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Selective laser trabeculoplasty

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Selective laser trabeculoplasty (Q-switched, frequency-doubled Nd:YAG laser (532 nm) applied to the trabecular meshwork. Image: Ellex Medical Pty Ltd

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Dr P Pathmaraj
FRANZCO FRCS MRCOphth MBBS
Consultant Ophthalmologist, Vision Eye Institute and Royal Victorian Eye and Ear Hospital Melbourne

Selective laser trabeculoplasty (SLT) is a form of laser surgery that is used to lower intraocular pressure (IOP) in glaucoma. There are reports of progression to blindness in 25 per cent of glaucoma patients under treatment for 20 years.1 There are many reasons why patients with glaucoma go blind, of which one is non-compliance with medical therapy. SLT averts the compliance (Table 1), side-effects (Table 2) and inconvenience issues associated with medical treatment.

The introduction a decade ago of SLT to the treatment regimen for glaucoma has changed the treatment algorithm in the management of glaucoma.

Complicated prescription regimens

Medication costs

Unpleasant side-effect (see Table 2)

Physically unable to place eye-drops

Forgetfulness and changes in routine

 Table 1. Reasons for non-compliance

Allergic reaction

Ocular surface disease

Hyperaemia, increased pigmentation of lids and iris

Reduced blood pressure and pulse rate

Shortness of breath in patients with respiratory disorders

Fatigue and drowsiness

 Table 2. Side-effects with eye-drops

Who is a candidate for SLT?

Any patient with glaucoma whose trabecular meshwork is visible on gonioscopy is a candidate for this procedure. SLT helps to lower IOP in open angle glaucoma or high-risk ocular hypertension as well as stabilise the intraocular pressure fluctuation, which causes the damage to the optic nerve.

How does it work?

SLT uses a Q-switched (that is, pulsed), three nanosecond frequency-doubled Nd:Yag, 532 nm wavelength green laser. Low energy and larger spot size allow the ophthalmologist to easily focus the laser onto the trabecular meshwork (TM) and distribute the laser energy evenly, so that all the target cells receive equivalent doses of laser energy. The laser energy targets pigmented trabecular meshwork endothelial (TME) cells. As this targeting is highly specific, the laser energy does not cause coagulative damage to the TM or other collateral thermal effects to the TM.

The treated TME cells release cytokines, which bind with Schlemm’s canal endothelial (SCE) cells, and open the cellular tight junction barrier that has been acting as a control site for aqueous outflow. The opening of the SCE barrier results in increased aqueous drainage leading to decreased IOP. Additionally, the newly released cytokines attract circulating monocytes to the laser site. These monocytes becomes macrophages, which perform phagocytosis of cellular debris in TM so that aqueous is drained easily from the anterior chamber in to Schlemm’s canal, resulting in reduction of IOP.

Why is it called ‘selective’?

A high degree of selectivity of target tissue by the laser and the use of a brief pulse duration, which last for just nano-seconds, produce minimal pain and scar tissue.

What are the risks?

One key aspect of SLT is a favourable side-effect profile, even when compared with glaucoma medication. Postoperative inflammation is common but generally mild and treated with observation or non-steroidal anti-inflammatory drops. There is a five per cent incidence of elevated IOP after laser, especially in heavily pigmented TM.2 By using lower laser energy, high IOP is averted in patients at risk. The elevated IOP can be managed easily by glaucoma medications and usually resolves after 24 hours but rarely may require surgery to control the IOP.

How effective is it?

SLT is effective as primary, replacement or adjunctive treatment. SLT lowers the IOP by about 30 per cent when used as initial therapy. This effect may be reduced if the patient is already on glaucoma medication. Treating 360 degrees of trabecular meshwork gives the greatest IOP lowering effect. It must be also noted that the patients most likely to have a significant response are those with the highest base line pressures, regardless of any prior medical or therapeutic intervention.

The clinical efficacy of SLT is comparable to the IOP lowering of the most commonly used class of medication, the prostaglandin analogue (PGA). In a study conducted by Dr Nagar and colleagues,2 it was found that there was a slightly higher reduction in IOP as a result of commonly used PGA latanoprost but it was explained that SLT offers the benefit of being a one-time intervention not requiring ongoing patient compliance. The authors noted that as a rule of thumb, with SLT they expected a 20 per cent drop in IOP when the base line pressure was 20 mmHg and a 30 per cent drop for a base line pressure of 30 mmHg. In another study,3 it was found that approximately 93 per cent of patients were expected to respond to SLT as primary therapy, and more than 50 per cent of all patients receiving SLT as replacement therapy will not need any medication post-treatment

How long does it last?

The effect will generally last from one to five years and in rare cases, longer than that. There is a higher non-responder rate among normal tension glaucoma (NTG) versus primary open angle glaucoma (POAG) (25-30 per cent versus 20-22 per cent) as well as a lower five-year survival rate (30 per cent versus 50 per cent). SLT is useful in NTG patients only if there is concern about either compliance or intolerance to PGA or significant IOP fluctuation. Duration of response also seems to be shorter in pseudoexfoliation compared to POAG.

What happens if effectiveness of SLT diminishes?

If SLT is effective for more than six to 12 months at lowering IOP but its effectiveness wears off over several years, the procedure can be repeated. The second treatment may not be effective as the first and may not have long-lasting impact. Glaucoma medication can be used if the effect diminishes over time.

What happens if it doesn’t work?

If SLT fails to lower the IOP, then the glaucoma is treated by other means such as medications or conventional surgery (trabeculectomy). The laser does not affect the success of these other types of treatment.

What is the cost?

Because the procedure is an accepted form of glaucoma treatment and is a TGA approved treatment, Medicare covers most of the cost but co-payments may vary.

Is there a need to use glaucoma medication after SLT?

Some patients can be controlled with just SLT; others require additional IOP reduction and may need to use glaucoma medication as well. SLT can be thought of as equivalent to one glaucoma medication. Just as some patients will require more than one glaucoma medication to control their IOP, some may require laser plus one or more medications. It is important to remember that SLT is not a cure for glaucoma, just as medication and surgery are not. Whatever method is used, appropriate follow-up and testing are required to monitor the progress of glaucoma to prevent blindness.

 

  1. Hattenhauer MG, Johnson DH, Ing HH. The probability of blindness from open-angle glaucoma. Ophthalmology 1998; 122: 477-485.
  2. A Decade of Selective Laser Trabeculoplasty: A supplement to Ophthalmology Times Europe; September 2011.
  3. Jindara LF, Gupta A, Maglino EM. Poster presented at the American Academy of Ophthalmology Annual Meeting, November 2007.


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