Christopher S. Connor, MD Hanover, N.H.
If you've avoided switching to topical anesthesia because you prefer the
anesthesia and akinesia that a retrobulbar block provides, but wish to avoid
the potential complications attendant with this approach, consider the technique
that I and a growing number of physicians are using. For the past four years,
I've been administering retrobulbar blocks with a cannula rather than a
needle, using the sub-Tenons pathway to distribute the anesthesia. Done
correctly, this approach is as safe, economical and rapid as topical anesthesia,
but more effective, more efficient, more versatile and more patient friendly.
In this brief article, I'll explain the technique and then list some of
the advantages I've observed.
Sub-Tenon's anesthesia
The first step to this technique is to assess how long the anesthesia needs
to last. This information can help you choose an anesthetic appropriate
for the procedure.
For most routine cataract cases, I use 1% lidocaine without epinephrine.
If the case looks to be more complex, and I feel the case will take longer,
I use 2% lidocaine. For corneal tranplants or retinal surgery, which take
longer and require some post-op pain control, I mix 0.5% or 0.75% marcaine
in with the lidocaine. Typically, 2 to 3 cc of anesthetic is all that's
necessary for this technique.
An assistant places whichever anesthetic I choose in a 3-cc syringe attached
to a blunt-tipped cannula (See sidebar). He or she then applies one drop
of topical anesthesia to the eye and then preps and drapes the patient as
usual.
Once seated, I focus the microscope and then use a Vannas scissors to punch
a tiny hole through the conjunctiva, about 6 mm from the inferior/nasal
limbus (Figure 1). I use this quadrant because it's closest to the posterior
pole. I then use the scissors to dissect down through Tenon's to the bare
sclera. It's very important to get the cannula under all layers of Tenon's
capsule; this virtually guarantees that the anesthetic will end up in the
muscle cone and provide an excellent block.
Following dissection, I insert the cannula through the opening. The syringe
should be positioned straight up, so that it's essentially parallel to the
optic nerve (Figure 5). Positioning the cannula in this way ensures that
the anesthetic will encircle the globe. I then slowly inject all the anesthetic,
remove the cannula, cauterize the Tenon's and conjunctiva closed, and proceed
with surgery. Here's a little tip: Following the injection, keep an eye
on the conjunctiva. No chemosis means that you've injected under all layers
of Tenon's.
The sub-Tenon's advantage
In the four years that I've used this technique, I've found that it offers
some significant advantages to over other forms of anesthesia, including:
Safety. Because you're using a blunt-tip cannula and not a needle, there's
no risk of inadvertently penetrating the globe or the optic nerve.
Versatility. Unlike topical anesthesia, this technique allows you to customize
the length of anesthesia to the procedure. At our clinic, we've successfully
used sub-Tenon's anesthesia on a whole spectrum of ophthalmic procedures,
including cataract, glaucoma, strabismus and retinal procedures. We've used
it to supplement topical anesthesia when a patient experiences pain or complications
arise. We've also found that this approach helps to control post-op pain
when it's used as a supplement to general anesthesia in trauma or pediatric
cases. In these cases, we use a dose of 3cc of 0.75% marcaine.
Adjustability. You can titrate the degree of the block to achieve full sensory
anesthesia, including the optic nerve and a complete motor block. Or, you
can achieve a more limited block by using a lower concentration and amount
of lidocaine. This is particularly helpful in monocular patients, because
there's almost immediate post-op visual recovery.
Efficiency. The sub-Tenons approach is nearly 20 minutes faster than a traditional
retrobulbar block. There's no need for a compression device to soften the
eye because the volume of anesthesia used is so small, between 2 and 3 ccs.
Also, the effect of the anesthesia is almost instantaneous; there's no waiting
around as there is with retrobulbar anesthesia.
Reduced patient anxiety. Patients are significantly less anxious when I
explain that there will be no needle. Also, because of the rapid onset,
you can administer the anesthesia after the patient has been prepped and
draped, so the patient is unaware of what you're doing. I've found that
this allows me to reduce or even eliminate IV sedation in most patients.
In addition, the surgeon no longer needs to worry about the photophobia
and other sensations many patients experience under topical anesthesia.
Cost effective. With this approach, there's no
need for an anesthesiologist or nurse anesthetist in the pre-op area either
giving a retrobulbar or peribulbar injection or a nurse administering topical
anesthetic drops. Also, the cannula is reusable.
There is one minor disadvantage. Occasionally, patients exhibit a small
subconjunctival hemorrhage at the injection site which rapidly clears after
surgery.
We've all heard a lot recently about the advantages of topical anesthesia
for cataract, glaucoma and strabismus surgery. At our operating room, we've
had great success going "back to the future" with retrobulbar
anesthesia delivered through a sub-Tenon's approach. It's as efficient as
topical anesthesia, but more effective, less expensive and easier on patients.
Dr. Connor is an assistant professor at Dartmouth Medical School and the
director of clinical services in ophthalmology at Dartmouth-Hitchcock Medical
Center. He specializes in anterior segment surgery and takes a special interest
in instrument design.
Dr. Connor wishes to thank Scott Greenbaum, MD, for getting him interested
in this approach to anesthesia.
The Connor Cannula
The cannula that I use is 15 mm in length, with a blunt tip and a slight
curve. It has three sideports and no central bore (Figures 2, 3 and 4),
so the anesthesia is broadcast around the globe as you inject.
Among the advantages I think this cannula offers:
The 15-mm length. Too many cannulas on the market are unnecessarily long.
This longer length can lead to damage of the vortex veins.
It has a 23-ga caliber. This smaller gauge helps to avoid a rush of anesthetic
fluid, which can cause pain during the injection. The larger gauges also
leave big buttonholes in the conjunctiva, while any smaller than 23 gauge
will make it too difficult to inject the anesthetic.
It has a gentle curvature. If the curve of a cannula is too great, it can
increase the likelihood of hooking a muscle or a vortex vein.
The tri-port design and smooth, rounded end. The smooth tip allows you to
gently dissect underneath Tenon's capsule without causing any localized
trauma. The tri-port gives a forward 60-degree angled spray, allowing for
an even, multidirectional distribution of anesthetic creating a more complete
block.
The cannula is available with a complimentary video from Bausch & Lomb
Surgical. I have no financial interest in it.
Other ophthalmic companies, including Alcon Surgical, offer other sub-Tenon's
injection cannulas.