Twenty-four-hour rhythm of ocular perfusion pressure in non-arteritic anterior ischaemic optic neuropathy

Purpose: In non-arteritic anterior ischaemic optic neuropathy (NAAION) patients, circulatory insufficiency within the optic nerve has previously been hypothesized to be related to nocturnal systemic hypotension. The main objective of this prospective cohort study was to investigate the nyctohemeral variations in ocular perfusion pressure (OPP) in NAAION patients. Methods: In 20 patients with NAAION, the intraocular pressure (IOP) was measured using the Tono-Pen XL (TM) electronic tonometer every hour for 24 hr. Blood pressure (BP) was evaluated over 24 hr. Mean OPP was calculated with the following formula: OPP (sitting position) = (0.74 x mean BP) - IOP and OPP (lying) (position) = (0.84 x mean BP) - IOP. A nonlinear least squares dual-harmonic regression analysis approach was used to model the 24-hr rhythms of OPP data. Results: On average, a 24-hr amplitude of 4.7 +/- 2.6 mmHg was found for OPP. The patients were classified as either having a diurnal OPP rhythm(i.e. with a diurnal acrophase, 10% of the cases), a nocturnal OPP rhythm (45%) or absence of OPP rhythm (45%). Four patients had a nocturnal reduction in OPP (mean, -11%). Conclusion: The physiological nocturnal rhythm of OPP was maintained in 45% of the NAAION patients. The nocturnal reduction in OPP seen in 20% of the patients was within the range of OPP where optic nerve blood flow autoregulation is still fully operative. A high prevalence of obstructive sleep apnoea syndrome in our population (71%) may explain the low frequency of systemic nocturnal hypotension.