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Josephine Shallo–Hoffmann, Mary Faldon, Ronald J. Tusa; The Incidence and Waveform Characteristics of Periodic Alternating Nystagmus in Congenital Nystagmus. Invest. Ophthalmol. Vis. Sci. 1999;40(11):2546-2553.
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purpose. To investigate the incidence and waveform characteristics of periodic
alternating nystagmus (PAN) in congenital nystagmus (CN).
methods. In a prospective study, 18 patients with CN without associated sensory
defects agreed to undergo eye movement documentation using binocular
infrared oculography. Two of the 18 had a diagnosis of suspected PAN
before entering the study. The patients sat in a dimly lit room and
viewed an LED (4 mm in diameter) located in the primary position, at a
distance of 100 cm. During an 8-minute recording, patients were read a
story of neutral interest to hold attention at a constant level. PAN
was defined as a left-beating nystagmus, a transition phase, a
right-beating nystagmus, and a final transition phase; the sequence was
results. Seven of the 18 patients had PAN (median cycle: 223 seconds, range
180–307 seconds). The periodicity of the cycles for each adult patient
was regular, although the phases within a cycle were often asymmetric.
Six of the seven patients had an anomalous head posture (AHP), and in
five the AHP was in only one direction. Except for one patient, the PAN
waveforms had an increasing slow-phase velocity in at least one phase
of the cycle; in the other phase they were linear.
conclusions. The occurrence of PAN in CN is not as rare as previously thought and
can be missed because of the long cycles and the use of only one AHP.
The AHP was dependent on, and could be predicted from, the waveforms
containing the longest foveation times. Although the waveforms and
foveation times may differ among the phases of the PAN cycle, the
periodicity of the cycle was usually regular and therefore predictable.
Identification of PAN is essential in cases in which surgical treatment
is considered for correction of AHPs.
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