Subhyaloid haemorrhage from proliferative diabetic retinopathy after Nd:YAG laser iridotomy
- Dr Hamush
- Accepted 23 May 2000
Editor,—Various vascular and haematological disorders can cause a subhyaloid premacular haemorrhage leading to decrease in vision.1 2 So far though, a premacular haemorrhage has not been recognised as a complication of peripheral laser iridotomy (PI). The following is a case report of such an occurrence with consideration of possible mechanisms.
CASE REPORT
A 58 year old diabetic woman complained of sudden reduction in vision immediately after a prophylactic Nd:YAG laser peripheral iridotomy to her left eye. Fundus examination revealed a dense premacular haemorrhage. The iridotomy was performed through an Abraham lens using a Coherent 7970 laser. Ten shots were needed with energy of 3–5.5 mJ.
The patient had proliferative diabetic retinopathy for which she underwent extensive peripheral retinal photocoagulation with residual neovascularisation at the disc (NVD). Several weeks before her current event, she presented with a spontaneous premacular haemorrhage following a Valsalva manoeuvre. She refused any surgical or laser intervention and the haemorrhage had completely resolved at the time of the PI.
COMMENT
The Nd:YAG laser creates a PI by photodisruption of tissues. Three mechanisms are involved in this process: (1) very high irradiances strip electrons from atoms creating a gaseous state of electrons and ions called “plasma”; (2) the plasma expands rapidly outwards creating shock and acoustic waves that mechanically disrupt adjacent tissue; and (3) latent stress in the tissues causes additional disruption when the laser makes an incision.3 The peak pressure in the shock wave associated with plasma formation may exceed 1000 atm.4
Several investigators have previously reported retinal and choroidal damage following argon or Nd:YAG laser peripheral iridotomy.5 This included peripheral retinal and foveal photocoagulation, choroidal and retinal detachment, and cystoid macular oedema and unexplained visual loss. Most of the complications were attributed to radiant laser energy rather then mechanical stress. In our patient, this is a remote possibility in view of the good laser focus on the targeted iris tissue through the Abraham lens. A likely explanation is a rebleeding of fragile NVD secondary to the posterior propagation of shock waves. This mechanism has been contemplated to explain retinal complications after posterior capsulotomy6as well as after excimer laser photorefractive keratotomy.7 8 Experimentally, a pressure wave of 9–16 bar (130–230 psi) has been measured by Vogel et al 18 mm from the focal point of a Q switched Nd:YAG laser using a pulse energy of 5 mJ.9 Changes in intraocular pressure during the procedure or while removing the contact lens is another possible explanation although unlikely; the patient, already experienced with laser procedures, was very cooperative. The iridotomy was technically easy and performed with particular care since this was her only seeing eye.
The Nd:YAG laser is used in the management of premacular haemorrhages.10 In our patient, performing the PI with the same laser was the cause of such a one. Regardless of the mechanism(s) involved, the procedure should be performed with special care particularly in predisposed patients such as those with proliferative retinopathy and/or with a recent bleed. Using the least amount of energy is an obvious precaution. Pretreatment of the chosen iris site with the argon laser in a photocoagulative mode to stretch or thin the iris bed might also be helpful in lowering the Nd:YAG laser energy required.
