Phase portrait analysis of eye movement waveforms in congenital nystagmus eyes

Background

The characteristics of eye movement waveforms in congenital nystagmus eyes is so complicated that the time-series waveform can hardly demonstrate the oscillational kinetic property of the eye movement of nystagmus, which is not convenient for the clinical diagnosis and evaluation.The eye movement system is a nonlinear feedback control system, and phase portrait analysis is a useful method in describing the characteristics of movement in a nonlinear system.

Objective

This study was to establish the phase portrait analysis method of the eye movement waveforms in congenital nystagmus eyes and provide a new technique for the clinical diagnosis and evaluation of congenital nystagmus.

Methods

A prospective series-case observational study was performed.This study complied with the Declaration of Helsinki and was approved by Ethic Commission of Tianjin Eye Hospital.Written informed consent was obtained from each patient before examination.Twenty-five patients with congenital nystagmus were enrolled and received examination of video-oculography (VOG) in Tianjin Eye Hospital from April 2012 to February 2013, including 12 patients with congenital motor nystagmus (CMN) and 13 patients with latent nystagmus (LN). The VOG data were format-converted and processed by a Matlab routine.VOG-based phase portraits of eye movement cycles were drawn and related parameters were measured from the phase portraits for further analysis and comparison.Main measurement indexes were cycle position shift (CPS), slow phase peak velocity (SPV), standardized slow-phase peak (SSPV), fast phase peak velocity (FPV) and standardized fast-phase peak velocity (SFPV). Two principal types of waveforms, increasing-velocity type and decreasing-velocity type were compared.

Results

The phase portraits of the waveforms of congenital nystagmus were cycles of reciprocating trajectories.The slow phases were in dense ribbon region, and the fast phases were in sparse annular region, and all the trajectories travelled in clockwise.The CPS, SPV and FPV of increasing-velocity type were (4.646±1.565), (223.821±114.049), (767.481±263.560)° per second, respectively.The CPS, SPV and FPV of decreasing-velocity type were (9.373±4.189), (357.531±154.300) and (1 148.706±541.362)° per second, respectively.The SPVs and FPVs of both increasing-velocity type and decreasing-velocity type showed strong correlation with CPS, respectively (the increasing-velocity type: r_SPV-CPS=0.685, P=0.000; r_FPV-CPS=0.680, P=0.000; the decreasing-velocity type r_SPV-CPS=0.783, P=0.000; r_FPV-CPS=0.803, P=0.000). FPVs were significantly larger than SPVs in both types of waveforms (t =6.558, P=0.000; t=5.068, P=0.000). The SSPV of increasing-velocity type was (48.062±15.365) ° per second, which was slightly larger than (41.099±17.027)° per second of decreasing-velocity type, with no significant difference between them (t=1.070, P=0.296). The SFPV of increasing-velocity type was (171.186±47.825)° per second, which was larger than (125.317±38.266)° per second of decreasing-velocity type, showing a significant difference between them (t=2.658, P=0.014).

Conclusions

Phase portraits can visualize the cyclically dynamic features of congenital nystagmus in a direct way.It facilitates the measurement of eye movement amplitude, speed and other parameters.The phase portrait analysis method provides a novel useful tool in the clinical diagnosis and treatment efficacy evaluation of congenital nystagmus.