rectus and oblique muscle weakening for large vertical head positions.146,235 The former approach has the potential to induce bilateral disconjugate torsion, which is usually asymptomatic and can be minimized by simultaneous horizontal transposition of the vertical rectus muscles. The latter approach is more effective but also tends to produce a more significant gaze paresis. The resulting vertical null shift, just as an accurate horizontal null shift, should preclude the need for a gaze sift to see better and thereby eliminate torticollis.

Head tilts resulting from torsional null positions are rare but well recognized in infantile nystagmus.500 Head tilts resulting from a torsional null point can be treated by “torsional Kestenbaum procedures” that involve transposition of the vertical or horizontal rectus muscles or oblique muscle surgery to produce bilateral ocular torsion.115,131,133,513,559 Conceptually, one can view this procedure as taking off all vertical rectus muscles, rotating the eyes in the direction of the head tilt and then reattaching the muscles. Thus, in a child with a right head tilt, the goal of surgery would be to surgically induce clockwise torsion in both globes (as viewed from the patient’s perspective, not the examiner’s perspective) to induce a leftward environmental tilt that is compensated for by straightening the head.559 (This rationale suggests that rightward tilt in the subjective visual vertical may underlie both the head tilt and the nystagmus damping in this position).

The optimal treatment paradigm for more complex cases of torticollis associated with infantile nystagmus is the subject of ongoing investigation. For multiplanar torticollis, Hertle278 has recommended performing initial surgery to correct the largest plane of torticollis. This approach broadens the null zone in all planes so that they often don’t need additional surgery. The parents should, of course, be told that additional surgery may be necessary.278 Rarely, children with alternating horizontal null zones affecting both fields of lateral gaze may benefit from large four-muscle recessions, ­taking care to recess the medial rectus muscles slightly less than the lateral rectus muscles (unless an additional artificial divergence effect is desired).

Surgery to Improve Vision

Nystagmus surgery is an underutilized means of improving vision in patients with infantile nystagmus. Patients with infantile nystagmus report that they are slow to see, and take longer to recognize people in a crowd.565 Recognition time may be reduced by nystagmus surgery, even when visual acuity is unchanged. Cosmetic considerations often also weigh heavily in the patient’s mind. Many patients are happy with a medical or surgical treatment that reduces the intensity of their nystagmus, even when visual acuity is

unchanged. As mentioned above, examination of pre-and- postoperative Snellen visual acuity does not provide an accurate measure of patient satisfaction. Surgical procedures can also increase the horizontal range of clear vision and decrease the recognition time for objects of interest. As stated by Dell’Osso, “patients who used to have an island of clear vision now have an ocean.”286 Patients also see faster, more efficiently, and feel more confident, all of which are probably more important than improving visual acuity by one line.565 Since affected patients develop foveation periods by three months of age, it is unclear whether early surgery is necessary to minimize the amblyogenic effect of the oscillation. The optimal age for surgery to improve vision has not been established.

The goal of surgery is not to reduce the amplitude or frequency of infantile nystagmus (although these benefits occur), but to improve the quality of foveation over a wide range of gaze. In this regard, it is important for the clinician to check visual acuity in primary position, right gaze, and left gaze pre-and-postoperatively. To quantitatively assess this function, Dell’Osso et al328 have developed the expanded NAFX, a mathematical function that incorporates duration, positional accuracy, and velocity accuracy of the foveation period. Because the NAFX applies to only one position of gaze, a postoperative improvement does not reveal the true degree of visual benefit that comes from improved sidegaze acuity and quicker target recognition and target acquisition, but the later may be determined directly from eye movement data. Also, the NAFX is the foundation for the only known method to estimate a priori the percent improvement in visual acuity that will result from a four-muscle surgical procedure (e.g., the tenotomy procedure).137 This estimation has never before been possible and is impossible from visual acuity data alone.

Tenotomy with Reattachment

First devised by Hertle and Dell’Osso142,151 tenotomy with reattachment involves taking the four horizontal extraocular muscles off the globe and then reattaching them to their original insertions. Despite the early studies that showed little therapeutic effect,138,402,403 recent studies have suggested that this procedure may improve the NAFX in the null position to broaden the field of vision of the null position, and to decrease the latency of object recognition in the visual field.282,563,565 According to Dell’Osso and Hertle, this procedure allows patients to see better off to the side (because they can look around without increasing their nystagmus and degrading their vision) and refoveate faster (normal individuals foveate a moving object in about 200 ms, whereas the same act takes 1–1.5 s in an individual with infantile nystagmus).286 By decreasing their foveation time, tenotomy is said to improve their real-world level of functioning.

Tenotomy can be viewed as a neurosurgical procedure rather than an orthopedic procedure.286 Presumably, the afferent proprioceptive receptors are irritated by tenotomy, causing them to increase their baseline firing rate. The feedback loop to the brain relaxes the signal that keeps the muscles in a steady state tension, thereby putting the muscles on a lower slope of firing and tension. Unlike large four-muscle recessions, tenotomy does not reduce saccadic amplitudes or velocities, so the brain is not driven to compensatorily increase the signal.564 Despite its strong proponents, the supporting data, its muscle-sparing reversibility, and its surgical simplicity, the clinical value of four-muscle tenotomy with reattachment as an isolated procedure to treat infantile nystagmus remains highly controversial. It represents a paradigm shift in nystagmus surgery that prior to recent anatomical discoveries,92,542 was considered implausible.142 Because over 90% of children with infantile nystagmus have an abnormal head position that needs to the addressed, free tenotomy with attachments is indicated in less than 10% of cases.279

Four Muscle Recession

Numerous authors have advocated simultaneous large recessions of four horizontal rectus muscles in the treatment of infantile nystagmus. In this procedure, the medial rectus muscles must be recessed less than the lateral rectus muscle to avoid postoperative exotropia, a complication that has been observed and remains problematic, especially in binocular patients, where diplopia results. Diplopia from exotropia in lateral gaze also remains a problem in some patients regardless of whether the medial rectus muscles are recessed a little less than the lateral rectus muscles, and the long-term effects of this procedure on lateral gaze accuracy are unknown. Results suggest that it generally improves acuity measurements by an average of one line and produces considerable subjective visual improvement without inducing oscillopsia or diplopia.

In evaluating the purported benefits of this and other surgical procedures for infantile nystagmus, it should be remembered that increased foveation time (rather than decreased intensity of the nystagmus) is the fundamental correlate of acuity. Also, many early investigators recessed all four horizontal rectus muscles equally, which may have induced large exophorias, so it is difficult to assess the degree to which any postoperative improvement resulted from an unintentional artificial divergence effect. Although this procedure seems more intuitively appealing than free tenotomy without recession, Dell’Osso has cautioned that four muscle recessions may actually be counterproductive because it produces hypometric saccades, necessitating that

the ocular motor centers work to increase the central gain of horizontal eye movements and thereby work against the surgery. Also, it has been demonstrated that the obligate four-muscle tenotomy and reattachment alone (that is built into the maximal-recession procedure) accounts for the therapeutic improvements claimed, rendering the problematic aspects of large recessions unnecessary. Thus, despite its advocates, four-muscle recession seems to confer little objective or functional benefit in patients with infantile nystagmus.71

Artificial Divergence Surgery

Cüppers first advocated strabismus surgery in infantile nystagmus to diverge the eyes, requiring active convergence for fusion, which dampens the nystagmus.124 Artificial divergence surgery is the best single operation for improving visual acuity in infantile nystagmus. Unfortunately, it is suitable only in about 10% of cases – because of the fact that only about 50% of the total group of infantile nystagmus patients damp with convergence and, of those, many have overt strabismus or poor fusion.279 In planning artificial divergence surgery, it is important to first confirm the presence of fusion and stereoacuity in the preferred head position and to quantitate fusional convergence amplitudes by placing 7-10 PD base-out prisms before the eyes. It is also important to observe damping of the nystagmus during convergence. Bimedial recessions of 3 or 4 mm are sufficient to induce a large exophoria that allows the patient to use fusional convergence, thereby damping the nystagmus.

Confirming the presence of these features minimizes the risk of overcorrection with loss of fusion. Overcorrection to exotropia negates any surgical benefit and requires reoperation to reduce the deviation in an attempt to convert the exotropia to an exophoria. Spielmann515 has coined the term “pseudo-latent infantile nystagmus” to describe the dramatic increase in infantile nystagmus intensity when convergence is blocked by monocular occlusion. This finding provides indirect evidence of active convergence in the binocular state and therefore indicates an ideal result following artificial divergence surgery.514 Medical and surgical treatments of infantile nystagmus are summarized in Table 8.4.

Zubcov et al608 compared the efficacy of the artificial divergence procedure of Cüppers124 with the Kestenbaum– Andersen procedure in improving visual acuity. They found that a visual improvement of approximately one line is seen in about half of patients following either procedure and that combining the two procedures further improves vision. In practice, artificial divergence surgery can be combined with two-muscle recession (by recessing the medial and lateral rectus muscles the same amount) to simultaneously treat the torticollis and improve visual