Examination of visual acuity and blindness (CN II)

A discussion of the lesions of the visual pathways takes place elsewhere.

Similarly, pupillary abnormalities deserve their own page.

Here, the focus is primarily on the optic nerve itself.

Visual acuity in the conscious patient

In order to accurately assess visual acuity, one requires a conscious and cooperative patient. One does not routinely see the Snellen chart waved around in the ICU. However, this is the better method of testing the visual acuity. "How many fingers am I holding up" and looking at the bedside clock are rather crude methods, but certainly enough for a rapid bedside identification of major deficits. Other crude methods (eg. the identification of routine objects) can also be employed, but ultimately, nothing beats the Snellen chart in terms of reproduceability and accuracy.

However, generally speaking, rather than focusing on subtle deficits of long and short distance vision, the pragmatic intensivist usually just wants to know whether the patient is blind or not.

Visual acuity in the unconscious patient

One does not need the patient to be either conscious or cooperative in order to establish whether they are blind. There are several tests one can perform to assess the unilateral retina:

Sustained visual pursuit

Close one of the patient's eyes. The other eye, remaining open, gets an object moved in front of it.

If one can convince oneself that the patient's eyes track that object, at the very least the optic nerve must be intact (as well as most of the rest of the visual pathway). The patient in a minimally conscious state may demonstrate the capacity for visual tracking, as one of the features of their slow recovery.

The tracking of objects is a reflex mediated by the superior colliculus, in the tectum of the midbrain.

Startle response

Provided they have not recently received a dose of muscle relaxant, one can rely on the "startle" response to establish whether the patient's optic nerve is still working.

Close one of the patient;s eyes. Rapidly confront the other eye with your finger. A patient with intact optic nerve function up to the level of the optic tract will blink. This reflex is also mediated by the superior colliculus

Light reflex

The other two tests in this section are fairly unreliable. Is the patient tracking the object, or are their eyes roaming uselessly? On the other hand, the light reflex is fairly unequivocal. if the pupil constricts briskly in reponse to light, the optic nerve is intact. The light reflex is discussed in greater detail elsewhere.

Interpretation of lost visual acuity