Walsh & Hoyt: Differential Diagnosis of Acquired Trochlear Nerve Palsy: Skew Deviation

Although trochlear nerve paresis is the most common cause of acquired vertical diplopia, other causes include ocular myopathies (e.g., dysthyroid eye disease), disorders of the neuromuscular junction (e.g., myasthenia gravis), and skew deviation. The ocular myopathies and disorders of the neuromuscular junction are described in Chapters 22 and 21, respectively. Skew deviation is discussed in Chapter 19, but we chose to discuss it in this chapter as well in order to contrast the findings with those seen in patients with trochlear nerve palsy. Skew deviation is a vertical misalignment of the visual axis caused by defective supranuclear inputs. The hypertropia may be constant for all positions of gaze (comitant), or it may vary and may even alternate depending on the direction of gaze.