight on your theatre trolley with your feet dangling over the side, resting on a stool. You will then need to curl your back as much as possible to help open up the spaces between your vertebrae. 

2.  Your anaesthetist sterilises a wide area of your back with antiseptic liquid. It feels freezing cold!

3.  Sterile drapes are then positioned to maintain sterility throughout the procedure. 

4.  After palpating the spines along your back to identify the most open space at the correct level, your anaesthetist infiltrates local anaesthetic into the skin and subcutaneous tissues. This is not the spinal injection. It simply numbs the pathway the spinal needle will traverse. The injection stings. 

5.  Next, your anaesthetist carefully pushes the spinal needle through the numbed area, aiming it between the two spines of the chosen intervertebral space. This is typically below the spinal cord’s termination in the lower lumbar spine. This is good news because the danger of impaling the fixed spinal cord and causing significant damage to it is entirely averted. Yes, the horse’s tail nerve roots are floating lazily in the vicinity, but being more mobile, they tend to deflect safely away from an approaching needle. 

6.  On its journey towards the subarachnoid space wherein lies the CSF bathing the horse’s tail, the spinal needle pierces skin, subcutaneous tissue, three ligaments (the supraspinous ligament, interspinous ligament and ligamentum flavum) and two of the three meningeal membranes (the dura mater and the arachnoid mater). 

7.  Obviously, the anaesthetist cannot see where the tip of the spinal needle is headed. Successful performance of spinal anaesthesia relies on good knowledge of the relevant anatomy and a feel for the structures the spinal needle passes through. This is obviously a bit tricky the first time you ever perform a spinal. It’s like many things in life. Practice really does make perfect. I performed a couple of thousand spinals during my anaesthetic career. By the end, I was getting quite good at them, relying as much on instinct as feel.

Junior anaesthetist alert!! Don’t worry if a seemingly 15-year-old pimply youth introduces themselves as your anaesthetist and tells you they will be performing your spinal. Anaesthetists are very risk-averse. In fact, we are the most risk-averse of all the medical specialities because when the shit hits our fan, the resulting cesspool leaves absolutely everyone submerged in filth. So, we don’t take risks. Ever. And certainly not with the nerves in your back. All junior anaesthetists are directly supervised by a senior colleague to ensure patient safety, ready to step in at a moment’s notice and take over if necessary. 

8.  Although every patient’s back is different, and some have more regular and easier anatomy than others, it’s usually the case that the spinal needle will be gripped tightly as it passes through the dense ligamentum flavum, after which there is a sudden loss of resistance as the needle passes through it and also immediately pierces the dura and arachnoid mater. 

9.  At this point the central stilette of the spinal needle is removed, and the gratifying sight of gin clear CSF dripping from the hub of the spinal needle confirms that the tip is in exactly the right place – the subarachnoid space. 

10.  A crazily low volume of local anaesthetic produces a satisfactory spinal anaesthetic block, just 2-4ml. Sometimes an opioid such as fentanyl is added to the mixture. It is amazing how such a tiny dose of local anaesthetic can have such a profound and far-reaching effect. There are three reasons for this. First, lots of spinal nerve roots are packed tightly together in a small space, so the entire nerve supply to your lower body is within easy reach. Second, the local anaesthetic is injected directly into the CSF and easily spreads in the fluid surrounding the nerve roots. Finally, the spinal nerve roots in the subarachnoid space are bare, lacking the thick connective tissue protective layer that invests peripheral nerves. As a result, the local anaesthetic penetrates the spinal nerve roots much more easily. 

A straightforward spinal anaesthetic – and most are straightforward – is all done and dusted in 5-10 minutes from start to finish. You lie back down on your trolley and wait for things to happen. You won’t be waiting long.

The effects of a spinal anaesthetic

Spinal anaesthesia comes on like grease lightning. You usually notice the first effects within a minute or two. Your legs become warm and tingly. The tingling turns into numbness and heaviness. You find it difficult to lift your legs up in the air. It’s not long before you can’t lift your legs off the trolley at all and you’re limited to wiggling your toes. Even that ability is lost after another few minutes. The loss of sensation is so profound that even those neurological sensors broadcasting the position sense of your lower limbs (what is termed proprioception) are taken out. With no idea where your legs are, your brain makes it up, and many patients report that their legs are floating in the air. Your spinal is fully cooked and ready for surgery only 5-10 minutes after the local anaesthetic is injected.

Your anaesthetist will assess you thoroughly to ensure your spinal anaesthetic is fully effective before you are taken into theatre. Your sensation will be tested to touch, cold and pinprick. Your motor power will also be checked to confirm you are fully paralysed below the waist.

What operations is a spinal anaesthetic suitable for?

Hip and knee replacements figure near the top of the list, as do Caesarean sections and TURPs (TransUrethral Resection of the Prostate). However, a spinal anaesthetic is a good choice for many operations below your belly button, including hernia repair, operations on the penis and testicles, foot surgery, and a variety of gynaecological procedures.

Okay. I believe you when you say I won’t feel any pain with a spinal anaesthetic on board, but I still don’t much fancy the idea of being awake during my operation

I often heard this concern expressed whenever I discussed the option of spinal anaesthesia with patients at my preoperative visit. Being awake shouldn’t be a big deal for most patients having shorter operations lasting, say, half an hour to an hour. Your anaesthetist can easily keep you amused and distracted. And of course, women having babies by Caesarean section definitely want to be wide awake so they don’t miss the birth. However, staying awake is more of an issue for longer procedures such as hip and knee replacements, which take two or three hours and involve noisy drilling, hammering, pushing, and pulling (as well as profane language from your orthopaedic surgeon). And a few patients will say no to a spinal even for short procedures.

Fear not, though. Your friendly anaesthetist is on your side and well aware that being awake for the duration can be a deal-breaker. Plus, they don’t want you staying wide awake for long periods of time, either! If you’re awake, you need to be kept entertained. It means engaging you in polite conversation for the next three hours. Cue sedation.

Sedation is marvellous. Marvellous for patients. Marvellous for anaesthetists. Sedation comes in a variety of different flavours. I used to favour either midazolam or propofol, depending on the situation. I gave midazolam as intermittent small boluses, whereas I administered propofol as a continuous intravenous infusion by syringe pump.

The sedation induced by midazolam is rather like the effects of a couple of pints of beer or glasses of wine. A patient at first relaxes, then becomes euphoric, disinhibited and loquacious. Finally, they nod off. They might wake up again after a while, express surprise at where they are, and chat merrily away, whether I am paying attention or not. I might give them another bolus of midazolam, depending on how close we are to the end of surgery and how interesting their conversation is.

Propofol sedation is an entirely different thing. I reserved this for the longer joint replacement procedures. The depth of sedation achievable with propofol administered by continuous intravenous infusion can be controlled remarkably precisely, which means patients can be deeply sedated safely – more deeply than you can safely go with midazolam. In a continuous, dose-dependent manner, propofol produces first anxiolysis, then a state of calm, next a light doze similar to regular sleep. The final stage is hypnosis, although at a lighter plane than general anaesthesia. Patients sedated with propofol, even deeply sedated, may be startled awake by a loud noise because deep sedation is not general anaesthesia. Or, they may wake up for no reason at all. And because they are not in a state of general anaesthesia, they will certainly wake up, screaming in agony, while the surgeon saws, drills and hammers into their hip bone if their spinal anaesthetic is completely ineffective. That never happens, though, and patients usually snooze contentedly through most, if not all, of their operation, blissfully unaware of their surroundings.

After two or three hours of propofol sedation, patients wake up bright and breezy after their hip and knee replacements, full of the joys of spring. And, of course, they are absolutely 100% pain-free, which is not the case for patients waking up after a general anaesthetic.

As an aside, propofol infusions can provide general anaesthesia if you up the dose, usually in conjunction with remifentanil, an ultra-short-acting opioid. As I mentioned in my last post, any member of the TIVA brigade will be only too delighted to explain at great length why it is (they think) their kind of anaesthetic is so much better for you.

Okay, okay. A spinal anaesthetic really does sound good, but what about side effects and complications?

No drug, medical or surgical procedure is without its shopping list of side effects and complications. The calculation that must always be made before deciding to go ahead with any intervention, whether that be simply popping a daily pill or undergoing a major operation, is: how much do the intended benefits of proceeding with Intervention A outweigh its potential risks, compared with proceeding with Intervention B, or with doing nothing at all? In the case of surgery, if spinal anaesthesia is Intervention A, then Intervention B is usually a general anaesthetic, which has its own shopping list of side effects and potential complications.

Relatively common (and trivial) side effects associated with spinal anaesthesia include pain and discomfort during the insertion procedure itself, low blood pressure, difficulty passing urine after your operation, shivering, itching, failed block, headache and backache. Rarer (and potentially more serious) complications include unsatisfactory block during surgery, infection, spinal haematoma and nerve injury.

‘That’s quite a long list,’ I hear you cry. ‘Dr Tony had me on board there for a while, but now I’m not so keen.’

Let me play devil’s advocate. The alternative to a spinal, a general anaesthetic, has the following potential serious complications: hypoxia caused by unanticipated problems maintaining your airway and breathing, resulting in permanent brain damage from oxygen starvation; aspiration pneumonia and emergency admission to the ICU; anaphylactic shock; malignant hyperthermia; waking up in the middle of your operation while paralysed; death. For most patients, these risks are so rare they’re almost theoretical. The same is true of the serious complications after spinal anaesthesia.

Let’s take a look at both the more common side effects and rarer complications in a bit more detail.

Pain and discomfort during the procedure (insertion of the spinal) – the thought of this can easily put many patients off opting for a spinal. After all, with a general anaesthetic all you have to do is lie back and think of England as you drift effortlessly and painlessly into the hinterland between life and death. Although you will endure the pain and discomfort of having an intravenous cannula inserted, of course. And you can expect to emerge from your general anaesthetic with quite a lot of pain and discomfort consequent on you having had a mad axeman viciously attack your hip bone while you weren’t paying attention. Yes, you’ll be given morphine to combat the pain, but there’s no denying that you will suffer at least a degree of postoperative pain after major surgery such as a hip or knee replacement. It therefore seems illogical, to me at least, to forego the benefit of missing out on any and all postoperative pain for a minimum of several hours, perhaps for as long as twenty-four hours, because you’re worried about a teeny bit of pain and discomfort for a few moments while a spinal needle is inserted. Man up! Or woman up!

Your anaesthetist will reassure you that the skin and subcutaneous tissues of your back are much less sensitive than most other parts of your body, and that having a spinal anaesthetic is no more painful than having a drip inserted.

Low blood pressure – this is a common, almost expected, side effect because a spinal block also knocks out the motor nerves that supply muscles in the wall of your arteries. When your arteries lose contact with these nerves, the muscle loses tone, your arteries dilate and your blood pressure nosedives. You may feel temporarily light-headed or sick. Your anaesthetist will be on the lookout for this, monitor your blood pressure closely, and have a couple of syringes of different drugs ready to treat significant hypotension.

Difficulty passing urine – this can affect some patients because the spinal numbs the nerves supplying sensation and motor function to your bladder. It’s usually only a temporary postoperative problem, although patients occasionally need to be catheterised for a day or two if they absolutely can’t pee.

Shivering – this harmless but annoying side effect sometimes occurs following a drop in core body temperature caused by the dilatation of peripheral blood vessels in the lower body I spoke about above and the resulting increased loss of body heat by radiation.

Itching – this is much more likely if an opioid drug such as fentanyl is added to the local anaesthetic mixture. Your anaesthetist has drugs such as injectable antihistamines to treat this if it feels like any army of invisible ants is on the march just under your skin.

Failure of the spinal block to properly develop – this isn’t common. It happened to me (I suppose I should say it happened to my poor sod of a patient, really) no more than a couple of handfuls of times during my career. Either your anaesthetist cannot find a way into your subarachnoid space wth the tip of their spinal needle or the block simply fails to develop as expected after the apparently successful injection of local anaesthetic. There are a variety of reasons why this happens, and they are not always clear. However, the more experienced your anaesthetist is, the less wonky your spine is, and the slimmer you are, the more likely it is that a working spinal anaesthetic can be achieved.

A failed spinal is usually more disappointing than disastrous. You’ve psyched yourself up for the procedure, convinced of the benefits, and then you can’t have a spinal after all. You don’t need to worry, however, that you’ll only find out that your spinal hasn’t worked after the surgeon gets to work. Your anaesthetist will diagnose the problem in the anaesthetic room, and you’ll end up having a general anaesthetic instead.

Headache – this sounds like a pretty minor side effect on the face of it, but a so-called dural puncture or spinal headache can be quite nasty and incapacitating for several days. Although I’ve had to classify it in the relatively common group (a 1 in 100 risk is officially defined as common), I can hardly remember any of my patients suffering from a spinal headache despite performing about 2,000 spinal anaesthetics during my anaesthetic career (I’m excluding obstetric patients here – they are a special case).

If you’re unlucky enough to develop a spinal headache that doesn’t settle after a day or two, there is a highly effective treatment for it, though it does involve another injection in your back.

Backache – almost always this is minor and self-limiting, caused by minor trauma and bruising in the area your spinal needle passed through. Don’t worry about it.

Unsatisfactory block during surgery – this usually only happens when an operation unexpectedly goes on for a lot longer than the spinal anaesthetic was designed to last. The effective duration of a spinal depends mainly on the dose of local anaesthetic injected and ranges from 1-4 hours. Having your spinal anaesthetic wear off before your operation has finished sounds totally horrendous – the stuff of nightmares. Although I wouldn’t recommend it, an unsatisfactory spinal block during surgery is not quite as bad as it sounds. You don’t fall off a cliff edge between experiencing no pain whatever followed immediately by full-on agony. If you’re sedated, which you probably will be, you stir in your sleep and perhaps wake up, informing your anaesthetist that you can feel a bit of discomfort. Faster than you can think, ‘I’m going to sue the bastard for this,’ you’ll be under a full general anaesthetic. Anaesthetists don’t like being sued. There’s a good chance you’ll remember nothing about the incident later and be surprised when your anaesthetist fesses up. The other good news is that, even if the spinal fails to provide satisfactory anaesthesia towards the end of your operation, it may still provide you with good postoperative analgesia.

Infection – this doesn’t sound too alarming until I mention that the infection we are talking about is either meningitis or a spinal abscess. Calm down! These complications are rare. Anaesthetists are scrupulous with their aseptic technique. None of my patients ever suffered from an infective spinal complication.

Spinal haematoma – how bad can a bruise be? Very bad, it turns out, if it develops in the confined space of your vertebral canal. The pressure generated on spinal nerve roots by a haematoma can cause permanent neurological damage if not swiftly diagnosed and surgically decompressed. It’s another rare bird, although if you have a blood clotting disorder or take anticoagulant medication, the risk is higher. That is why we don’t routinely perform spinals (or epidurals) on such patients unless there are pressing reasons to carry on regardless.

Nerve injury – because we know our anatomy, we’re not overly worried about this. A spinal needle enters the subarachnoid space well below the termination of the spinal cord, and so the risk of serious permanent nerve injury is rare, between about 1 in 10,000 and 1 in 100,000. Although nerve roots in the horse’s tail may occasionally be pranged during the performance of a spinal, most postoperative problems such as tingling, numbness or weakness in a particular area almost always resolve over time.

Who is spinal anaesthesia a good choice for?

The straight answer is that a spinal anaesthetic is a good choice for most patients. I would certainly choose a spinal over a GA if I needed a hip or knee replacement, or a TURP, for instance. However, there are subgroups of patients whose risk of a general anaesthetic is higher than the general population who particularly benefit from a spinal. These include: patients with significant breathing problems such as brittle asthma, chronic bronchitis and emphysema; morbidly obese patients; the frail and the elderly; obstetric patients having a Caesarean section; certain patients with heart disease.

Before a doctor can proceed with any medical or surgical intervention, patients must be fully briefed on their options so they can make an informed choice and give consent. This is often easier said than done. I always found discussing the pros and cons of spinal anaesthesia, and its advantages over general anaesthesia, challenging. Life was much easier when you simply offered what your patient was expecting rather than persuading them to have a procedure they initially had misgivings about.

Most patients obviously haven’t benefitted from five or six years at medical school. Yet, they must absorb a shedful of technical information, understand it, process it, and make a considered decision. And they have to accomplish this prodigious feat during a preoperative visit that will last at most fifteen or twenty minutes and which needs to cover a lot else besides.

Impossible! I know, but it’s informed consent, don’t you know? Society demands it. Patients demand it. Solicitors litigating on their clients’ behalf absolutely demand it. Failure to properly obtain informed consent is the easiest way for them to win a medical negligence case. I’ll have much more to say on this topic in a future post.

Even if patients are given reams of written information at their preoperative assessment clinic visit and consult widely with Dr Google, Professor Wikipedia and Regius Professor ChatGPT, informed consent for spinal anaesthesia remains difficult. I found that patients fell into three groups. The first group, accounting for three-quarters or more of all the patients I came across, was open-minded and, after listening to what I had to say, opted to go with whatever I recommended as best for them. A second, smaller group had an almost conscientious objection to being awake during their operation, and nothing I said would change their mind. The third group comprised those relatively few remaining patients who could understand the issues well and thought everything through carefully before arriving at their own, well-considered decision.

Are there any patients a spinal is not suitable for?

Some patients present particular problems and risks, and for whom a spinal anaesthetic is therefore not a good idea. At the top of the list is patient refusal. After that we have: skin or soft tissue infection near the proposed injection site (risk of meningitis or spinal abscess); significant clotting abnormality (risk of spinal haematoma); inability to cooperate or sit/lie still (risk of pranging something you ought not to have); certain neurological conditions (because of concerns that subsequent exacerbation of any underlying condition may be attributed to the spinal); certain cardiac conditions (risk of cardiovascular collapse and cardiac arrest); systemic sepsis or dehydration (risk of cardiovascular collapse); anatomical abnormality of the lower back such as severe kyphoscoliosis (higher risk of failure).

So, I think I understand what a spinal anaesthetic is now. What’s an epidural?

The short answer is that epidurals are similar to, but different from, spinals. There’s an old adage in anaesthetics dating all the way back to 1898, when the first ever spinal anaesthetic was performed: it isn’t the length of your needle that’s important. It’s where you put your tip that counts.

(Alright, you got me. I just made it up.)

In what way are they similar?

Both spinals and epidurals involve injecting local anaesthetic into the lower back (epidurals can also be performed higher up the spine). Both procedures therefore interfere with nerve signals passing to and from the brain via peripheral nerves supplying the lower body. Many of the side effects and complications of a spinal anaesthetic are also side effects and complications of epidurals.

So what are the main differences?

The main difference is where the local anaesthetic is injected. We learnt that with a spinal, the local anaesthetic mixture is injected deep into the subarachnoid space, where the nerve roots are exposed and vulnerable. As I am sure you all already know full well, having enrolled on online courses in ancient Greek and Latin since my post on medical terminology, an epidural is so called because the local anaesthetic is deposited on top (outside, superficial to) the dura mater, the outermost of the three meningeal layers that invest the brain and spinal cord. The nerve roots in the epidural space are protected by a thick connective tissue sheath, making them less sensitive to local anaesthetic drugs.

Several consequences follow from this:

1.  Whereas the effects of a spinal anaesthetic are felt within a minute or two, an epidural block often takes half an hour or longer to become fully effective. 

2.  A much larger dose (and volume) of local anaesthetic mixture is needed for an epidural block. 

3.  Even so, the intensity of nerve block produced by an epidural is less than that with a spinal. Patchy blocks, rare with spinal anaesthetics, are sometimes a problem with epidural blocks. Muscle weakness rather than paralysis is the norm, too. 

4.  Spinal anaesthetics almost always involve a single injection of local anaesthetic drug, after which the needle is immediately withdrawn. Nothing is left in your back. An epidural block, on the other hand, usually involves threading a fine, flexible catheter down the needle into the epidural space where it is left. Afterwards, a continuous infusion of local anaesthetic can be given through the catheter, extending its duration of action. 

From what you’re saying, a spinal seems to win hands down. Why bother with epidurals?

It’s a case of horses for courses. The more perspicacious among you will have noted that while I talk about spinal anaesthesia, I talk about epidural blocks rather than epidural anaesthesia. In a nutshell, a spinal is preferred when the aim is to produce surgical anaesthesia that permits surgery to proceed painlessly and provides optimal operating conditions for the surgeon. Although it is possible to produce surgical anaesthesia with an epidural (as evidenced by legions of labouring women who have an epidural inserted to relieve their labour pains only to be rushed to the operating theatre for an emergency Caesarean section using that same epidural as the anaesthetic), an epidural is usually preferred when the primary aim is to produce analgesia rather than anaesthesia.

And finally…

Phew! You’ve made it to the end. I hope this post has been useful. It’s a complex topic. If one day you find yourself being asked if you want to have a spinal or an epidural, hopefully you’ll now be in a better position to make an informed choice. Even so, to stop this post becoming an anaesthetic tutorial for candidates preparing for the FRCA exam, I have omitted or simplified quite a lot.

If I might offer a word of advice, unless you have solid reasons to act to the contrary, always say yes if an anaesthetist suggests you will benefit from a spinal anaesthetic or epidural block.

Check out my website at https://www.drtonymccluskey.com for more details about my books.

Vocation, Resuscitation, and Resignation are available to purchase as eBooks or paperbacks at https://www.amazon.com/dp/B0DJ3273KH.

(https://www.amazon.co.uk/dp/B0DJ3273KH for readers in the UK).