Magnetic resonance imaging studies of the volume of the rabbit eye with intravenous mannitol

Effect of mannitol on globe and orbital volumes in humans

Purpose:

To determine the effect of intravenous mannitol on globe and orbital volumes.

Methods:

Retrospective chart review of a consecutive series of Cleveland Clinic Neurosurgical Intensive Care Unit patients who underwent computed tomographic imaging before and after intravenous mannitol administration. Volume measurements were performed according to a previously described technique by averaging axial image areas. Measurements before and after mannitol administration were compared using paired t-test.

Results:

Fourteen patients (28 eyes) met inclusion criteria. Average globe volume decreased 186 mm3 (-2.5%, p = 0.02) after mannitol administration, while average orbital volume increased 353 mm3 (+3.5%, p = 0.04). Average globe volume change for subjects with follow-up scan less than 4.7 hours (mean 1.9 hours; range 0.2-4.5 hours) after mannitol administration was -125 mm3 (-1.7%, p = 0.24) and average orbital volume change was +458 mm3 (+5.1%, p = 0.11). Average globe volume change after mannitol administration for those with follow-up more than 4.7 hours (average 13.9 hours, range 4.9-24.7 hours) was -246 mm3 (-3.3%, p = 0.05) and orbital volume change was +248 mm3 (+2.2%, p = 0.24). Dividing the study population into groups based on mannitol dose did not yield any statistically significant change.

Conclusions:

Human globe volume decreases after intravenous mannitol administration, while orbital volume increases. These volume changes occur during the time period when intraocular pressure normalizes, after the pressure-lowering effects of the drug. This novel volumetric information improves our understanding of mannitol’s mechanism of action and its effects on human ocular and periocular tissues.

Whole-globe biomechanics using high-field MRI

The eye is a complex structure composed of several interconnected tissues acting together, across the whole globe, to resist deformation due to intraocular pressure (IOP). However, most work in the ocular biomechanics field only examines the response to IOP over smaller regions of the eye. We used high-field MRI to measure IOP induced ocular displacements and deformations over the whole globe. Seven sheep eyes were obtained from a local abattoir and imaged within 48 h using MRI at multiple levels of IOP. IOP was controlled with a gravity perfusion system and a cannula inserted into the anterior chamber. T2-weighted imaging was performed to the eyes serially at 0 mmHg, 10 mmHg, 20 mmHg and 40 mmHg of IOP using a 9.4 T MRI scanner. Manual morphometry was conducted using 3D visualization software to quantify IOP-induced effects at the globe scale (e.g. axial length and equatorial diameters) or optic nerve head scale (e.g. canal diameter, peripapillary sclera bowing). Measurement sensitivity analysis was conducted to determine measurement precision. High-field MRI revealed an outward bowing of the posterior sclera and anterior bulging of the cornea due to IOP elevation. Increments in IOP from 10 to 40 mmHg caused measurable increases in axial length in 6 of 7 eyes of 7.9 ± 5.7% (mean ± SD). Changes in equatorial diameter were minimal, 0.4 ± 1.2% between 10 and 40 mmHg, and in all cases less than the measurement sensitivity. The effects were nonlinear, with larger deformations at normal IOPs (10-20 mmHg) than at elevated IOPs (20-40 mmHg). IOP also caused measurable increases in the nasal-temporal scleral canal diameter of 13.4 ± 9.7% between 0 and 20 mmHg, but not in the superior-inferior diameter. This study demonstrates that high-field MRI can be used to visualize and measure simultaneously the effects of IOP over the whole globe, including the effects on axial length and equatorial diameter, posterior sclera displacement and bowing, and even changes in scleral canal diameter. The fact that the equatorial diameter did not change with IOP, in agreement with previous studies, indicates that a fixed boundary condition is a reasonable assumption for half globe inflation tests and computational models. Our results demonstrate the potential of high-field MRI to contribute to understanding ocular biomechanics, and specifically of the effects of IOP in large animal models.

Preoperative Mannitolization Can Decrease Corneal Endothelial Cell Damage After Cataract Surgery

PURPOSE

To evaluate whether preoperative mannitolization can change ocular biometry and affect postoperative corneal endothelial cell density.

METHODS

Bilateral sequential cataract surgery was performed in 38 patients. Preoperative mannitolization was done in one eye of each subject. We checked the change in preoperative ocular biometry before and after intravenous mannitolization. We compared the postoperative corneal endothelial cell density between eyes with mannitolization and without mannitolization at postoperative week 1, 2, 5, and 8. We evaluated the relationship between change in ocular biometry and change in postoperative corneal endothelial cells in eyes that underwent preoperative mannitolization.

RESULTS

After mannitolization, eyes exhibited decreased intraocular pressure, axial length (AL), and vitreous chamber depth (VCD) and increased anterior chamber depth (ACD) and lens position (LP) compared to before mannitolization (p < 0.05). Preoperative mannitolization has a tendency to decrease the intraoperative use of phaco energy in eyes with moderate nucleosclerosis. Eyes with preoperative mannitolization showed less loss of postoperative corneal endothelial cells than eyes without preoperative mannitolization (p < 0.05). The ACD, LP, and AL changes by mannitolization were all negatively correlated with corneal endothelial cell loss (p < 0.05).

CONCLUSION

Preoperative mannitolization can decrease postoperative loss of corneal endothelial cells. The protective effect of preoperative mannitolization on the corneal endothelium may be due to the decreased need for phaco energy and changes in ocular biometry such as ACD, AL, and LP.

Simulating intravitreal injections in anatomically accurate models for rabbit, monkey, and human eyes

Purpose

To develop models for rabbit, monkey, and human that enable prediction of the clearance after intravitreal (IVT) injections in one species from experimental results obtained in another species.

Methods

Anatomically accurate geometric models were constructed for rabbit, monkey, and human that enabled computational fluid dynamic simulation of clearance of an IVT injected bolus. Models were constructed with and without the retrozonular space of Petit. Literature data on clearance after IVT injection of substances spanning a range of molecular weight up to 157 kDa were used to validate the rabbit model.

Results

The space of Petit had a significant increase on the clearance of slowly diffusing substances cleared by the anterior pathway by reducing the bottleneck for drug efflux. Models that did not include this zone could not accurately predict the clearance of slowly diffusing substances whose clearance was accelerated by intraocular pressure-driven convection.

Conclusions

The ocular anatomy must be carefully reconstructed in the model to enable accurate predictions of clearance. This method offers an alternative means for scaling experimental data from one species to another that may be more appropriate than other simple approaches based entirely upon scaling of compartment volumes and flow rates.

Simulating dissolution of intravitreal triamcinolone acetonide suspensions in an anatomically accurate rabbit eye model

Purpose

A computational fluid dynamics (CFD) study examined the impact of particle size on dissolution rate and residence of intravitreal suspension depots of Triamcinolone Acetonide (TAC).

Methods

A model for the rabbit eye was constructed using insights from high-resolution NMR imaging studies (Sawada 2002). The current model was compared to other published simulations in its ability to predict clearance of various intravitreally injected materials. Suspension depots were constructed explicitly rendering individual particles in various configurations: 4 or 16 mg drug confined to a 100 μL spherical depot, or 4 mg exploded to fill the entire vitreous. Particle size was reduced systematically in each configuration. The convective diffusion/dissolution process was simulated using a multiphase model.

Results

Release rate became independent of particle diameter below a certain value. The size-independent limits occurred for particle diameters ranging from 77 to 428 μM depending upon the depot configuration. Residence time predicted for the spherical depots in the size-independent limit was comparable to that observed in vivo.

Conclusions

Since the size-independent limit was several-fold greater than the particle size of commercially available pharmaceutical TAC suspensions, differences in particle size amongst such products are predicted to be immaterial to their duration or performance.

Manganese-enhanced MRI of the DBA/2J mouse model of hereditary glaucoma

PURPOSE

To test the hypothesis that manganese-enhanced magnetic resonance imaging (MEMRI) is a sensitive approach for measuring of age-related ocular changes in experimental pigmentary glaucoma.

METHODS

Four groups of light-adapted mice were studied using MEMRI: young (2-3 months), C57BL/6 (negative controls), and DBA/2J mice and aged (10-11 months) C57BL/6 and DBA/2J mice. In all mice, eye perimeter, optic nerve head width, iridocorneal angle, ciliary body area, and total and inner retinal thickness, and a surrogate of retinal ion regulation (intraretinal uptake of manganese) were assessed from MEMRI data and compared. Axon counts were obtained from optic nerves harvested from MEMRI-assessed eyes.

RESULTS

As the C57BL/6 and DBA/2J mice aged, differential and significant changes in ocular perimeter, retinal thickness, iridocorneal angle, ciliary body area, and optic nerve head width were readily measured from MEMRI data (P < 0.05). In C57BL/6 mice, only inner retinal thickness and perimeter were correlated. In DBA/2J mice, ocular perimeter was correlated with total and inner retinal thickness, ciliary body area, optic nerve head width, and iridocorneal angle. Comparison of young and aged mice revealed a subnormal intraretinal manganese uptake (P < 0.05) in aged DBA/2J mice, but not in aged C57BL/6 mice. Manganese uptake did not correlate with the ocular perimeter. Axon density in the optic nerve correlated with MEMRI-measured optic nerve head width (P < 0.05).

CONCLUSIONS

These studies provide a baseline of noninvasive MEMRI-detectable changes associated with age in a common animal model of hereditary glaucoma that may be useful in the longitudinal evaluation of therapeutic success.

Intracameral Vigamox (moxifloxacin 0.5%) is non-toxic and effective in preventing endophthalmitis in a rabbit model

PURPOSE

To determine whether Vigamox (moxifloxacin 0.5% ophthalmic solution) can be safely injected intracamerally to prevent Staphylococcus aureus endophthalmitis in a rabbit model.

DESIGN

Animal study.

METHODS

The safety and bactericidal-effectiveness of Vigamox were evaluated in three stages using 189 New Zealand White rabbits. (Stage 1) The toxicity of two intravitreal doses of Vigamox (moxifloxacin 500, 250 microg) was compared with vancomycin (1 mg) and saline. (Stage 2) A reproducible rabbit model of Staphylococcus aureus endophthalmitis was established. (Stage 3) The bactericidal effect of intracameral Vigamox (moxifloxacin 500, 250, 125, 50 microg) was compared with vancomycin (1 mg) and saline. Intracameral antibiotic therapy commenced immediately after Staphylococcus aureus intravitreal challenge (5000 cfu). Toxicity was evaluated by masked clinical examination using a slit-lamp, an indirect ophthalmoscope, and corneal-ultrasound pachymetry. The clinical examination included the exterior eye, cornea, anterior chamber, vitreous, and retina. The presentations were graded on a severity scale of 0, 0.5, 1, 2, and 3. The bactericidal efficacy was determined using intracameral colony counts.

RESULTS

In the toxicity studies without bacterial challenge, the clinical scores of rabbits injected intracamerally with Vigamox were statistically equivalent to rabbits given intracameral vancomycin or saline. In the efficacy studies, eyes treated intravitreally with Vigamox, at all doses, or vancomycin were negative for Staphylococcus aureus and nontreated controls remained culture-positive.

CONCLUSIONS

Vigamox appears to be nontoxic for intracameral injection and effective in preventing experimental endophthalmitis in the rabbit model. Further studies will determine the clinical role of intracameral Vigamox for surgical prophylaxis and postoperative therapy.