The College of Optometrists

Why do we get a nasal step or arcuate step with glaucoma??....am a bit confused....something to do with the layout of the fibres...The nasal step occurs near the horizontal raphe...but what abt the fibres affected???

Why do field screeners, well the majority, only test the central 30 degrees..??....is it cos most of the field defects are likely to occur in the central 30 deg...???

ThanQ

Look in kanski. It shows you what the arrangment of the nerve fibre layers are. The inferior and superior portion of the disc correspond to the nasel step areas of of the retina.and as u said, the nerve fibres dont cross forming the horizontal raphe. These areas are more likely to get damaged in glaucoma and least likley being the macula (unless its nomo/low tensive glaucoma) You would also possibly get arcuate field defects.

Nasal fibres do not cross H.raphe, and at this line the fibres are staggered.

Nasal fibres vulnearable in glaucoma as they are the most crowded at disc, so less structural and vascular support.

The reason you get a nasal step in glaucoma is due to the distribution of the retinal nerve fibres. As Mike correctly points out the superior fibres are staggered relative to the inferior fibres. They do not cross the horizontal raphe and so the associate visual field defect also respects the mid line.

The reason that nasal visual field defect often appear first is due to the size of the receptive fields in the temporal retina - ie they are large (a single ganglion cell will collect responses from several thousand photoreceptors). Therefore, if a single ganglion cell responsible for the temporal retina is lost then a relatively large area of the visual field is affected. This is compared to the macular where receptive field size is very small (a single ganglion cell receives impulses from a few photoreceptors) and so a central visual field defect represents a significantly larger loss of neurons. Additionally, neurons that serve the temporal retina (nasal field) are the longest fibres in the retina and emanate from the poles of the disc where there is less connective tissue both of which make them more susceptible to damage.

With regard to the central 30degs, I think this stems back to the Bjerum screens, and statistically visual field defects occur most frequently in the central 30degs. Also, your visual performance reduced dramatically with eccentricity meaning results of retinal sensitivity measurements beyond the central 30dgs become unreliable.

You can be asked to draw the retinal nerve fibre arrangement in the IT exam! As well as the nerve fibre pathways in the visual pathway.