Atropine induces narrowing of the superior and inferior iridocorneal angle and ciliary cleft in normal horse eyes.

Objective Summary

• This study examined how atropine eye drops affect the drainage angles and clefts of the eye in healthy horses using detailed ultrasound imaging.
• The research found that atropine causes specific areas of the eye’s drainage system to narrow, which may impact eye pressure and suggests caution in certain eye conditions.

Introduction and Purpose

• The iridocorneal angle (ICA) and ciliary cleft (CC) are important structures in the eye involved in fluid drainage and intraocular pressure (IOP) regulation.
• Atropine is a commonly used ophthalmic medication that can influence pupil size and possibly eye anatomy and pressure.
• The study aimed to investigate atropine’s effects on the ICA and CC in normal equine (horse) eyes using ultrasound biomicroscopy (UBM), a high-resolution imaging modality.

Methods

• Participants: 12 normal adult horses without known eye disease.
• Measurements included:
  • Intraocular pressure (IOP).
  • Vertical and horizontal pupil diameters.
  • Ultrasound biomicroscopy imaging of ICA and CC at four quadrants of the eye: superior, temporal, inferior, and nasal.
• Parameters measured in the ICA and CC:
  • Pectinate ligament distance (PLD): Distance across a key structure of the drainage angle.
  • Ciliary cleft length (CCL): Length of the fluid drainage space.
  • Height of opening distance (HOD): Vertical opening dimension of the cleft.
  • Ciliary cleft area (CCA): Cross-sectional area of the cleft.
  • Pectinate ligament angle (PLA): Angular measurement relevant to the angle’s openness.
• Measurements were taken before and 2 hours after administering 1% atropine ophthalmic solution.

Results

• Intraocular Pressure (IOP):
  • Mean IOP decreased significantly from 14.17 ± 2.34 mm Hg to 12.67 ± 1.14 mm Hg post-atropine.
• Pupil Diameter:
  • Although not explicitly detailed in the abstract, pupil size likely increased due to atropine’s known mydriatic effect.
• Iris-Corneal Angle and Ciliary Cleft Measurements:
  • No significant changes in the nasal or temporal quadrants after atropine.
  • Superior quadrant:
    • Significant decreases in CCL, CCA, HOD, and PLA—indicating a narrowing of the drainage angle and ciliary cleft.
    • Specific numeric reductions, e.g., CCL went from 2.74 ± 0.52 mm to 2.62 ± 0.43 mm.
  • Inferior quadrant:
    • Significant decreases in PLD, HOD, and PLA—also indicating narrowing.
    • For example, PLD decreased from 1.46 ± 0.23 mm to 1.39 ± 0.17 mm.

Interpretation and Clinical Significance

• The findings demonstrate that atropine causes localized narrowing of the superior and inferior drainage structures in the horse eye.
• This narrowing could potentially reduce aqueous humor outflow, which is contrary to the observed decrease in IOP but may be balanced by other atropine effects such as ciliary muscle relaxation.
• Because narrowing of the ICA and CC can affect fluid drainage, atropine use should be carefully considered in horses with glaucoma or elevated intraocular pressure.
• These results suggest that while atropine lowers IOP in normal eyes, it may pose risks in eyes predisposed to or suffering from glaucoma due to structural narrowing of key drainage pathways.

Conclusions

• Atropine eye drops induce constriction of the superior and inferior iridocorneal angles and ciliary clefts in normal horse eyes.
• Veterinarians should exercise caution when using atropine in horses at risk for glaucoma because the induced angle narrowing could exacerbate outflow obstruction.
• This study highlights the importance of advanced imaging techniques like UBM to understand drug effects on ocular anatomy in veterinary medicine.