The nystagmus blockage syndrome. Congenital nystagmus, manifest latent nystagmus, or both?

We have carefully studied, by quantitative oculography, a patient with the nystagmus blockage syndrome (NBS), and two patients with a similar disorder of eye movements that might be mistaken clinically for NBS. Our recordings revealed two distinctly different abnormalities present in a single patient with NBS. Our NBS patient exhibited congenital nystagmus (CN) waveforms when viewing at distance; the CN did not damp with convergence on a near target. When the patient allowed one eye to become esotropic, however, the nystagmus damped considerably and abruptly changed from CN to manifest latent nystagmus (MLN). This peculiar transition from CN to MLN has not been described previously. The appearance of MLN in a case with ongoing CN suggests that two different mechanisms may underlie NBS, since the only other case documented with eye movement recordings showed no transition to MLN. Because the diagnosis of NBS usually is made on evidence of clinical signs alone, it is probable that these two types have been combined indiscriminately and presented as one syndrome. In addition, our discovery of two mechanisms discernable only by quantitative recording suggests that NBS has been diagnosed inappropriately in patients with clinically similar but oculographically different eye signs. Further quantitative studies are required to fully define NBS and to determine if these are the only two mechanisms found in this syndrome.