Bardet-Biedl Syndrome clinical outcomes

Bardet-Biedl Syndrome (BBS) is a rare genetic condition that affects many parts of the body. Its effects can include:

• obesity
• vision loss
• additional fingers and/or toes (polydactyly)
• learning disabilities
• kidney abnormalities

BBS is seen in about 1 in 100,000 of the general population of the UK.

This national service was established in 2010. Children are referred to their nearest specialist centre – either Great Ormond Street Hospital (GOSH) or Birmingham Children’s Hospital. A key service aim is to provide both earlier diagnosis and intervention to promote optimal health.

The service at GOSH offers dedicated multi-disciplinary (MDT) care of children and young people with BBS, and their families, including Specialist Nursing, Dietetics, Ophthalmology, Nephrology, Endocrinology, Clinical Genetics, Speech and Language Therapy and Psychology. Each year, GOSH sees around 60 children and young people (of which approximately 10 are new referrals), who attend our MDT clinic for expert assessment by a team that specialises in each organ system commonly affected by this syndrome. Our service is delivered in partnership with BBS UK to ensure our children, young people and their families have access to ongoing support. Additional MDT telephone clinics started in 2019 to help manage the increasing demand for our service and additional funding was acquired in 2025 to extend this service.

In collaboration with the UCL Great Ormond Street Institute of Child Health, the team is committed to translating excellence in research into effective treatments of BBS in children and young people.

Early obesity is a major clinical feature of BBS. Managing weight gain in BBS can be challenging due to both lower energy requirements and hyperphagia (constant drive to eat). Specialist dietetic support is an integral part of this service; dietetic advice focuses on portion control strategies and promotion of activity to address underlying energy imbalance.

Body Mass Index (BMI) is a clinically useful metric of obesity for patients over two years of age as it’s easy to measure and it relates an individual’s weight to their height. As ideal BMI varies throughout childhood, we use a BMI z-score (standard deviation score - SDS) as our outcome measure to allow comparison to children and young people in the general population by age.

BMI data from our service showed for children attending the first year of this specialised service, 88% of patients were living with obesity, 12% living with overweight and none had a healthy BMI. Data from multi-disciplinary clinic attendances in 2024 to 2025 shows 59% of patients with a confirmed diagnosis of BBS on genetic testing were living with obesity, 27% living with overweight and 14% with a healthy BMI. This highlights that controlling BMI is possible in BBS and that increasing BMI is not inevitable in BBS.

It is notable that our data shows around 9% of our population were able to maintain a healthy BMI between 2017 to 2020. After the COVID-19 pandemic this was noted to have reduced, reflecting the impact of this time. Currently BMI scores for children seen in 2024/2025 have returned to pre-pandemic scores with an increase to 14% of this group having a healthy BMI.

BBS is characterised by increasing weight with age. However, limited data is available in the literature demonstrating weight-related outcomes in children and young people. We report results for children over 2 years of age with a diagnosis confirmed by genetic testing seen in clinic between April 2023 to March 2025 with a BMI above the healthy BMI range. Reduced, maintained, or increased BMI z-score categories apply to children living with overweight or obesity, where the aim is to limit weight gain and reduce BMI over time. As we are comparing BMI z-score changes over a significant period, slight variations in BMI z-score are expected. Therefore, BMI maintenance has been defined as a BMI z-score within the range of +/- 0.05 SDS to ensure we only report significant change. BMI z-score reduction in children living with overweight and obesity has been linked with positive health changes including changes in body composition, blood pressure and insulin sensitivity.

The data shows in 2024/2025, 56% of children living with overweight or obesity reduced or maintained their BMI z-score since their last BBS MDT outpatient appointment. 44% had an increase in BMI z-score.

In 2023/2024, 61% of children living with overweight or obesity reduced or maintained their BMI z-score since their last BBS MDT outpatient appointment. 39% had an increase in BMI score.

Though there is limited data nationally and internationally on children and young people with BBS, studies in adults have reported that 72% to 92% of adults with BBS are living with obesity. Pomeroy et al reported that 56% of children are living with overweight/obesity by the age of 2 and from 2 and over, 70% of children with BBS are living with obesity. Our data challenges the reported increasing weight with age as some children have maintained a healthy BMI and about 60% have been able to reduce or maintain their BMI z-score.

We report results for children over 2 years of age with a diagnosis confirmed by genetic testing of BBS1 or BBS10 seen in clinic between April 2023 to March 2025. These are reported as they are the most common identified genotypes (genetic material) and are known to have different general characteristics (e.g. children with BBS1 typically achieve a greater adult height than children with BBS10) so we would expect BMI outcomes to vary between these genotypes. Limited data is available for both genotypes. Due to these small numbers this information is presented as percentages only for information governance reasons. Reduced, maintained, or increased BMI z-score categories apply to children living with overweight or obesity, where the aim is to limit weight gain and reduce BMI over time.

Additional data is provided for children with BBS1 and BBS10 as these are the most common genotypes seen in our clinic. During 2024/25 reduction or maintenance of BMI was achieved by 53% children living with BBS1 and 62.5% with BBS10. In 2023/24, 64% of the children living with overweight or obesity with BBS1 reduced or maintained their BMI z-score and 58% of the children living with overweight or obesity with BBS10 reduced or maintained their BMI z-score. Of note, data collected between 2023 and 2025 showed children with both BBS1 and BBS10 were able to maintain a healthy BMI highlighting this outcome is possible for both genotypes.

Notes:

• BBS is a highly variable syndrome so direct comparison between children and young people attending each year is not possible.
• Diagnosis confirmed by genetic testing is available for about 80% of our patients. Data for children and young people with a presumed clinical diagnosis has been excluded as aspects of this syndrome (e.g. vision changes) evolve with time, affecting the timing a clinical diagnosis can be confirmed.
• Hospital weight and height data was not available for comparison for all children attending their follow up reviews in the 2023/24 reporting period limiting the available change in BMI z-score data.
• Due to increasing demand for our service, time between MDT outpatient clinic reviews continues to increase. For the latest reporting period, reviews on average are every 2 years.
• BMI data presented is for children aged over 2 years of age at both clinic visits as BMI is an accepted measure only from this age.
• Numbers reported in the reduced or maintained BMI z-score may include children reducing their BMI to within the healthy BMI range.
• Data is collected from all MDT clinics; including the transition clinic to adult care as this is the end point for paediatric care.

The establishment of the national BBS service has also allowed significant data collection across the service leading to joint publications and improvements in patient care. Further details can be found in the reference section below.

We will continue to collect and review the data about the service we provide, and to seek to add more measures that tell us about our effectiveness.

We have developed a questionnaire to look at hyperphagia (excessive hunger) and food related behaviours in our clinic. This tool allows us to measure this in a consistent way. The questionnaire will be available electronically (via MyGOSH) so parents/carers can complete this before clinic appointments. We hope this will provide useful information that we can share with families attending our clinic.

A new clinic offering medication targeting the hyperphagia seen in BBS started in 2024. Eligibility criteria includes confirmed genetic diagnosis, age and documented hyperphagia. Outcome data is being collected and will be published in the future.

More information about the Bardet Biedl Syndrome Service can be found via our specialty homepage.
Bardet-Biedl Syndrome UK (BBS UK) offers information, advice and support to adults and children with Bardet-Biedl Syndrome and to their families and carers. 

Beales PL, Cetiner M, Haqq AM, et al. Hyperphagia in Bardet–Biedl syndrome: Pathophysiology, burden, and management. Obesity Reviews. 2025;26(7): e13915. doi:10.1111/obr.13915

Forsythe E, Mallya UG, Yang M, Huber C, Cala ML, Greatsinger A, et al. Burden of hyperphagia and obesity in Bardet-Biedl syndrome: a multicountry survey. Orphanet J Rare Dis. 2023;18(1):182.

Forsythe E, Mallya UG, Yang M, Huber C, Cala ML, Greatsinger A, et al. Caregiver burden in Bardet-Biedl syndrome: findings from the CARE-BBS study. Orphanet J Rare Dis. 2023;18(1):181.

Varughese R, Pujari D, Hatton E, Dyakova T, Sparks K, Flack S, et al. Endocrine outcomes in Bardet- Biedl syndrome from a large single-centre paediatric multidisciplinary clinic. Endocrine Abstracts. 2023.

Pomeroy J, Krentz AD, Richardson JG, Berg RL, VanWormer JJ and Haws RM. Bardet-Biedl syndrome: Weight patterns and genetics in a rare obesity syndrome. Pediatric Obesity. 2021;16:e12703.

Forsythe E, Kenny J, Bacchelli C, Beales PL. Managing Bardet-Biedl Syndrome- Now and in the Future. Front Pediatr. 2018;6:23.

Mujahid S, Hunt KF, Cheah YS, Forsythe E, Hazlehurst JM, Sparks K, et al. The Endocrine and Metabolic Characteristics of a Large Bardet-Biedl Syndrome Clinic Population. J Clin Endocrinol Metab. 2018;103(5):1834-41.

Kenny J, Forsythe E, Beales P, Bacchelli C. Toward personalized medicine in Bardet-Biedl syndrome. Per Med. 2017;14(5):447-56.

Forsythe E, Sparks K, Best S, Borrows S, Hoskins B, Sabir A, et al. Risk Factors for Severe Renal Disease in Bardet-Biedl Syndrome. J Am Soc Nephrol. 2017;28(3):963-70.

Forsythe E, Beales PL. Bardet-Biedl syndrome. Eur J Hum Genet. 2013;21(1):8-13.

Poster Presentation: S. Flack, C. Peters, K. Sparks, P. Beales. Does Attendance at a Paediatric Multidisciplinary Specialist Bardet-Biedl Syndrome Annual Review Clinic Lead to a Reduction in Body Mass Index? 2017 International Conference on Nutrition and Growth