Head-down posture induces PERG alterations in early glaucoma

Abstract

Purpose: To probe susceptibility of retinal ganglion cells (RGC) to physiological stressors associated with moderate head-down body tilt in patients with suspicion of glaucoma or early manifest glaucoma (EMG).

Methods: One hundred nine subjects with best corrected visual acuity (BCVA) ≥ 20/20 and no disease other than glaucoma [glaucoma suspects (GS)=79, EMG=14, normal controls (NC)=16 and comparable age range were tested. Noncontact intraocular pressure (IOP), pattern electroretinogram (PERG), and brachial blood pressure/heart rate measurements were performed in 3 consecutive conditions (∼0038 min apart): seated (baseline), -10-degree whole body head-down tilt (HDT), and seated again (recovery). PERG amplitude and latency, IOP, and systolic/diastolic blood pressures, heart rate, calculated mean central retinal artery pressure, ocular perfusion pressure, and systolic/diastolic perfusion pressures were evaluated.

Results: During HDT, IOP significantly (P<0.001) increased in all groups approximately to the same extent (approximately 20%). PERG amplitude did not change in NC but decreased significantly (P<0.001) in patients (GS, -25%, EMG -23%). PERG phase become delayed in NC (-1.6%, P=0.04) but more so in patients (GS, -2.7%, P<0.001; EMG, -6.0%, P<0.001). The proportion of patients with PERG alterations significantly (P<0.05) exceeding those occurring in age-adjusted and baseline-adjusted NC were, GS: amplitude 20%, phase 15%; EMG: amplitude 14%, phase 50%. All measures recovered baseline values after HDT.

Conclusions: Moderate HDT induces temporary worsening of RGC function in a subpopulation of GS and EMG patients. This noninvasive protocol may help disclose abnormal susceptibility of RGCs in a subset of the patients at risk of glaucoma.

Figure 1

A custom made, adjustable PERG stimulus (A) in combination with a Skytron Elite surgical table (B) allowed PERG recording in subjects oriented −10° head-down.

Figure 2

Mean IOP (average of the two eyes) measured in different conditions of body posture in normal controls and in patients with either suspicion of glaucoma or manifest glaucoma in the early stages. Labels on the x-axis: Bas, baseline seated; Tilt, whole body −10 degrees head-down; Rec, recovery seated. In all panels, errors bars represent the SEM. P-values for statistical differences with baseline are marked with asterisks (*, <0.05; **, <0.01; *** ≤0.001).

Figure 3

Example of raw PERG waveforms recorded simultaneously from both eyes of a patient with early manifest glaucoma at baseline-seated condition (left panel), during −10° head-down body tilt (middle panel), and after seated repositioning (right panel). Green waveforms represent the right eye; red waveforms represent the left eye. Waveforms were analyzed with Discrete Fourier Transform to isolate the frequency component corresponding to the reciprocal of the pattern-reversal period of 61. 4 ms (16.28 Hz) and measure its amplitude in μV and phase in π radians. In the legend, CV is the coefficient of variation of two consecutive responses whose average constitutes the waveforms shown.

Figure 4

Mean PERG amplitude (average of the two eyes) measured in different conditions of body posture in normal controls and in patients with either suspicion of glaucoma or manifest glaucoma in the early stages. Labels on the x-axis: Bas, baseline seated; Tilt, whole body −10 degrees head-down; Rec, recovery seated. In all panels, errors bars represent the SEM. P-values for statistical differences with baseline are marked with asterisks (*, <0.05; **, <0.01; *** <0.001).

Figure 5

Mean PERG phase (average of the two eyes) measured in different conditions of body posture in normal controls and in patients with either suspicion of glaucoma or manifest glaucoma in the early stages. Labels on the x-axis: Bas, baseline seated; Tilt, whole body −10 degrees head-down; Rec, recovery seated. In all panels, errors bars represent the SEM. P-values for statistical differences with baseline are marked with asterisks (*, <0.05; **, <0.01; *** <0.001).

Figure 6

Standardized baseline and age adjusted deviations from normal (z-scores) for PERG amplitude and phase changes during −10 deg head down body tilt.