Management decisions in IDEX present a challenge to the clinician that is not encountered for most other types of childhood onset strabismus. Any intervention must endeavour to improve unacceptable misalignment and poor BSV for distance fixation whilst preserving or improving BSV demonstrable for near fixation. Decisions to intervene are currently guided by both the size and control of the deviation. Where the distance angle is less than 20 prism dioptres, conservative treatment is thought to be more appropriate and surgical intervention not indicated.11–14
In IDEX where the distance angle is greater than or equal to 20 prism dioptres, the control of the deviation is considered the important factor in determining whether or not to intervene.
Control is assessed by noting how frequently the strabismus is observed at home and in the clinic and the ease with which it is controlled on cover testing.12
Observation of the strabismus for >50% of waking hours and poor control on cover test is thought to suggest the need for surgical correction11
but despite this, most clinicians remain unclear as to when intervention is appropriate. Measures of control have not previously been standardised and this may be in part due to the difficulties inherent in relying on parental reports of the frequency of the strabismus.
The NCS incorporates both objective and subjective measures of control into a simple grading system that differentiates and quantifies the various levels of severity in IDEX. The measures of reliability and repeatability found in this study indicate that the NCS is a consistent and robust method of rating severity that can be used accurately in clinical practice. It provides a classification of cure (NCS 0 or 1) and also enables any change over time to be more easily monitored.
We used the NCS to try and establish an appropriate threshold for surgical intervention and thus address the current uncertainties surrounding this issue. There does not seem to be a significant management dilemma for those patients at each end of the scale. For those with well controlled IDEX (NCS 2) surgical intervention may be difficult to justify. We found a significant spontaneous cure rate (39%) in this subgroup and generally surgery was not undertaken. On the other hand, in very poorly controlled IDEX (NCS 7) the clinical picture is one where surgery would generally be indicated. However the majority (86%) of patients were moderately affected with NCS 3–6. Of these only 16% spontaneously cured compared with 53% who were cured following surgery. This suggests that surgical intervention is necessary to achieve a cure in this subgroup. The spontaneous cure rate is significantly lower for NCS
3 than for NCS 2 (χ2
0.047) suggesting that NCS 3 is an appropriate threshold for surgery. The cure rate from surgery in the moderately affected IDEX group (NCS 3–6) of 53% is comparable with that reported in other studies.15–17
In a condition that is generally asymptomatic with normal BSV for near fixation, this may portray surgery as being of dubious benefit, especially in view of the risk of overcorrection with loss of pre-existing binocular function. Surgery however appears to be driven mainly by parental and peer awareness of the strabismus rather than clinician concern for function, and it may be that improvement in alignment without achieving a “cure” is an acceptable outcome. Information obtained from the use of the NCS can be used to counsel patients and parents preoperatively about the likely outcome following surgery.
There is clearly a need for a well planned, prospective clinical trial of surgical intervention in this group of patients to address questions regarding the nature of the condition itself, the most appropriate management for different severities of IDEX, and realistic goals of treatment.