• Sometimes cataract surgery results in a post-operative refractive "surprise". Perhaps we were aiming for plano and the eye ended up with a -3.00 D refractive error. In these cases we would like to pinpoint the cause of the error. We will want to confirm that the axial length was correctly measured.

• It is always a good idea to have K readings and A-scan measurements on both eyes before cataract surgery is performed. Since both eyes have similar measurements in most people, this provides a double-check of the measurement. If the fellow eye has an IOL inserted by another doctor, you may not have access to the IOL specifications and/or the measurement information.

• It is sometimes necessary to replace an IOL that was inserted many months or years ago. Even if you have IOL specifications and measurement information from the previous surgery, it is nice to have the confirmation of a current measurement. It is necessary to have a new measurement if a scleral buckling procedure, which lengthens the eye, has been done since the IOL was inserted.

There are three question to be answered, or factors to be considered, when performing a pseudophakic A-scan. They have to do with the material the lens is made of, the measuring protocol to be used, and the accuracy of the measurement.

The type of material is important because the velocity of sound is a function of the material that the sound is passing through. A-scans don’t actually measure length, they measure how long it takes a sound beam to bounce off an object (anterior lens, posterior lens, and retina) and return to the probe. The instrument is pre-programmed with the velocity of sound factors for the aqueous, the lens material, and the vitreous.  Many instruments use an average speed calculation.   The instrument performs a calculation that results in a length measurement.

The instrument performs different calculations for a eye with a natural lens (phakic), an eye without a lens (aphakic), and an eye with an IOL (pseudophakic).  Using the wrong mode will cause an error in the measurement.  The material that an IOL is made of will make a difference in a pseudophakic calculation.    When performing a pseudophakic scan,  you must also take the lens material into consideration to avoid measurement errors.

PMMA — Many IOLs have been made from PMMA (polymethylmethacrylate) plastic. This is the same plastic that the original hard contact lenses were made of.  Sound travels faster through PMMA than it does through the natural lens. Most A-scans have a pre-programmed mode for performing PMMA pseudophakic measurements.

Silicon — Silicon IOLs are foldable. Folding an IOL allows it to be inserted through a smaller incision. A smaller incision heals faster and induces less post-op astigmatism. Sound travels much slower through a silicon lens than it does through the natural lens. Not taking this into account could result in a -3.00 D post-op refractive error.

Acrylic — Some foldable lenses are now being made of Acrylic plastic.  Sound travels faster through acrylic than it does through the natural lens.

Protocol has to do with how the instrument goes about performing the measurement. Some A-scan instruments use a protocol for phakic scans that individually measures the cornea/anterior chamber, the lens, and the vitreous. This protocol cannot be used for pseudophakic scans because IOLs are so highly reflective that they produce "reverberation" echoes. Instead of two lens spikes, there may be three or four lens spikes.

The protocol you use will depend on the instrument you have and type of IOL material you are dealing with.

Disclaimer:   The author believes that the compensation factors listed in this section have been obtained from reliable sources and have been accurately transcribed.  However, the author has not personally tested each factor for reliability.  Use these factors at your own risk.  It is recommended that you obtain confirmation from other sources before using these factors in your calculations,  or use one of the available IOL calculation software packages that deal with these situations.

PMMA protocol:  Many A-scans have a "Pseudophakic mode" that is pre-programmed for PMMA IOLs. Read your manual and use this mode if it is available.  The pseudophakic mode will only be good for PMMA lenses unless your A-scan biometer has settings for other materials.

If a pseudophakic mode is not available, you can measure the eye in the aphakic mode and add a standard compensating factor of .4 mm to the resultant axial length. For example: If the aphakic scan gave a result of 24.53 mm, add .4 mm to give a corrected length of 24.93 mm for an eye with a PMMA IOL.

Silicon protocol:   If your biometer does not have a pseudophakic/silicone mode, or if you are unsure, simply use the aphakic mode and subtract a compensation factor of .8 mm.  If the aphakic scan gave a result of 24.53 mm, subtracting .8 mm would give a corrected length of 23.73 mm.

Acrylic Protocol:  If your biometer does not have a pseudophakic/acrylic mode, or if you are unsure, simply use the aphakic mode and add a compensation factor of .2 mm.  If the aphakic scan gave a result of 24.53 mm, adding .2 mm would give a corrected length of 24.73 mm.

Protocol for other materials and unusual IOL thickness:  The above protocols will not provide an accurate measurement for every silicone or acrylic IOL, depending upon the manufacturer and the lens power.  To insure accuracy, try to get the correction factor from the lens manufacturer.

References:  

Jack Holladay, MD

The A-Scan Challenge,  Rhonda Waldron, MMSc, COMT, CRA, ROUB, RDMS,  Eye Scan Consulting, 404-286-9067.

Special care must be taken when performing pseudophakic scans because of the presence of reverberation echoes. Your instrument may mistake one of these echoes for the retina echo and produce an inaccurate axial length.

The figure to the right depicts anterior (A) and posterior (B) lens echoes from an IOL and two reverberation spikes (C, and D).

Some instruments use measuring "gates" that measure the first spike to reach a threshold height within a specified distance along a baseline. Make sure a reverberation spike does not appear within the retina gate.  In the figure to the right a reverb spike is within the retina gate at point A.  The gate would need to be moved or resized to avoid this spike.  Otherwise the instrument will read the spike at point A as being the retina spike.

Some instruments use "lights" that attach to spikes that reach a threshold height, no matter where they occur on the baseline. In the aphakic mode there may only be two lights; one for the cornea, and one for the retina. Make sure the retina light attaches to the retina spike and not a reverb spike.   In the figure to the right the retina light has attached to a reverb spike (A).  You will need to manually move it to the retina spike (B).

It is sometimes easier to identify reverberation spikes if the gain is turned down. The manual to your instrument will tell you how to adjust gates or move lights, if necessary.

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