Stephen P. Christiansen

Human CHN1 Mutations Hyperactivate α2-Chimaerin and Cause Duane's Retraction Syndrome

Duanes retraction syndrome (DRS) is a complex congenital eye movement disorder caused by aberrant innervation of the extraocular muscles by axons of brainstem motor neurons. Studying families with a variant form of the disorder (DURS2-DRS), we have identified causative heterozygous missense mutations in CHN1, a gene on chromosome 2q31 that encodes α2-chimaerin, a Rac guanosine triphosphatase–activating protein (RacGAP) signaling protein previously implicated in the pathfinding of corticospinal axons in mice. We found that these are gain-of-function mutations that increase α2-chimaerin RacGAP activity in vitro. Several of the mutations appeared to enhance α2-chimaerin translocation to the cell membrane or enhance its ability to self-associate. Expression of mutant α2-chimaerin constructs in chick embryos resulted in failure of oculomotor axons to innervate their target extraocular muscles. We conclude that α2-chimaerin has a critical developmental function in ocular motor axon pathfinding.

The Infant Aphakia Treatment Study: Design and Clinical Measures at Enrollment

OBJECTIVEnTo compare the use of contact lenses and intraocular lenses (IOLs) for the optical correction of unilateral aphakia during infancy.nnnMETHODSnIn a randomized, multicenter (12 sites) clinical trial, 114 infants with unilateral congenital cataracts were assigned to undergo cataract surgery with or without IOL implantation. Children randomized to IOL treatment had their residual refractive error corrected with spectacles. Children randomized to no IOL treatment had their aphakia treated with a contact lens.nnnMAIN OUTCOME MEASURESnGrating acuity at 12 months of age and HOTV visual acuity at 4 1/2 years of age.nnnAPPLICATION TO CLINICAL PRACTICEnThis study should determine whether either treatment for an infant with a visually significant unilateral congenital cataract results in a better visual outcome.nnnRESULTSnEnrollment began December 23, 2004, and was completed January 16, 2009. The median age at the time of cataract surgery was 1.8 months. Fifty patients were 4 to 6 weeks of age at the time of enrollment; 32, 7 weeks to 3 months of age; and the remaining 32, more than 3 to less than 7 months of age. Fifty-seven children were randomized to each treatment group. Eyes with cataracts had shorter axial lengths and steeper corneas on average than the fellow eyes.nnnCONCLUSIONSnThe optimal optical treatment of aphakia in infants is unknown. However, the Infant Aphakia Treatment Study was designed to provide empirical evidence of whether optical treatment with an IOL or a contact lens after unilateral cataract surgery during infancy is associated with a better visual outcome.

Extraocular Muscle Force Generation after Ricin-mAb35 Injection: Implications for Strabismus Treatment

PURPOSEnRicin-mAb35 is an immunotoxin targeted against skeletal muscle. Previously, we have shown that injection of ricin-mAb35 into rabbit extraocular muscle results in long-term muscle loss, and we have proposed this as a potential treatment for strabismus. In this study, we assessed the effects of ricin-mAb35 injection on extraocular muscle force generation.nnnMETHODSnRicin-mAb35, 0.2 microg/kg in a volume of 0.1 mL, was injected into 1 superior rectus muscle in 16 adult rabbits. The contralateral superior rectus was injected with an equal volume of normal saline. Muscle force generation was assessed in vivo at 1, 6, and 12 weeks. Isometric length-tension curves were developed. Single-twitch tension, peak tetanic force generation, and fatigue rate were determined at optimal preload. Data from treated and control muscles were compared with the paired t test.nnnRESULTSnForce generation declined in ricin-mAb35 treated muscles at each postinjection interval. At 12 weeks, mean tetanic tension (200 Hz) in treated muscles was 13.8 mN/cm(3) compared with 27.7 mN/cm(3) in saline-injected controls (P =.02), a reduction of 50%. Single-twitch tension at 12 weeks was reduced 33% compared to controls (P =.04). Similar effects were noted at 1 and 6 weeks. Fatigue rate was not greater in treated muscles at any postinjection intervals.nnnCONCLUSIONSnInjection of ricin-mAb35 results in sustained weakness in extraocular muscle, although additional studies will be required to determine the duration of physiologic effect. These results confirm our histological analysis and suggest that ricin-mAb35 may be a more long-term alternative to botulinum toxin A for the treatment of strabismus.

Myotoxic effects of the skeletal muscle-specific immunotoxin, ricin-mAb35, on orbicularis oculi muscle after eyelid injections in rabbits.

Purpose: The authors recently demonstrated that a single injection of the immunotoxin ricin-mAb35 has potent and long-lasting myotoxic effects in extraocular muscles. The myotoxicity of injected ricin-mAb35 was tested in the eyelids of rabbits to determine its potential for use in the treatment of benign essential blepharospasm and other dystonias. Methods: The immunotoxin ricin-mAb35 was injected in one eyelid of adult rabbits. After 1 week, 1 month, or 6 months, the rabbits were euthanized, and the eyelids were prepared for histologic examination of inflammatory cell infiltrate with immunohistochemical localization of cd11b and myosin heavy chain isoform expression. Muscle loss was quantified by analysis of muscle fiber cross-sectional area and total myofiber number. Results: Within the first week after a single injection of ricin-mAb35, some edema developed, which resolved by the second week. Otherwise, the eyelids were normal in appearance. A short-lived inflammatory response was seen at 1 week, but this resolved 1 month after treatment. One week after injection, there was a significant decrease in the total number of orbicularis oculi myofibers in the ricin-mAb35–treated eyelids. This myofiber loss remained significant 1 month later and was maintained 6 months after the initial injection. Conclusions: Direct injection of the immunotoxin ricin-mAb35 resulted in significant, acute muscle loss in the orbicularis oculi of rabbits that was maintained for up to 6 months. Physiologic studies are needed to demonstrate concomitant loss of muscle strength, but these results suggest that ricin-mAb35 injection holds promise as a muscle-weakening agent in the eyelid.

Anterior ciliary circulation from the horizontal rectus muscles

Purpose: To quantify the anterior ciliary arteries that accompany horizontal rectus muscles and to correlate these findings with analysis of arterial cross-sectional area in distal tendons of these muscles. Patients and Methods: In 31 consecutive patients, previously unoperated horizontal rectus muscles were photographed intraoperatively. Major and minor anterior ciliary arteries (ACAs) were counted from the photographs by a masked independent observer and compared between medial and lateral rectus muscles. Cross sections from medial and lateral rectus muscle resection specimens were stained for elastin to quantify arterial volume in the distal tendons. Results: Twenty lateral and 22 medial rectus muscles from 31 patients were included. There was no significant difference in the mean number of major anterior ciliary arteries between lateral rectus muscles (2.25 ± 0.79, range 1–4) and medial rectus muscles (2.05 ± 0.65, range 1–3) (p = 0.46). The mean number of minor anterior ciliary arteries was 6.7 in both medial and lateral rectus muscles (p = 0.97). Resected rectus muscle tendons showed no significant difference between lateral and medial rectus muscles for arterial vessel number or cross-sectional area. Conclusions: These results show the contribution from the lateral rectus muscle to the anterior segment circulation may be more robust than is commonly taught.

Extraocular Muscles: Extraocular Muscle Anatomy

The extraocular muscles (EOMs) are responsible for complex and coordinated eye movements that allow fixation of the visual world on analogously located sites of the retinae. There are six EOMs in each orbit: two oblique muscles and four rectus muscles. Histologically, two distinctly different layers can be seen: an outer orbital layer, where the myofibers are extremely small, and an inner global layer. The EOMs are quite distinct from noncranial skeletal muscles. They have one of the fastest contraction rates, the smallest motor units, and contain amongst the smallest myofiber cross-sectional areas. In non-cranial skeletal muscle, generally two main fiber types are described, fast and slow, whereas in EOM there is a much greater diversity. At least eight myosin heavy chain isoforms are expressed, in contrast to the three or four expressed in body and limb skeletal muscle. The following isoforms of myosin heavy chains are expressed in EOM: slow or type 1, slow-tonic, fast or type 2a, type 2b (nonhuman), type 2x/d, neonatal, developmental, α-cardiac, and EOM-specific. These MyHC isoforms display complex expression patterns in EOM, and the majority of myofibers co-express multiple isoforms. These hybrid myofibers are the norm rather than the exception in normal adult EOM. In addition to hybrid myofibers, there are also mismatched myofibers, as single myofibers are seen to co-express various molecules associated with only slow or only fast myofibers in non-cranial muscle. These include myosin light chains, troponin, and molecules in various metabolic pathways such as succinate dehydrogenase and α-glycerophosphate dehydrogenase. There are two patterns of myofiber innervation in the EOM. The majority of fibers are singly innervated, and about 10% of the fibers in both orbital and global layers are multiply innervated. The complexity of fiber types, innervation pattern, and mismatched expression of various molecules set these muscles apart from non-cranial skeletal muscles and suggest that there is a continuum of skeletal muscle fiber types within the EOM. This would be reflected in a continuum of shortening velocities. n nEOMs undergo continuous remodeling throughout normal adult life, apparently adding and removing nuclei in regions of individual myofibers. These muscles have an exceptionally large number of myogenic precursor cells, a percentage of which are activated and dividing even in aging eye muscles. The control of this process is currently unknown; however, it may be responsible for the eye muscles unusual recalcitrance to the effects of denervation.

Botulinum toxin pretreatment augments the weakening effect of injection with ricin-mAb35 in rabbit extraocular muscle

PURPOSEnTo examine force generation and duration of effect of botulinum toxin pretreatment, followed by injection of ricin-mAb35 in extraocular muscle.nnnMETHODSnIn normal adult rabbits, one superior rectus muscle was injected with either 5 units botulinum toxin or 1/50 maximally tolerated dose for rats (rMTD) of ricin-mAb35. Additional rabbits were first injected with 5 units botulinum toxin, and after 1, 2, or 4 weeks the same muscle was injected with either 1/10 or 1/50 rMTD ricin-mAb35. In each treatment group, the contralateral muscle was injected with an equal volume of saline. After 12 weeks (1/50 rMTD) or 6 months (1/10 rMTD), the rabbits were euthanized. Both SR muscles were removed and assayed physiologically, using an in vitro apparatus.nnnRESULTSnTwelve weeks after treatment with either botulinum toxin or immunotoxin alone, only ricin-mAb35-treated muscles were weaker than control muscles at tetanic stimulation frequencies. Pretreatment with botulinum toxin prior to injection with immunotoxin, especially at shorter intervals between injections, resulted in significant decreases in twitch and tetanic force generation compared with controls and muscles treated with ricin-mAb35 only or botulinum toxin only. At 6 months, force generation was decreased from control only in muscles treated with the higher dose of ricin-mAb35. Botulinum toxin pretreatment did not augment this effect at 6 months.nnnCONCLUSIONSnUpregulation of postsynaptic nicotinic acetylcholine receptors caused by botulinum toxin pretreatment amplifies the reduction of force generation in extraocular muscle following secondary injection of the immunotoxin ricin-mAb35 within 3 months of treatment.

Horizontal Misalignment in Patients With Unilateral Superior Oblique Palsy

PURPOSEnTo determine the frequency and distribution of horizontal misalignment in patients with unilateral superior oblique palsy (SOP) and to determine the indications for combining horizontal with vertical strabismus surgery.nnnMETHODSnPatients included in the study had a vertical heterophoria or tropia that fit Parks three-step test for SOP and had no previous strabismus surgery or other ocular motility disturbance. Ocular motility and alignment were recorded. Outcomes between patients who had vertical surgery alone and those who had combined vertical and horizontal surgery were compared using the Students t test.nnnRESULTSnOf 205 patients, 121 (59.0%) had a horizontal misalignment in addition to vertical strabismus. Ninety-six patients (46.8%) required strabismus surgery. Of these, 29 had 8 prism diopters (PD) or more horizontal deviation. Twenty-two had vertical combined with horizontal surgery (V+H group). Although their initial deviation was greater, these patients had better surgical outcomes than patients who had vertical surgery alone (V group). The V+H group had a final mean horizontal deviation of 2.18 PD compared with 6.85 PD in the V group (P < .01). Postoperative vertical alignment in the V+H group was also superior with a final mean vertical deviation of 3.7 versus 6.8 PD for the V group (P = .12).nnnCONCLUSIONnThese results indicate that horizontal misalignment is common in patients with SOP. Patients with 8 PD or more horizontal deviation benefited from surgical correction of the horizontal deviation in addition to the vertical surgery.