Diagram showing a LogMar chart as used druing eye examinations to measure visual acuity.  Hermansky-Pudlak Syndrome (HPS) causes reduced visual acuity. Diagram showing a Schnellen chart as used during eye examinations to meaures visual acuity.  Hermansky-Pudlak Syndrome (HPS) causes reduced visual acuity.

The LogMar chart is also commonly used for measuring visual acuity.  It has a number of advantages over the Snellen chart that make it a more reliable and accurate test.  The results are written as a decimal number and not a fraction, eg: 0.78 which is equal to a Snellen of 6/36 or 20/120.  The patient stands 6 metres from the chart and reads from top to bottom.  The more letters and whole lines that can be read the lower the LogMar score is.  For instance a LogMar of 0.0 (zero) is equal to a Snellen of 6/6 (or 20/20) whereas a LogMar of 1.0 is equal to 6/60 (20/200).  Click here for a conversion table.


The name, LogMar can be split into two, the ‘MAR’ part refers to ‘Minimum Angle of Resolution’.  Put simply, this refers to the smallest sized image that can be seen and recognised (or the smallest letter in this case).  The ‘Log’ part of the name refers to ’Logarithm’ which is the mathematics used to calculate the score and convert it into a decimal number.  The maths can seem over complicated but once a score has been calculated it can be used to make more accurate comparisons.

In contrast, in the case of Hermansky-Pudlak Syndrome and other types of albinism, there is typically some improvement in visual acuity during the early years but it typically falls far short of the acuity of normal vision.  There may be small measurable improvement beyond the early years  if there is some pigment growth in the irises and retinas of the eye.  This is unlikely to make a noticeable difference during practical daily activities.


Visual acuity is not the same as either ‘refractive errors’(long and short sighted-ness) or ‘astigmatism’. Refractive errors and astigmatism are defects in the way that light is focused on to the retinas at the back of each eye (Read more).  They are caused by imperfections in the shape of either the eyeball, the cornea (at the front)of the eye), or the lens.  They result in blurred or distorted images being seen.  These can typically be ‘corrected’ by the wearing of prescription glasses or contact lenses.


The terms ‘best corrected vision’ and ‘best corrected visual acuity’ refer to the results from tests of visual acuity taken when a patient is wearing lenses that correct for refractive errors and astigmatism.


However, glasses and contact lenses cannot ‘correct’ the reduced visual acuity of HPS and other types of albinism.  For this reason some people with HPS and albinism find there is little benefit to wearing them.


How is visual acuity measured.


Patients and parents may find that ophthalmologists and opticians quote different results from eye tests.  This is because visual acuity can be measured with a number of different test charts.  Each chart has its own scoring system.  It is possible to convert and compare results between the different approaches.  Click here for a conversion table.


The most common approach is known as the ‘Snellen chart’.  The results are expressed as a fraction.



For example, good visual acuity is 6/6 (or 20/20)  when a person can read the eighth line of a Snellen chart.  In the case of HPS:


What is Visual acuity ?


The term ‘Visual Acuity’ refers to how much detail can be seen at a set distance (usually 6 metres).  The better a persons’ visual acuity is the more fine detail they can see at the set distance.  Acuity is typically low for people affected by HPS and albinism.


The amount of detail that can be seen is directly related to the number of cells found in the retinas (photo-receptors called rods and cones) and the arrangement of nerve fibres that carry information to the back of the brain (primary visual cortex).  In both HPS and other forms of albinism the retinas and optic nerves are immature and mis-routed.  In particular the retinas lack a structure called the foveal pit which is the part responsible for sensing (seeing) fine detail.


Visual acuity is low in normal newborn babies and typically improves dramatically during the first 18 months in line with expected development.  It typically continues to improve up to the age of around 4 years and remains stable throughout life - provided that eye health is good.

Diagram showing a Schnellen chart as used during eye examinations to meaures visual acuity.  Hermansky-Pudlak Syndrome (HPS) causes reduced visual acuity.
Diagram showing a LogMar chart as used druing eye examinations to measure visual acuity.  Hermansky-Pudlak Syndrome (HPS) causes reduced visual acuity.

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Features of the Visual impairment of Hermansky-Pudlak Syndrome