Sir,

Aqueous misdirection following pars plana vitrectomy (PPV) is rare, with four cases described to date.1, 2, 3 We report such a case following PPV for macular hole repair.

Case report

An 85-year-old female underwent uneventful PPV, internal-limiting membrane peel and gas tamponade (C2F6), combined with cataract surgery. Post-operative facedown posturing was not advised.

On day 1, axial shallowing of the anterior chamber (AC) and an intraocular pressure (IOP) of 66 mm Hg were noted. Mannitol and acetazolamide were administered along with maximum topical therapy. Gas was also released from a scleral port site, lowering the IOP to 5 mm Hg. Within 24 h, the IOP had crept up again to 80 mm Hg, which required further gas release and AC reformation. Laser peripheral iridotomies (PIs) were attempted but limited by corneal oedema. As IOP still remained refractory, further gas was removed (equalised to atmospheric pressure) and a surgical PI created. She also received 180° of cyclocryotherapy. These had little effect, with IOPs continuing to range between 55 and 65 mm Hg, and the presumptive diagnosis of aqueous misdirection was made. Anterior vitrectomy with zonulo-hyalo-iridectomy was performed.

The IOP remained low on no medications, but unfortunately the eye had become phthisical. Fellow eye gonioscopy revealed iridocorneal touch, so prophylactic cataract surgery was performed.

Comment

Aqueous misdirection is characterised by elevated IOP and central shallowing AC without pupillary block or choroidal abnormalities.1 Cases refractory to medical and laser therapy undergo PPV. As PPV is a form of treatment, aqueous misdirection was not suspected early on, and instead we treated the more common complication of gas-related IOP rise.

Incomplete removal of the anterior hyaloid, which inhibits communication between the AC and vitreous cavity, would explain this paradox. A higher rate of recurrence of aqueous misdirection has been described in patients with PPV alone for the same reason, and nowadays PPV with zonulo-hyalo-iridectomy is recommended.4

Experiments performed by Epstein et al5 suggested that at normal perfusion pressures the vitreous and anterior hyaloid offered very little resistance to forward flow of aqueous, but at higher pressures there was an increased resistance. Certain aspects of PPV, such as gas overfill or expansion, may simulate these increased perfusion pressures and result in aqueous entrapment.5

The patient developed a phthisical eye 3 months after the initial vitrectomy procedure. This is most likely related to the cyclocryotherapy, resulting in a non-functioning ciliary body and reduction of aqueous production. Outcomes of cyclocryotherapy are often unpredictable and are associated with a higher incidence of hypotony in comparison to cyclodiode.6 Further, the prolonged high pre-treatment IOP may have caused ciliary body ischaemia, which in turn may lead to ciliary body shutdown following cyclocryotherapy. The reason for choosing cyclocryotherapy was that cyclodiode was not yet available at our unit.

Clinicians should consider the possibility of aqueous misdirection after vitrectomy. Some surgeons even advocate the routine disruption of the anterior hyaloid in cases at risk of aqueous misdirection.