- Clinical phenotype
- Key investigations
- Current research
- Further information and support
- Cone/Cone-rod dystrophy: for patients
|Genes involved (OMIM No.)||35 genes identified to date with the most common being:|
|Systemic features||Some examples include:|
|Molecular diagnosis||Next generation sequencing|
|Therapies under research|
Cone or cone-rod dystrophies encompasses a group of progressive inherited retinal dystrophies (IRDs) characterised by predominant impairment of cone-mediated vision. In cone-rod dystrophies, this is usually accompanied or followed by subsequent rod dysfunction manifesting as nyctalopia and peripheral visual field loss. Disease onset is usually within the first two decades of life and the main presenting features are:
- Reduced visual acuity (VA) not corrected fully by glasses
- Colour vision disturbance
35 genes have been identified currently that cause cone/cone-rod dystrophies and as a result, there is significant inter- and intrafamilial phenotypic variability in terms of age of onset, severity of visual dysfunction, disease progression and clinical findings. Generally, cone-rod dystrophies are more severe than cone dystrophies with an earlier mean age of disease onset and a faster rate of visual decline. The mean age of achieving legal blindness (BCVA 6/60 or worse) in cone-rod dystrophy and cone dystrophy are 23 and 48 years old respectively according to a longitudinal study. Compared to retinitis pigmentosa (RP), patients with cone- and cone-rod dystrophies tend to suffer from severe visual loss at an earlier age but this is highly variable.[2,4]
Due to the significant genetic heterogeneity, the fundal features are highly variable as well but patients usually have symmetrical ocular findings. The macula is predominantly affected but there might be more widespread involvement in the periphery. The following features may be observed:
- Macula: Blunted foveal reflex, RPE disturbances or bull’s eye lesion which progresses to macular atrophy over time
- Retinal periphery: Flecks (ABCA4-retinopathies), bone-spicule pigmentation and chorioretinal atrophy
- Optic disc: Appears normal or there may be subtle temporal disc pallor in the early stages; waxy disc pallor in more advanced stages
- Blood vessel attenuation
In the early stages of the disease, cone and cone-rod dystrophies may be difficult to distinguish from macular dystrophies based on fundoscopic appearance alone but they can be differentiated with electrophysiological studies. Although pattern electroretinogram (ERG) responses are reduced in both, full-field ERG tends to be relatively preserved in isolated macular dystrophies.
On the other hand, advanced cone-rod dystrophies may be difficult to distinguish from advanced RP based on fundoscopy and ERG findings. In such situation, the initial symptomology and sequence of events are usually the only way to differentiate both phenotypes, along with genetic testing to identify the causative gene.
Other associated ocular features
Apart from the retinal changes, patients may have the following ocular features which can lead to further visual deterioration:
Associated extraocular features
Cone/cone-rod dystrophy is usually an isolated ocular finding. However, in minority of cases cone-rod dystrophies may be associated with other systemic abnormalities. Some examples include:
- Alstrom syndrome
- Bardet-Biedl syndrome
- Spinocerebellar ataxia Type 7 (SCA7) — An autosomal dominant neurodegenerative disorder characterised by progressive cerebellar ataxia and visual loss. Central visual disturbance usually precedes neurological features. The macula appears normal initially but progresses to RPE pigment disturbances and then to geographic atrophy over time.
Pathogenic mutations in 35 genes have been identified so far that account for about 60% of cases. The most common causative genes are:
- ABCA4 (60% of autosomal recessive cases); biallelic null mutations are associated with cone-rod dystrophies
- GUCY2D (35% of autosomal dominant cases)
- RPGR (73% of x-linked cases); most pathogenic variants causing cone/cone-rod dystrophies are located at the 3’ end of the ORF15 exon
Many of the identified causative genes encode proteins involved in photoreceptor morphogenesis/development and the phototransduction cascade.
|Phototransduction||CNGA3, CNGB3, GUCA1A, GUCY2D, OPN1MW, OPN1LW, PDE6C, PDE6H|
|Photoreceptor morphogenesis/development||AIPL1, ADAM9, CDHR1, CERKL, CRX, KCNV2, PROM1, PRPH2, RAX2, SEMA4A|
|Photoreceptor ciliary development and transport||C21orf2, CEP78, POC1B, RAB28, RPGR, RPGRIP1|
|Neurotransmitter release||CACNA1F, CACNA2D4, RIMS1, UNC119|
|Signalling pathway||PITPNM3, TTLL5|
|Visual cycle||ABCA4, RDH5|
|Ion transport||CNNM4, KCNV2|
Further information about each gene can be found on OMIM and Medline Plus.
In the early stages, there is delayed implicit time in the cone-specific 30 Hz flicker response. This is then followed by a decrease in amplitude in both 30 Hz flicker and single photopic flash (a- and b-waves) responses, which gradually deteriorate over time.  The rod responses are decreased as well in cone-rod dystrophies but not to the extent of the cone responses. Both responses are eventually extinguished in advanced stages.
It is important to note that about one-third of patients diagnosed with cone dystrophies may actually be early cone-rod dystrophies as it can take up to 10 years for rod dysfunction to manifest on ERG.  In most cases, there is usually a decreased response in pattern ERG due to predominant macular involvement.
KCNV2-retinopathies display a characteristic ERG finding where abnormal cone responses are accompanied with “supranormal” rod responses. However, this is not associated with enhanced rod function. 
2) Fundus autofluorescence imaging (FAF)
FAF can reveal areas of RPE dysfunction and atrophy which may not be obvious on fundoscopy. The FAF findings are highly variable, where the abnormalities may be limited to the macula and/or involve the peripheral retina as well. 
Some FAF appearances may be characteristic of specific genotypes. For example, ABCA4-retinopathies tend to have areas of decreased AF centrally accompanied by a heterogeneous “speckled” background which may extend beyond the vascular arcades. There is usually peripapillary sparing of AF as well. 
On the other hand, some cone-rod dystrophy patients harbouring RPGR mutations may have a progressively enlarging parafoveal ring of increased AF, which is associated with reduced rod and cone sensitivities. 
The advent of wide-field FAF has enabled clinicians to better predict generalised retinal function by measuring the area of abnormal AF. This has been shown to correlate well with visual fields and ERG findings. 
3) Optical coherence tomography (OCT)
The overall macular structure including the outer retinal layers and RPE integrity can be ascertained through OCT. Abnormalities of the outer retinal layers (disruption or loss), specifically the ellipsoid zone (EZ) and external limiting membrane (ELM) are frequently observed in the foveal or perifoveal regions. Over time, the foveal outer nuclear layer (ONL) undergoes thinning as well. Outer retinal layer disruption in the peripheral macula is more variable.
Some specific characteristics can be observed with certain genotypes. For instance, EZ disruptions are usually limited to the fovea in PDE6C and ABCA4-retinopathies tend to have hyper-reflective deposits above the RPE. 
4) Kinetic perimetry
A central scotoma is usually present which may be accompanied with peripheral scotoma in cone-rod dystrophy cases.
As cone/cone-rod dystrophies may be the first manifestation of syndromic cases, careful systemic enquiry during history taking and an increased awareness of potential extraocular involvement are crucial in identifying patients that may require input from other specialities. In children, consider early referral to paediatricians for a development assessment and screening for any systemic features.
Genetic testing can also flag up patients that may require systemic screening but clinical findings must be taken into context when interpreting the results as some genes are associated with both syndromic and non-syndromic cone-rod dystrophy (e.g. C8orf37).
Similar to RP, most cases tend to present with no family history (sporadic). Therefore, genetic testing should be undertaken to obtain a molecular diagnosis which can help facilitate genetic counselling, advice on prognosis and future family planning, direct further clinical management and aid in clinical trial participation.
This can be achieved through a variety of next generation sequencing (NGS) methods:
- Targeted gene panels (retinal)
- Whole exome sequencing
- Whole genome sequencing
Most sporadic patients are found to have autosomal recessive inheritance from genetic testing and further parental segregation of causal variants. 
- Correcting any refractive errors
- Referral to low vision services
- Directing patients to supporting organisations
- Tinted glasses/contact lens for photophobia
- Encourage the use of assistive technology that may improve quality of life
- Encourage a healthy diet consisting of fresh fruit and green leafy vegetables
- Vitamin A supplementation should be avoided in those with ABCA4 mutations
A multidisciplinary approach is required if a child is affected by syndromic cone-rod dystrophy such as Alström syndrome and Bardet-Biedl syndrome.
Visual impairment can have a negative impact on a child’s early general development. Therefore, timely referral to practitioners familiar with developmental surveillance and intervention for children with visual impairment (VI), such as developmental paediatricians as well as a Qualified Teacher of children and young people with Visual Impairment (QTVI) is crucial to optimise their developmental potential.
The Developmental Journal for babies and young children with visual impairment (DJVI) is a structured early intervention programme designed to track developmental and vision progress from birth to three years of age. It is mainly used by qualified healthcare professionals working in services providing support to babies and young children with VI in conjunction with the child’s parents.
Children with VI may be referred to specialist services such as the developmental vision clinic in the Great Ormond Street Hospital for Children or other specialist developmental services for further management.
Family management and counselling
Cone/cone-rod dystrophies can be inherited in the following Mendelian pattern:
- Autosomal recessive (most common)
- Autosomal dominant
- X-linked recessive
Patients and families require genetic counselling and can seek advice for family planning including prenatal testing and preimplantation genetic diagnosis. However, most patients tend to be simplex cases on presentation and thus may make counselling challenging prior to genetic testing.
Emotional and social support
Eye Clinic Liaison Officers (ECLOs) act as an initial point of contact for newly diagnosed patients in clinic. They provide emotional and practical support to help patients deal with their diagnosis and maintain independence. They work closely with the local council’s sensory support team and are able to advise on the broad range of services provided, such as visual rehabilitation, home assessment, work and access to qualified teachers for children with visual impairment (QTVI) among other services.
Referral to a specialist centre
In the UK, patients should be referred to their local genomic ophthalmology (if available) or clinical genetics services to receive a more comprehensive genetic management of their conditions (genetic testing and genetic counselling) and having the opportunity to participate in clinical research.
Current research in cone/cone-rod dystrophies
Much of the current research in cone/cone-rod dystrophies are focused on elucidating the remaining causative genes and their molecular mechanisms, understanding the natural history of the disease and establishing optimum clinical trial endpoints. Findings from ongoing research in ABCA4-retinoapthies could pave the way for the development of novel therapies for one of the most common causes of cone-rod dystrophies. Other studies that may play similar roles include:
- Gene therapy for X-linked RP due to RPGR mutations (NCT 03116113, NCT 03316560, NCT 03252847)
- Gene therapy for Leber Congenital Amaurosis due to GUCY2D mutations (NCT 03920007)
- Gene therapy for achromatopsia due to CNGA3 (NCT 02610582, NCT 03758404, NCT 02935517) and CNGB3 (NCT 03001310, NCT 02599922) mutations
- Research Opportunities at Moorfields Eye Hospital UK
- Searching for current clinical research or trials
Further information and support
- Retina UK
- Alström syndrome UK
- BBS UK
- Royal National Institute of Blind People (RNIB)
- Guide Dogs for the Blind Association
- Look UK
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- Gill JS, Georgiou M, Kalitzeos A, Moore AT, Michaelides M. Progressive cone and cone-rod dystrophies: clinical features, molecular genetics and prospects for therapy. Br J Ophthalmol. 2019;103(5):711-720
- Stockman A, Henning GB, Michaelides M, et al. Cone dystrophy with “supernormal” rod ERG: psychophysical testing shows comparable rod and cone temporal sensitivity losses with no gain in rod function. Invest Ophthalmol Vis Sci. 2014;55(2):832-840
- Boulanger-Scemama E, Mohand-Saïd S, El Shamieh S, et al. Phenotype Analysis of Retinal Dystrophies in Light of the Underlying Genetic Defects: Application to Cone and Cone-Rod Dystrophies. Int J Mol Sci. 2019;20(19)
- Robson AG, Michaelides M, Luong VA, et al. Functional correlates of fundus autofluorescence abnormalities in patients with RPGR or RIMS1 mutations causing cone or cone rod dystrophy. Br J Ophthalmol. 2008;92(1):95-102
- Oishi M, Oishi A, Ogino K, et al. Wide-Field Fundus Autofluorescence Abnormalities and Visual Function in Patients With Cone and Cone-Rod Dystrophies. Investigative Ophthalmology & Visual Science. 2014;55(6):3572-3577
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