Square-wave jerks: a disorder of microsaccades?

A patient with presenile dementia had instability of ocular fixation caused by small rapid horizontal eye movements (square-wave jerks). High resolution recordings of these eye movements were characteristic of microsaccades, which ordinarily occur during fixation, in all respects except amplitude. In addition, the patient showed complete absence of normal microsaccades. We suggest that microsaccades have become altered in our patient to form a clinical disorder of fixation.