Assessment of the Occuity PM1 Pachymeter

The cornea is a thin, transparent structure at the front of the eye; its surface, as well as the overlying tear film, is responsible for the majority of the eye's refractive power.

Accurate and precise measurement of corneal thickness is important in the diagnosis and management of many ocular diseases. In particular, the central corneal thickness (CCT) measurement is a key parameter to consider when planning for procedures such as refractive surgery, and in the diagnosis of glaucoma.

Refractive error is the leading cause of reversible visual impairment worldwide; it is therefore unsurprising corrective refractive surgery is one of the most widely performed procedures globally. In the United Kingdom, the most frequently performed refractive surgery are laser assisted surgical procedures, such as laser assisted in situ keratomileusis (LASIK), Laser epithelial keratomileusis (LASEK), and Photorefractive Keratectomy (PRK). All of these techniques involve ablating the cornea with a laser to achieve a specific shape. Having an accurate measurement of corneal thickness is therefore of utmost importance in the planning stages, as excessive ablation of the cornea can lead to corneal ectasia, a significant complication of refractive surgery .

Glaucoma is a progressive disease which causes irreversible vision loss, and with ever aging populations, it is now one of the leading causes of irreversible blindness worldwide. Although it is considered to be multifactorial in aetiology, with both genetic and acquired factors contributing to its development and progressions, it is still widely accepted that the intraocular pressure (IOP) is the only modifiable factor in its management to prevent or slow further visual loss. (5).

IOP measurement is typically done in hospital clinics using a Goldmann applanation tonometer. The pressure reading is affected by the corneal thickness - a thick cornea will give a falsely elevated IOP reading, which may result in a patient with healthy eyes being commenced on unnecessary treatment, while conversely a thin cornea may give a falsely low pressure reading, which can result in patients with glaucoma not being started on sight-saving treatment. It has been estimated that a 10% increase in CCT can result in an apparent IOP increase of 3.4mmHg (2).

Ultrasound Pachymeter The ultrasound pachymeter (UP) is currently the gold standard method , as well as the most widely used to measure CCT. In comparative device studies, the ultrasound pachymeter has been shown to provide the most repeatable measurements . In addition, it is a small and portable handheld device, which makes it very convenient and timesaving to use during clinic appointments.

However, there are several negative aspects to the UP. First, it is necessary for the device to make direct contact with the cornea, which introduces the risk of corneal abrasions and subsequent infection. Second, anaesthetic eye drops need to be used for the patients to tolerate the corneal touch, and studies have shown that after instillation of anaesthetic eye drops, the CCT measurement is increased compared to before the use of anaesthetic, which can lead to a systematic over-reading of IOP. Third, the UP can be difficult to use, as it is necessary for the probe to contact the cornea perpendicularly, and exactly at the centre of the cornea to achieve the most accurate reading, which is difficult to achieve even with good staff training and experience. Lastly, despite the use of anaesthetic, measurement using this device is uncomfortable, and sometimes poorly tolerated due to the corneal touch, especially by the paediatric population.

While there are optical devices available on the market which can measure CCT without the instillation of anaesthetic or corneal touch, they tend to be bulky machines which are not portable and require specialist training to use. This makes their use in busy hospital clinics and community optometry practices less feasible.

There is a clear need to develop a device which can measure CCT accurately without necessitating corneal touch and use of topical anaesthetic, as well as being easily portable. There is no such device available on the market currently, and in our search in the clinical trial databases and Occuity did not identify any active trials investigating similar devices.

Occuity PM1 Pachymeter The Occuity device is a new handheld, non-contact, optical pachymeter, which utilise a low power, eye safe near infrared (1310nm) beam to measure the corneal thickness.

The device has an intuitive touchscreen display interface, and a series of 9 LEDs which provides the illumination needed to align the device in front of the eye. Once alignment is achieved, the measurement can take place by pressing the start button on the touchscreen display. During preliminary testing at the parent company, it has been shown the alignment and measurement process takes less than 10 seconds in total, after which a reading will be shown on the digital display of the device. The device is reported to have high reliability (variation of approximately 10um with repeat readings of the same eye), and comparable results to the Lenstar LS900 Biometer during initial testing.

The LED light system consist of a partial ring of 9 white LEDs at a radius of 8.75mm and spaced at 30° intervals. The total power is 2.4 mWm-2 which is will within the 10 mWm-2 blue light hazard eye safety limit. The maximum power of the infrared beam is 50µW, which given the expected exposure time of less than 10 seconds, is considered Class 1 eye safe according to BS EN 60825.

Lenstar LS 900 (Haag-Streit AG, Koeniz, Switzerland) The Lenstar is an optical device which utilises an 820mm super-luminescent diode and allows for high resolution measurements of the structures in the eye, including CCT. Its use is well established in the hospital clinic setting, and measurements do not require the use of anaesthetic drops or corneal touch. A drawback of this device is that it is large and heavy, and therefore not portable or as fast to use, unlike the ultrasound pachymeter or the Occuity PM1 device.

Pentacam (Oculus Optikgerate GmbH (Germany) Very similar to the Lenstar, the Pentacam is an optical device which also utilises an 820mm super-luminescent diode and allows for high resolution measurements of the structures in the eye, including CCT. Its use is well established in the hospital clinic setting, and measurements do not require the use of anaesthetic drops or corneal touch. A drawback of this device is that it is large and heavy, and therefore not portable or as fast to use, unlike the ultrasound pachymeter or the Occuity PM1 device.