CONE DYSTROPHY


The retina or light sensitive membrane in the eye must remain intact and healthy to provide good vision.

Two types of cells are light sensitive and detect light, projecting their response back to the brain via the optic nerve to be decoded and interpreted as "sight".

The Cones are cells which are responsible for daylight vision, they distinguish fine detail, colour and form. They are concentrated close to the fixation area of the eye- the macula. Their function enables us to recognise faces, to read print and distinguish colours.  

 The Rods are more numerous. They are very sensitive cells so are useful in dim lights to maintain our night vision. Our peripheral (or side vision) depends on their function. They detect movement and shapes helping us to navigate in crowds or around obstacles.

If their function deteriorates we have difficulty distinguishing fine detail, the change in colour vision can be more subtle. Because the eye is then depending on the rods- very sensitive cells- bright lights become a problem. Vision tests fail because interpretation of detail fails.

 INHERITANCE:

The cone dystrophies are a heterogeneous group of inherited disorders- they may be dominantly or recessively inherited. In the former case normally other members of the family are known to be affected. While in the recessively inherited group there is usually no positive family history- however siblings may also have the problem (each have a 1 in 4 chance of contracting the disorder). The children of an affected person are unlikely to be affected.

SYMPTOMS:

Photophobia- dislike of lights, reduced visual acuity and abnormal colour vision.

AGE OF ONSET:  between 10 and 30 years

     DIAGNOSIS: 

Retinal change - Early a change in central areas may produce a "Bullseye pattern".  In some forms of the condition diffuse pigmentary changes occur later.

Tests of retinal function including Ganzfeld electroretinography (ERG) which shows reduced or absent cone responses is essential for the diagnosis. Tests for colour vision are complementary (Farnsworth-Munsell 100-hue test).

Visual Electrophysiology of Cone Dystrophy

GENETIC TESTING:

Cone dystrophy and cone-rod dystrophy  (as in Stargardt's disease, Best's disease, pattern dystrophy) have been mapped to different regions of the genome, however the underlying genetic mutations await identification. At the present time genetic testing of individual patients with Cone Dystrophy is of limited value. This may change at any time. Similarly electrophysiology is becoming increasingly important as genetic therapies are being trialled. Accurate phenotyping will be necessary to  enable patients to be assessed for these new therapies.

PROGNOSIS:

The condition may be stationary or progressive. Serial tests are necessary to determine prognosis. Peripheral vision and independent mobility is usually maintained for many years. Reading can usually be maintained to a late age with a variety of spectacles aids.

THERAPY: Supportive aid is required. Regular checks to determine if lenses are required.There is no means of treatment at this stage. Gene therapy  may be available in the future.

References:


1. Simunovic MP. Moore AT, The cone dystrophies     Eye. 12 ( Pt 3b):553-65, 1998

2. Sadowski B. Zrenner E Cone and rod function in cone degenerations. Vision Research. 37(16):2303-14, 1997  

3. Molecular genetics of central retinal dystrophies A & N Z J  Ophthalmology. 24(3):189-98, 1996 Aug.

4. Sadowski B. Zrenner E . Differential diagnosis of cone dystrophies Ophthalmologe. 91(6):719-29, 1994 Dec.

5. Jacobson DM. Thompson HS. Bartley JA .  X-linked progressive cone dystrophy. Ophthalmology. 96(6):885-95, 1989 Jun.