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Emmetropisation after Molteno implant insertion for buphthalmos
  1. BRENDAN J T VOTE,
  2. J ROBERT FULLER,
  3. ANTHONY C B MOLTENO,
  4. TUI H BEVIN
  1. Department of Ophthalmology, Dunedin Public Hospital, Dunedin, New Zealand
  1. Dr Brendan Vote, Department of Ophthalmology, Auckland Public Eye Hospital, Auckland, New Zealand eye.vote{at}clear.net.nz

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Editor,—We report a series of four infant patients who underwent insertion of Molteno implants for control of buphthalmos and congenital glaucoma. In addition to long term control of their glaucoma, they each demonstrated a return to emmetropia from myopia. We are unaware of any similar case reports in the literature.

CASE REPORTS

All refractions discussed in cases were measured under cycloplegia (cyclopentolate 1% × 3) using a streak retinoscope and trial lenses.

Case 1

A male patient presented at 6 months of age with bilateral congenital glaucoma. Bilateral Molteno implants were inserted without complications and long term intraocular pressure (IOP) control was achieved. Refraction 6 months postoperatively revealed myopia of −10.5 D in both eyes. Subsequent refraction at age 2 (18 months postoperatively) revealed reduced myopia of −4.5D in both eyes. Refraction at age 6 (66 months postoperatively) revealed essentially emmetropic refraction (plano/−1DC × 180 in both eyes).

Case 2

A male patient first presented to our department at age 6 months. He was diagnosed with Sturge-Weber syndrome with bilateral trigeminal nerve distribution naevus flammeus involving both upper and lower lids. He was noted to have bilateral buphthalmos, but with clear corneas and normal discs. He showed no evidence of visual attention at presentation. Examination under anaesthetic (EUA) revealed IOP of 32 mm Hg right and 37 mm Hg left and horizontal corneal diameters of 14 mm in both eyes. A one plate Molteno implant with Vicryl tie, Sherwood slit, and donor sclera was inserted in the left eye at 8 months of age and the right eye at 9 months of age. EUA at 11 months of age (2 months postoperatively) revealed −6.5D (myopia) in both eyes and normal IOP. Horizontal corneal diameters were 14 mm right and 13 mm left. Further EUA at 20 months of age (12 months postoperatively) revealed corneal diameters of 14 mm right and 13 mm left, with normal discs and IOPs (10, 12 mm Hg right and left). His myopia had increased to −8.5D right and −8.0D left and he was prescribed −5D glasses in both eyes. At age 36 months of age (28 months postoperatively) the parents questioned the need for glasses, and EUA revealed the patient was essentially emmetropic (plano/+1.25DC × 045 right; plano/+1.25DC × 135 left). Other glaucoma parameters were unchanged and glasses were stopped. EUA at age 42 months (36 months postoperatively) revealed refraction of +0.5DS in both eyes which has remained stable on subsequent review to date (now 5 years postoperatively).

Case 3

A female patient presented at age 5 months with buphthalmos. EUA next day revealed horizontal corneal diameters 14 mm right and 13.5 mm left with IOPs of 33 mm Hg right and 30 mm Hg left. There was corneal oedema worse right than left and bilateral Haab striae. We proceeded to right one plate Molteno implant with Vicryl tie and slit, with a similar procedure in the left 1 week later. Subsequent EUA at 13 months (8 months postoperatively) revealed normal IOP (14 mm Hg in both eyes) with horizontal corneal diameters of 13 mm Hg right and 13.5 mm Hg left and normal discs. Refraction revealed myopia of −4.0D right and −4.5D left. EUA at 15 months (10 months postoperatively) revealed greater myopia of −7.0D right and −5.5D left and the patient was prescribed glasses (−4D right and −3D left). EUA at 21 months of age (16 months postoperatively) revealed myopia had reduced significantly to −1.75D right and −1D left with other parameters unchanged.

Case 4

A male patient presented aged 3 months with history of cloudy left cornea from 4 weeks of age. EUA revealed left corneal diameter of 12 mm and IOP of 50 mm Hg. The patient proceeded to a left one plate Molteno implant with Vicryl tie and Sherwood slit and donor sclera. At EUA 1 week later the left eye showed marked improvement with resolution of corneal oedema and normal IOP. Refraction at 4 months of age (1 month postoperatively) revealed +1.75DS right and −0.5DS left. Subsequent refraction 9 months of age (6 months postoperatively) revealed plano/+1DC × 090 in both eyes. Refraction at 24 months of age (21 months postoperatively) revealed +0.5DS right and +0.75DS/+1.0DC × 045.

COMMENT

Human adults show a non-Gaussian distribution of refractive errors with a predominance of refractions around emmetropia. Other ocular parameters such as corneal power, lens power, and axial length show a Gaussian distribution.1-3 Normal ocular development is towards emmetropia by corneal power, lens power, and axial length proportionally working in combination. This process has been called emmetropisation. Emmetropisation occurs by active and passive processes. The passive process is one of proportional enlargement of the eye and the active mechanism probably involves the feedback of image focus information from the retina.4

Congenital glaucoma has an incidence of 1 in 10 000 live births.5 In congenital glaucoma trabeculodysgenesis causes elevated IOP. The eye in utero and in the neonatal period enlarges in response to elevated IOP. It is likely that globe expansion is most sensitive to abnormal IOP before 6–12 months of age and that ocular distensibility falls subsequently. Ocular enlargement through elevated IOP results in flattening of the cornea, deepening of the anterior chamber, flattening and posterior displacement of the lens, and an increase in axial length (especially vitreous length), often asymmetrically. The myopia due to increased axial length is only partially compensated by flattening of the cornea and lens and posterior lens position so that enlargement causes myopia which is a feature of congenital glaucoma.6-8 This suggests that congenital glaucoma disturbs emmetropisation as a result of ocular enlargement, but does not disrupt it as these partially compensatory measures in themselves are tending to emmetropise a buphthalmic eye that otherwise would be much more myopic if axial length alone increased. The comparison with X linked megalocornea where patients typically are emmetropic having symmetrically large but steep corneas with very deep anterior chamber depth and posteriorly placed lens but short vitreous length, both illustrate biometric differences between these two conditions7 and also that emmetropisation proceeds in other conditions with altered refractive parameters (the shorter vitreous length being the principal mechanism facilitating emmetropisation). Conversely other ocular conditions also disturb the emmetropisation process when the refractive parameters do not enlarge proportionally. Retinopathy of prematurity is often associated with myopia, but with a small eye but relatively smaller anterior segment with steep cornea and possibly forward positioned lens. While nanophthalmos with a steep cornea and small eye results in high hyperopia.

We report four patients (seven eyes) with congenital glaucoma and myopia. Their myopia increased initially after surgical normalisation of pressure suggesting that disturbed growth of the globe continues for a period of time (up to 12 months postoperatively in these patients). Figure 1 illustrates the refractive changes with time. After some months of normalised IOP there was a return to emmetropia. Thus emmetropisation must be robust enough to withstand the effects of IOP and ocular enlargement on the various parameters that determine refraction, and furthermore appropriately influence them to achieve emmetropia (or at least result in an eye that is not nearly so myopic as its abnormal axial elongation would suggest). This would provide further support for some active process as part of emmetropisation, rather than just passive proportional enlargement.

Figure 1

The time course of emmetropisation in four cases. Refractions have been shown as spherical equivalent and averaged between the two eyes (except case 4 which was a unilateral implant). A line of best fit has been drawn for each case.

Increased axial length (in particular vitreous length) in congenital glaucoma is related to the compliance of the connective tissue in children that allows distension of the globe. Furthermore, an increase of vitreous length indicates uncontrolled glaucoma and is a valuable method for follow up of glaucoma in children.7 Presumably the elasticity of scleral tissue returns vitreous/axial length relatively towards normal values on controlling IOP, until being proportionate with that buphthalmic eye's flattened cornea and lens plus posterior lens position—again allowing emmetropia.

While this department now routinely performs biometry and keratometry as part of serial congenital glaucoma assessment, unfortunately such measurements are unavailable for these cases. As a result we are unable to determine the mechanism(s) at play in the reduction of myopia in these cases. Serial measurements that include biometry and keratometry on similar cases elsewhere should allow a better understanding of the emmetropisation process in congenital glaucoma. The ability of the globe to emmetropise probably depends on the age at surgical intervention, the degree and duration of elevation of IOP and its subsequent control, and the patient's refractive genetic predisposition.

References

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Footnotes

  • Disclaimer: One of the authors (ACBM) has a financial interest in Molteno implants.

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