Robert S. Baker

Extraocular muscles: basic and clinical aspects of structure and function

Although extraocular muscle is perhaps the least understood component of the oculomotor system, these muscles represent the most common site of surgical intervention in the treatment of strabismus and other ocular motility disorders. This review synthesizes information derived from both basic and clinical studies in order to develop a better understanding of how these muscles may respond to surgical or pharmacological interventions and in disease states. In addition, a detailed knowledge of the structural and functional properties of extraocular muscle, that would allow some degree of prediction of the adaptive responses of these muscles, is vital as a basis to guide the development of new treatments for eye movement disorders.

Muscles of a different 'color' The unusual properties of the extraocular muscles may predispose or protect them in neurogenic and myogenic disease

The rules that govern many aspects of skeletal muscle structure and function are very different for the extraocular muscle allotype. The myoblast lineages present in the extraocular muscle primordia are permissive for generation of an unusually wide range of fiber types. The balance that is struck between genetic specification and activity dependent factors in shaping fiber phenotype to suit the demands of complex visuomotor systems is not yet well defined. Because skeletal muscle has high energy demands, diversity in fiber types is needed to maximize efficiency; greater diversity in fiber composition then indicates a more diverse functional repertoire. Together, the characteristics of small motor unit size, precise dependence of muscle force upon motor neuron discharge rate, high contractile speed but low tension development, and contractile protein heterogeneity contribute toward the high precision and diversity that is required for eye movements. Finally, the structural and functional characteristics and plasticity of the individual extraocular muscle fiber types play an important role in determining their response to disease or manipulation. The lack of uniform responses across the muscle allotypes in disease, or in response to pharmaceutical or surgical interventions, requires that we obtain a better understanding of the fundamental differences that exist between muscle groups.

Idiopathic intracranial hypertension (pseudotumor cerebri) in pediatric patients

Improved evaluation of visual features of idiopathic intracranial hypertension in pediatric patients has resulted in the demonstration of optic neuropathy which may be reversible. A wide variety of etiologic associations should be investigated to provide definitive therapy; however, nonspecific means often will be required to lower intracranial pressure. It is hoped that a double-blind controlled trial of medical therapies and another for surgical therapies will be conducted to elucidate optimum management of this condition.

Ocular motor abnormalities from head trauma

Head injuries cause the hospitalization of 200-300 persons per 100,000 population per year. Ophthalmologists provide diagnostic and therapeutic care to those trauma victims with damage to the globe, optic nerve, orbit, and ocular motor system. Eye movements can be affected by damage at any level of the central nervous system or peripheral motor unit. Comprehensive ocular motor assessment of the trauma patient can substantially contribute to the understanding of the patients injury, recovery, and rehabilitation. This review examines all aspects of head and face trauma that can lead to ocular motility disturbances.

Eye Injuries in Athletics and Recreation

Sports-related eye injuries represent a significant eye health hazard worldwide. In the USA, it is estimated that eye injuries from sports account for more than 100,000 physician visits per year at the cost of greater than

Corneal topographic changes after extraocular muscle surgery

175 million. The sports responsible for the greatest number of injuries are baseball, ice hockey, and racquet sports. In the USA there are over 189 million participants in these sports with a disproportionate number (one-third) of injuries involving the pediatric population. The most common injuries are abrasions and contusions, followed by more serious injuries, including hyphema. Substantial reduction in the incidence of eye injuries through prevention has been demonstrated in the sport of hockey which can serve as a model for other sports. Specific criteria for protective eyewear must be developed. Polycarbonate plastic lenses and frames which are sturdy and impact resistant provide optimal protection. Lensless goggles, street wear and spectacle correction glasses do not provide adequate eye protection. The ophthalmologists role in preventing sports-related eye injuries includes addressing particular sports participants needs, identifying one-eyed athletes, and informing patients of the need for protective eyewear.

Efficacy of gold weight implants in facial nerve palsy: quantitative alterations in blinking

PURPOSE Changes in refractive error have been reported after strabismus surgery. The influence of extraocular muscle tension on corneal topography is thought to be an important mechanism contributing to this alteration. This study investigates topographic changes after strabismus surgery in human beings. METHODS The corneal topography of 63 eyes of 43 patients who underwent either strabismus or optic nerve sheath fenestration (ONSF) surgery was measured before and after operation. The ONSF surgery involved removing and reattaching the medial rectus muscle, thus acting as a control for the strabismus procedures. Corneal power was measured centrally and at 1.5 and 3.0 mm in 8 meridians. Analysis of variance was performed to compare the difference in preoperative and post-operative corneal power for all procedures and comparing recessions, resections, combined recession and resection, and ONSF as grouped procedures. RESULTS All groups showed a significant change in preoperative to postoperative corneal power (P< .01). A significant change was seen between all grouped procedures except when the resection was compared with ONSF and compared with recession. CONCLUSIONS Change in the tension of 1 muscle does not produce a change in the adjacent quadrant as much as it produces a significant change in the entire corneal surface, which illustrates the interaction and interdependence all corneal positions have with respect to each other.

Perimetric Findings in Functional Disorders Using Automated Techniques

Deficient eyelid closure is a major visual threat to patients with unresolved facial nerve palsy. Gold weight implants assisted eyelid closure in patients with paresis of the orbicularis oculi, ameliorating patient complaints of dry eye, excessive tearing, and corneal epithelial breakdown. We used dynamic measures to assess the efficacy of upper eyelid gold weight implantation surgery for facial nerve palsy. The search coil technique was used to record spontaneous blinks bilaterally in six patients, before and after unilateral gold weight implantations into the upper eyelid in severe facial nerve palsy. In uncomplicated facial nerve palsy, the amplitude of blink down-phases for the paretic eyelid was 28.6 +/- 5.7% of the amplitude of the contralateral, normal eyelid. Following corrective surgery, closure of the paretic eyelid improved to 42.6 +/- 7.5% (P < 0.05). There was not a commensurate increase in the peak velocity of blink down-phases, suggesting that gold weight effects are mediated by a passive improvement in blink dynamics.

Botulinum A toxin stimulates neurite branching in nerve-muscle cocultures

Visual fields were assessed by automated static thresholding techniques in 15 consecutive patients with functional visual loss. Although field abnormalities were recorded in all patients, the pattern of abnormality was not distinguishable from organic loss. Furthermore, no form of reliability testing, including numbers of repetitions, false-positives or false-negatives, root mean square (RMS) evaluation, or percentage of fixation losses, was capable of characterizing functional defects. The authors conclude that automated perimetry, as currently practiced, cannot differentiate functional from organic visual field loss.

Absence of oculomotor and trochlear motoneurons leads to altered extraocular muscle development in the Wnt-1 null mutant mouse.

In addition to skeletal muscle paralysis, type A botulinum toxin commonly causes sprouting of motor axons in various experimental whole-animal systems. The use of type A botulinum toxin in clinical treatment of muscle spasm disorders is becoming increasingly popular. The eventual, unwanted return of involuntary activity in the treated muscles may be a consequence of such axon sprouting. We have developed a coculture model allowing the quantification of botulinum toxin-induced sprouting that shows promise for future studies on its mechanism and control. Chick embryo ciliary ganglion motor neurons were cocultured with chick leg muscle cells. The presence of type A botulinum toxin in the coculture medium was correlated with significantly increased branching frequency of neurites. Toxin-increased branching frequency occurred even when the neurons and muscle cells were separated from each other on the culture dishes, suggesting a presynaptic effect of toxin. Cocultures incubated in the presence of curare, a post-synaptic blocker, had control levels of neurite branching, ruling out the possibility that simple synaptic blockade causes sprouting but again supporting the hypothesis of a pre-synaptic activity of botulinum toxin.