IN-vision
Furthering Research into Infantile Nystagmus (IN)
Registered Charity
Therapies
No ‘cure’ for INS has yet been discovered; however there are a large number of interventions that have been shown to be effective in reducing the eye movements, moving the null zone, and/or improving visual function.
Contact lenses
Contact lenses provide superior refractive correction over conventional spectacle lenses for nystagmats due to the reduction in aberrations, and induced prismatic effect experienced as the eye moves away from the primary position. Allen and Davies (1983) found an increase in visual acuity of at least one Snellen line in seven out of eight nystagmats using contact lenses compared to when wearing spectacles. This was confirmed by Biousse et al. (2004). Contact lenses also appear to dampen nystagmus eye movements in INS, but this effect is not present when the eye is anaesthetised (Dell’Osso et al. 1988). This finding suggests that the presence of the lens touching the eye – rather than the optical effects of the lens – serve to reduce the movement, which implies that nystagmus intensity is partially governed by signals from nerves in this region. Dell’Osso, Leigh and Daroff (1991) went on to show that cutaneous stimulation of the ophthalmic division of the trigeminal nerve can cause a reduction of nystagmus intensity, using gentle touches, vibrations, pressing and rubbing of the forehead and upper eyelids.
Prism correction
The fact that many nystagmats have a convergent null zone led to the suggestion that prescribing base out prism (to drive convergence) could improve visual function. A study by Dickinson (1986) found no improvement in the contrast sensitivity function with this method; nystagmus intensity was, however, reduced. Rarely, the null zone is in the divergent position, in which case base in prism has been shown to be useful. A study by Dell’Osso (2002) found an improvement in visual acuity of two Snellen lines when prisms were used both to drive convergence and to place the eyes into a null zone of gaze. As ever when prescribing prism, care must be taken to balance the refractive and prismatic prescriptions with respect to the accommodative convergence : accommodation ratio (Dell’Osso 2002). Top of page
Surgery
Several surgical procedures have been advocated for INS. These include:
- Anderson-Kestenbaum surgery
- Artificial divergence
- Horizontal rectus recession
- Tonotomy
- Combinations of the above
Anderson-Kestenbaum surgery
In 1953, Kestenbaum devised a surgery intended to move the null zone towards the primary position, thus reducing any AHP present. This procedure involves a recession of the rectus muscles with action in the direction of the face turn, and resection of the antagonists. A form of this procedure is still used today and is effective in reducing AHPs (Lee 2002). Top of page
Artificial divergence
Using a similar principle to base out prisms, performing resections in both lateral recti induces a latent exophoria which is overcome by employing fusional convergence (in patients with sufficient fusional reserves). This serves to reduce the intensity of nystagmus. In a surgical study by Zubcov et al. (1993), three out of six patients undergoing artificial divergence surgery experienced a measurable improvement in visual acuity. Five patients underwent combined Anderson-Kestenbaum and artificial divergence surgery, four of whom gained two or more lines of Snellen acuity. Top of page
Horizontal rectus recession
The recession of all four horizontal recti has been shown to increase visual acuity whilst reducing nystagmus intensity. It has been suggested as an easier-to-perform alternative to Anderson-Kestenbaum surgery. In a study on ten subjects with INS undergoing this procedure, eight exhibited a reduction in the amplitude of their nystagmus with a concordant average improvement in visual acuity of one line.
Tenotomy
The ‘anterior tenotomy’ procedure was pioneered in 1999 (Dell’Osso et al.) on an achiasmatic mutant Belgian sheepdog with INS. The procedure was later performed on ten human subjects with no adverse events, leading to improved clinical visual acuity in five subjects, improved subjective visual function in nine subjects, and increased foveation periods in these same nine subjects. The procedure, which involves severing the horizontal recti, followed by reattachment at the original site, was first proposed in 1998 (Dell’Osso).
Combination surgery
Given the number of surgical approaches available, surgeons have recently been implementing combinations and modifications of the above procedures, with generally positive outcomes in both the nystagmus intensity, and clinical visual acuity (Bishop 2011; Kumar et al. 2011). Top of page
Pharmacological
Many medicines are known to reduce the intensity of nystagmus. In 2002, a list of fourteen treatments reported to improve the condition was published (Stahl, Plant, and Leigh). Of these, four have been shown to effectively dampen nystagmus intensity in INS (memantine, gabapentin, baclofen and cannabis). Memantine (20-24mg) and gabapentin (up to 2400mg) both reduce nystagmus intensity and improve visual acuity in INS, and have been validated in a controlled, double-masked randomised trial (Sarvananthan et al. 2006; Shery et al. 2006; McLean et al. 2007). One case report of a subject with IINS showed that smoking cannabis can cause a reduction in nystagmus intensity and an improvement in clinical visual acuity (Pradeep et al. 2008). Baclofen is often used in patients with periodic alternating nystagmus, as it is known to reduce nystagmus amplitude, improve visual acuity and alleviate AHP (Solomon et al. 2002; Comer et al. 2006).
Dexedrine, a stimulant used to treat attention deficit hyperactivity disorder, has been shown to increase foveation duration, improve stereopsis, reduce an exotropic deviation and improve visual acuity in a patient with INS associated with rod-cone dystrophy (Hertle et al. 2001).
A recent study demonstrated that brinzolamide, applied topically, can cause an improvement in NAFX in a patient with INS, providing hope for non-systemic approaches to nystagmus pharmaceuticals (Dell’Osso et al. 2011).
The mechanisms of the above medications are currently unknown, although their mode of action is suspected to be through sedation rather than specifically reducing the eye movements (Abel 2006). For a detailed account of the pharmacological treatments available for INS, the reader is directed to McLean and Gottlob (2009) and Strupp et al. (2011). Top of page
Other (biofeedback, acupuncture)
Biofeedback
Auditory biofeedback, derived from live eye movement recordings, provide nystagmats with direct feedback from their nystagmus, and with practice, patients are able to reduce the intensity of their nystagmus (Mezawa et al. 1990). This suggests that with practice, nystagmats can learn to consciously control their eye movements. After six half-hour sessions of auditory biofeedback, a study by Sharma et al. (2000) found nystagmus amplitude to be reduced by 51% and intensity reduced by 60%, with a subjective improvement in quality of vision. Abadi, Carden and Simpson (1980) found an objective improvement in visual acuity between 0.13 and 0.32 (logMAR). However, these improvements are generally not sustained following discontinuation of the therapy (Sharma et al. 2000), but in a study by Ciuffreda, Goldrich and Neary (1982), one subject was able to reduce their nystagmus eye movements to 50% of the pre-training level on demand, without biofeedback. Top of page
Acupuncture
In 1987 (Ishikawa, Ozawa, and Fujiyama), acupuncture was shown to reduce nystagmus intensity in nine out of 16 subjects tested. Blekher et al. (1998) showed that insertion of two acupuncture needles in the sternocleidomastoid muscles of the neck caused a significant increase in foveation durations in four out of six patients, which was observed five minutes following the end of treatment. In this experiment, one subject who was particularly responsive to the treatment later had a sham treatment administered, which caused exacerbation of his nystagmus. In this instance, foveation periods reduced at the times when the needle guide tubes were tapped, in contrast to an observed increase in foveation time during genuine acupuncture treatment. Top of page