The Kestenbaum-Anderson procedure is done in patients with nystagmus to mechanically shift the null point to primary position .
Patients with nystagmus frequently have a gaze direction in which the amplitude of their nystagmoid jerks are minimized compared to other gazes . This is called the null point, though insufficient data has been presented to explain this phenomenon. In order to bring a fixation target into their null point and thereby optimize their vision, these patients may adopt anomalous head positions (AHP). AHP is common in nystagmus and can be measured with an articulated protractor, called a goniometer . Reported incidences of AHP vary from 19% - 94% [4-6].
A surgical approach to treating the AHP associated with nystagmus was originally suggested independently by both Anderson  and Kestenbaum  in 1953 as a means of mechanically bringing the null point into primary position. Kestenbaum’s suggestion was to operate on all four horizontal muscles, and this is the approach most commonly used today . His initial recommendation was to perform 5mm resection and recessions on all four muscles, which is considered insufficient to treat AHP for a typical patient. This approach was amended in 1973 by Parks et al.  with the “5, 6, 7, 8” approach. Later researchers proposed an “augmented Parks” surgery, with doses being increased to match larger angles of head turn – 40% increase if the angle was greater than 30⁰, and 60% if the angle was greater than 45⁰ [11,12]. Many other authors have suggested varying surgical dosages [13,14].
Indications and Goals
Surgical intervention can be considered in patients with nystagmus with anomalous head positioning greater than 20⁰ . The goal of surgery is to reposition the eyes toward the head turn, away from the gaze direction of the null point. In other words, to weaken the ability to gaze in the direction of the null point. The Kestenbaum-Anderson procedure may be performed for patients with horizontal head turn. Though vertical head turn, and head tilt null points can occur, a horizontal head turn is the most common presentation . Furthermore, data suggests that up to 43% of patients can expect an improvement in best corrected visual acuity and a 60% decrease in recognition times [1,15-17].
A full ocular examination should be performed to assess the need for additional workup, especially in acquired nystagmus which may be related to a neurologic disorder, ocular disorder, or other afferent pathway disorder. Refraction with cycloplegia should be performed because astigmatism and anisometropia are common [9,14]. The head position should be carefully observed to ensure there is only a single null point and the null point is consistent over time. There is no optimal timing for the procedure. It is reasonable to delay those with congenital nystagmus until near school age and those with acquired nystagmus for at least a year with multiple visits to measure a repeatable degree of AHP. The degree of head turn should be measured using a goniometer for planning of surgical dosage. The goniometer is used by placing one arm of the device in the direction of the visual axis and the other in the axis of the head .
Procedure and Dosing
Horizontal Head Turn
The typical approach is essentially the same as what was originally suggested by Kestenbaum  with alterations in the dosage to match the degree of head turn. The surgery is performed with recession of the lateral rectus and resection of the medial rectus of the abducting eye. This is followed by recession of the medial rectus and resection of the lateral rectus for the adducting eye . In other words, moving the eyes away from the null point toward primary position. There are many approaches to surgical dosing [11-14, 18,19]. As mentioned earlier, the classic Parks dosing is “5,6,7,8” with each eye receiving the same 13 mm of surgery . One common method is referred to as “augmented Parks” and was originally suggested by Calhoun  and Nelson . For this approach, the “5, 6, 7, 8” dosing from Parks et al.  is augmented according to the degree of head turn. The original rule represented a 5mm (adducting eye) and 7mm (abducting eye) recession of the medial and lateral rectus respectively and a corresponding 6mm (abducting eye) and 8mm (adducting eye) resection of the other medial and lateral rectus respectively. For angles greater than or equal to 30⁰, a 40% increase is used (i.e., 7, 8.4, 9.8, 11.2mm). For angles greater than or equal to 45⁰, a 60% increase is used (i.e., 8, 9, 11.2, 12.8mm). If strabismus is also present in a patient with nystagmus when surgery for the AHP is planned, the surgical dosing can be modified. Align the fixating eye for nystagmus with dosing adjustments to the non-fixating eye for the strabismus . A detailed account of common techniques and instrumentation for horizontal extraocular muscle surgery is described elsewhere .
For chin elevation or depression as the main presentation of AHP, the same principle of repositioning the eyes away from the gaze direction of the null point – meaning in the direction of the head turn – should be used . For example, in order to correct chin elevation in nystagmus the inferior recti could be recessed 7-8mm each and the superior recti should be resected 7-8mm each . Other alternatives including inferior oblique anterior transposition combined with superior rectus recession have been described for chin down head position . A detailed account of common techniques and instrumentation for vertical extraocular muscle surgery is described elsewhere .
For pure head tilt, multiple different procedures have been described, including oblique muscle procedures and offset of the vertical recti muscles to provide torsional effect. Slanting of the insertions of the vertical recti and/or oblique muscles has also been advocated to either increase or decrease the torsional effect of the muscles . A detailed account of common techniques and instrumentation for cyclovertical extraocular muscle surgery is described elsewhere .
Success rate varies from 50-100%, so recurrence is possible [24,25]. Reoperation is considered safe and effective in these cases and can include resecting the previously resected muscles by another 5mm and adding a posterior fixation suture to the previously recessed muscles . Consecutive strabismus is possible following the Kestenbaum procedure. One study reported an incidence of 11%, with associated risk factors being severe bilateral amblyopia, a history of botulinum toxin treatment, and larger recessions of the four horizontal muscles .
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