Final answer:
The absence of eye movement on unilateral cover testing in a patient with horizontal diplopia suggests no heterotropia or heterophoria. However, further testing is required to conclude on ocular deviation or neuromuscular disorders like myasthenia gravis or internuclear ophthalmoplegia.
Step-by-step explanation:
A patient reporting horizontal diplopia with a worsening condition towards the end of the day and no eye movement noted on unilateral cover testing suggests that conjugate gaze is intact, and thus, it's unlikely that the patient has heterotropia or heterophoria. Since neither eye moves when the other is covered, it implies that the eyes are aligned when both are uncovered, ruling out constant (heterotropia) or intermittent (heterophoria) misalignment.
However, given that the diplopia worsens over time, this could indicate muscle fatigue or a neurological condition that impairs extraocular muscle function, such as myasthenia gravis or a subtle form of ocular misalignment that is not readily detected on cover testing. To reach a definitive conclusion about the presence of an ocular deviation such as internuclear ophthalmoplegia or other neuromuscular disorders, further testing is required. Additional evaluations might include other forms of strabismus testing, assessment of neurological function, and imaging studies, if indicated.