Oligocone trichromacy (ORPHA:75378)
A rare non-progressive form of cone photoreceptor dysfunction syndrome characterized by reduced visual acuity, normal fundus appearance and absent or reduced cone responses on electroretinography. In contrast to all other forms of cone dysfunction color vision is normal.
Epidemiology
The syndrome is very rare with less than 15 cases reported in the literature so far.
Clinical description
Individuals with this disorder have reduced visual acuity from a young age and usually have a mild degree of photophobia. Best corrected visual acuity is usually in the range of 6/12 to 6/36. Fundus examination is normal. Electroretinography demonstrates absent or markedly abnormal cone responses; rod responses are normal. Color vision testing is usually normal but some patients show a very mild reduction in color discrimination. This is in contrast to other forms of inherited cone dystrophy where color vision is severely impaired.
Etiology
The causative gene has not been identified. There have been a few reports of patients with a similar phenotype who have mutations in genes normally associated with achromatopsia but the phenotype is atypical and more suggestive of an incomplete form of achromatopsia. One suggestion is that oligocone trichromacy (OT) is a very mild form of incomplete achromatopsia but this is not supported by current molecular genetic data. The reason for the presence of normal color vision despite the reduced visual acuity and electrophysiological evidence of severe cone dysfunction is uncertain. It has been proposed that patients with OT may have reduced numbers of normal functioning cones with preservation of the three cone types in normal proportions thereby enabling normal color vision (trichromacy). This hypothesis is supported by the findings of adaptive optics scanning laser ophthalmoscopic imaging (AOSLO) of the cone mosaic in patients with OT where there are reduced numbers of functioning cones in the fovea.
Diagnostic methods
Diagnostic methods include color vision testing, retinal imaging and electroretinography (ERG). Molecular genetic testing is also helpful in excluding other forms of cone dystrophy.
Differential diagnosis
Differential diagnosis should include other cone dysfunction syndromes, namely achromatopsia, Blue cone monochromatism and X-linked cone dysfunction syndrome with myopia, where there is reduced or absent color vision. The disorder also needs to be distinguished from bradyopsia where extended ERG testing beyond the ISCEV (International Society for Clinical Electrophysiology of Vision) standard needs to be performed to distinguish this rare disorder from OT. Molecular genetic testing using next generation sequencing of retinal disease gene panels will also help distinguish these disorders (where the molecular genetic basis is known) from OT.
Genetic counseling
Although most reported cases are sporadic, the disorder is likely to be inherited as an autosomal recessive trait as a number of affected sibling pairs have been reported. One family has been reported in which there may be dominant inheritance with incomplete penetrance. Affected individuals and their families should be counselled on the basis that the inheritance is likely to be autosomal recessive (unless the family history suggests otherwise).
Management and treatment
There is no effective treatment for the underlying retinal disorder. Patients should be provided with accurate visual aids: eyeglasses that can be tinted if photophobia is a prominent feature. The minority of patients with more severe visual loss may benefit from low vision aids.
Prognosis
Although few patients with this disorder have been followed up long term, it appears that the prognosis is good as the disorder is usually non-progressive.

                            Last update: 
                            May 2019


                            - Expert reviewer(s): 
                            Pr Tony MOORE