Decentred ablation (decentration)

What is a decentred ablation?

A decentred ablation is a serious ablation defect where the laser ablation process is delivered to a location on the cornea not centred on the desired point.

What causes a decentration ablation? (posted: jan 2005)

There are several possible mechanisms which may result in a decentration. In older lasers without eye trackers, and particularly where the laser treatment is longer (for higher amounts of correction) patient failure to fixate on the target during the treatment can affect it. In lasers with trackers, surgeon failure to align properly may affect centration. There is also a phenomenon called parallax where the laser appears to centre properly but is not in fact. Additionally, degradation of a laser beam may cause a true or pseudo decentration. Sometimes an ablation defect where part of the ablation is not fully delivered can appear to be a decentration.

How might a decentration affect one's vision? (posted: jan 2005)

It depends in large part on the diameter of the ablation and the degree to which it is offset from the intended centre. If it is large and only slightly off centre, it may not be seriously symptomatic or may present symptoms only at night (typically ghosting, starbursts, haloes, etc). But more severe decentrations, i.e. >1.0mm, and particularly if the ablation is 6mm diameter or less, will typically cause double vision and other vision quality defects under all lighting circumstances and in many patients causes a loss of best-corrected vision acuity.

How is a decentration diagnosed? (Posted: jan 2005)

Decentred ablations can be detected in corneal topography. Note that depending on the resolution setting for the printout, it may be more or less visible. Wavefront aberrometry may also indicate the possibility of decentration (or other serious ablation defects) as a decentration would typically cause significant coma.

Can a decentration be treated surgically? (posted: jan 2005)

Not reliably. Relative success depends in part on the severity of the decentration. Attempts to correct decentrations of 2mm or more are among the least likely to succeed while slight decentrations may be more easily fixed because in those cases even simply an enlargement of the optical zone may reduce the visual symptoms.

The two most common approaches are topography-guided procedures and wavefront-guided procedures. The conventional wisdom is that topography-guided procedures are more appropriate for the larger defects visible on topography while wavefront is better at "refinements" such as reducing spherical aberration. However, there have also been some studies indicating that wavefront-guided procedures may help improve decentrations. Either type of procedure may be done under or over the flap depending on stromal thickness constraints.

A further possibility, used in extreme cases, is PTK.

What other treatment options are there for decentrations? (posted: apr 2005)

The safest and most reliable way to reduce or eliminate the symptoms of decentration is gas permeable contact lenses. They are difficult to fit on patients with any large ablation defects; they may require considerable experimentation; and some patients may not be able to tolerate contacts after surgery. But, for patients who can wear contacts, gas permeable lenses properly fit by an expert can not only reduce or eliminate symptoms in most patients but may in some cases also produced a temporary moulding effect which can improve vision quality even when the patient is not wearing the lenses.

Where can I get more information about decentrations?

Click here for LaserMyEye Encyclopedia entry, which may have additional articles and links.