Using Discrete Event Simulation to explore "what-if" waiting list scenarios in NHS Scotland

Background

General NHS Scotland context

Planned operations, across all specialties, are lower overall when compared to pre-COVID levels e.g. “Overall, the number of planned operations has been steadily increasing since the COVID-19 pandemic impacted service provision. In the 12 months from December 2023 to November 2024, there was a 3.3% increase in the number of planned operations compared to the previous 12 months. However, this remains 15.2% lower than those planned to take place in the 12 months from March 2019 to February 2020, just prior to when the pandemic began to severely impact service provision.” (1).

Attendances and admissions across all specialties are also lower than pre-COVID levels. In the 12 months from January 2024 to December 2024 there was a 2.4% increase in new outpatient attendances compared to the previous 12 months. This is still 11% lower than the number of attendances in the 12 months from April 2019 to March 2020 (2).

In the 12 months from January 2024 to December 2024 there was a 7% increase in inpatient or day case admissions compared to the previous 12 months. This is still 6.4% lower than the number of admissions in the 12 months from April 2019 to March 2020 (2).

Cataract case study context

Ophthalmology outpatient activity follows the same trends as described for the wider NHS Scotland context. Ophthalmology accounts for around 10% of all new and return outpatient activity (3). In the 12 months from January 2024 to December 2024 there was a 1.8% increase in ophthalmology new outpatient attendances compared to the previous 12 months. This is 7% lower than the number of attendances in the 12 months from April 2019 to March 2020 (2).

In contrast to the wider NHS Scotland context, ophthalmology inpatient/day case activity has increased above pre-COVID levels. In the 12 months from January 2024 to December 2024 there was a 21% increase in ophthalmology inpatient or day case admissions compared to the previous 12 months. This is 16% higher than the number of admissions in the 12 months from April 2019 to March 2020 (2).

It should be noted that ophthalmology has a significant volume of return outpatients with long term, sight-threatening conditions that require continued care and support.

Cataracts account for around 80% of all ophthalmic surgery activity (4). Cataract surgery was in the top five procedures performed in Scotland in 2023/24, accounting for 4.4% of all procedures across specialties (5).

It is anticipated that a significant proportion of patients waiting for cataract surgery will require surgery on both eyes (4). Currently most cataract surgeries are done one eye at a time, but surgeries that correct both eyes in the same surgery (“Immediate Sequential Bilateral”) are being carried out by a small number of surgeons in a few Health Boards (6).

As cataracts are the bulk of ophthalmic surgical activity, “single lists” where only cataract surgeries are performed are common. Other lists are called “mixed lists” which are usually comprised of a mix of more complex surgeries with gaps filled by cataract surgeries . Please note that in the model scenarios which follow, where surgical activity is increased, this increase is only applied to single lists.

High volume surgical lists are also possible for cataracts. According to guidance from the Royal College of Ophthalmologists, in a typical surgical list 6-8 patients will be seen, while a high volume list is defined as one where 10-14 patients are seen in a single surgical session (7).

Data on sub-specialty proportions is limited so it was not possible to account for the different resource requirements on a sub-specialty basis in this publication.

High level overview of a typical pathway for a cataract patient.

The first point of contact for a cataract patient is usually primary care. For cataracts (and other ophthalmology sub-specialties) primary care activity happens in the community via opticians and optometrists . The individual will then be referred to a New Outpatient clinic for consultation where a range of tests will take place e.g. measuring the curve of the cornea to help with surgery planning.

After the pre-assessment consultation if surgery is required the individual will be referred to the TTG waiting list, and then have the surgery. The tests carried out at an NOP consultation are valid for two years. If a patient is identified as requiring surgery on both eyes but just has one eye treated initially, they can be directly added to the TTG list to wait for the second eye surgery, following the initial surgical treatment. Note that this process is not standardised and therefore may not be followed in all Health Boards. After surgery the patient is discharged and will usually have post-operative refraction undertaken by community optometrists.

Waiting list definitions

The new outpatients (NOP) waiting list includes all patients waiting for their first appointment at a consultant-led clinic. These patients come from all referral sources covered by the 12 week waiting time standards under 'New Ways' (8, 9).

The treatment time guarantee (TTG) waiting list includes patients waiting for an inpatient or day case procedure that were added to the waiting list from 1 October 2012 onwards, where the TTG is applicable. The TTG comes from the Patient Rights (Scotland) Act 2011 which established a 12-week maximum waiting time for treatment of all eligible patients who are due to receive planned treatment deliver on an inpatient or day case basis (10).