You may have noticed some nurses and doctors carrying small penlight torches in their pockets. Known as neuro torches, they are used during a neurological assessment, to view your pupil’s response to light.
Normally, when the light intensity entering the eyes increases, impulses from the rods and cones on your retina travel via your optic nerve to the brain and then back along another nerve to the muscles of the iris causing it to constrict (this is known as miosis).
Conversely, when light intensity entering the eye decreases, signals from the retina travel via the optic nerve to a different part of the brain and back along a different nerve to the muscles of the iris causing it to dilate (mydriasis).
If the pupils do not respond appropriately to a direct light from the neuro torch, it can indicate problems affecting these nerve pathways (see below).
How is it used?
Ideally, your pupillary reflexes should be examined in a low light environment.
The nurse will ask you to fix your gaze on a target some distance behind them. They will then shine the torch for a few seconds into each eye observing the following:
Size and Equality. They are noting the diameter of your pupil size in millimetres. They may also describe your pupil using terms such as: pinpoint, small, midposition, large, or dilated.
Interestingly, up to 20% of the population have a slight difference in pupil size (known as Aniscoria) and this is considered a normal variant.
Shape: Again, your pupil shape may be documented as round, irregular, oval or keyhole. Causes of irregular pupils include eye surgery or the implantation of intra-occular lenses to treat cataracts.
Reactivity: This is the pupils expected constriction to the light source as I discussed above.
It may be described as:
At the same time the nurse will be looking for the pupil in the opposite eye to also constrict. This a normal response known as the consensual pupillary response.
Accommodation: Finally the nurse may ask you to focus on a distant object, and then refocus on a much closer one (perhaps their finger). Normally this will cause your pupils to constrict slightly.
Causes of abnormal pupils:
Unequal pupils: May be a normal variant (see above).
It can also be due to increased pressure in the skull (from bleeding for example) causing compression on arteries or nerves involved with iris function on one particular side. It can also indicate problems in the brainstem or spinal cord.
Dilated pupils: May be a result of drug use including antihistamines, hallucinogens, amphetamines, anticholinergics, dopamine or barbiturates.
Pupils may also become dilated during emotional arousal or increased mental effort.
Dilation can also be due to medication used to dilate the pupils during an eye examination.
In severe cases, it can indicate lack of oxygen reaching the brain or brain death.
Constricted pupils: may be caused by drugs such as opiates, pilocarpine or acetylcholine, or due to pressure on the nerve pathway to the constriction muscles of the iris.
In conclusion, the neuro torch is used to perform a quick pupil assessment giving a great deal of information about your neurological status. But it is important to understand that it is just one part of the overall examination, and any findings must be considered in the context of all the information obtained.
The Pupillary Response in Traumatic Brain Injury: A Guide for Trauma Nurses. Journal of Trauma Nursing.
Pupillary Responses. Stanford School of Medicine.