Impairment of ocular blood flow during regional orbital anesthesia.

BACKGROUND: Regional anesthesia for ophthalmic surgery has been associated with ischemic complications, such as central retinal vascular occlusion, optic atrophy and ischemic optic neuropathy. Impairment of pulsatile ocular blood flow (POBF) may occur with regional orbital anesthesia. In this study we quantified POBF in patients undergoing regional orbital anesthesia. METHODS: Eleven patients (12 eyes) with a mean age of 76.5 years having regional orbital anesthesia for cataract or retinal surgery at a private refractive surgical centre in Calgary had POBF monitoring before, during and 15 minutes after induction of anesthesia. RESULTS: There were no significant changes in intraocular pressure or heart rate during the induction phase or 15 minutes after induction of regional orbital anesthesia. However, ocular blood flow indices, including pulse amplitude, pulse volume and POBF, were significantly reduced following attainment of regional orbital blockade (p < 0.05). With time there was recovery in these variables, but they all remained significantly reduced from baseline 15 minutes later. INTERPRETATION: Ocular blood flow appears to be significantly impaired during regional orbital anesthesia, induced as described. There could be benefit in monitoring POBF to reveal otherwise undetectable deleterious effects on retinal circulation in patients having retrobulbar injections, orbital compression or digital manipulation of the globe.

Early macular dysfunction detected by focal electroretinographic recording in non-insulin-dependent diabetics without retinopathy.

The focal electroretinogram, which measures the functional integrity of the distal retina of the macula, was recorded with a hand-held stimulator-ophthalmoscope in 26 eyes from patients with non-insulin-dependent diabetes mellitus with normal fundus photography, and in 52 control eyes of similar age range. Implicit time and amplitude of the responses were studied as a function of the age, glycemic control through glycosylated hemoglobin measurement and duration of diabetes. Implicit time and amplitude were significantly delayed (F=5.05, p=0.028) and reduced (F=11.26, p=0.013) in diabetic patients without diabetic retinopathy compared to control subjects. Moreover, there was a significant relationship between the implicit time (r=0.57, p=0.002) and amplitude (r=-0.65, p=0.0004) with the duration of diabetes but not with hemoglobin Alc. These results strongly suggest an early macular dysfunction in non-insulin-dependent diabetes mellitus before the appearance of diabetic retinopathy.

The electroretinogram during orbital compression following intraorbital regional block for cataract surgery.

The electroretinogram (ERG) is a transient biopotential that reflects the electrical response of the distal retina to photostimulation. Disturbances in retinal circulation produce characteristic abnormalities in the ERG wave form. The objective of this study was to investigate the changes in the ERG produced by combined retrobulbar and peribulbar injections of a large volume (8 ml) of local anaesthetic, followed by ocular compression. Electroretinogram recordings were obtained from skin electrodes placed on the infero orbital ridge in response to stroboscopic flash stimulation in 34 adult patients undergoing cataract surgery: (a) prior to regional anaesthesia (baseline condition); (b) within one minute after regional anaesthesia of the orbit (block condition); (c) after ten minutes of orbital compression with a Honan's device at 30 mmHg. (compression condition); (d) and five minutes after removal of orbital compression (recovery condition). The ERG implicit times of both a- and b-wave increased (P < 0.001) after anaesthetic block. The amplitude of the a- and b-waves also decreased (P < 0.001) immediately following anaesthetic block and continued to decrease following application of the compression device (P < 0.01). Following removal of ocular compression the amplitude of the b-wave increased (P < 0.01). Only the a-wave implicit time (P < 0.005) decreased with release of ocular compression. These findings are compatible with the ERG changes of transient retinal ischaemia produced by ocular compression.

Contribution of Ca and Ca-activated Cl channels to regenerative depolarization and membrane bistability of cone photoreceptors.

1. Cone photoreceptors in several vertebrate species generate Ca-dependent regenerative depolarizations (e.g., Ca spikes lasting up to 2 s) in response to current injection or surround illumination and may remain in a state of prolonged depolarization (e.g., a permanent plateau near 0 mV) after these stimuli. This paper, while confirming the role of Ca channels in the regenerative depolarization, demonstrates that Ca-activated Cl channels either enhance or hinder prolonged depolarization, depending on the value of the chloride equilibrium potential (ECl). 2. Current- and voltage-clamp recordings obtained with the whole-cell patch-clamp technique were compared in 158 isolated tiger salamander cones to determine the contribution of specific ion channel types to the two forms of depolarizing response. Cones dialyzed with CsCl or KCl intracellular solution (such that ECl = 0 mV) that had sustained negative slope regions in their current-voltage (I-V) relations recorded under voltage clamp, were, under current clamp, bistable with respect to their resting potential. Injection of approximately 20-pA steps of depolarizing current resulted in transitions from the negative stable membrane potential (near -50 mV) to a long lasting plateau around 0 mV. Injection of 200-300 pA of hyperpolarizing current could then force a return to the negative stable resting potential, although once repolarization occurred, current injection had to be reduced or terminated to prevent damaging hyperpolarization of the cell. 3. The inward currents accounting for the negative slope region of the I-V relation were carried in Ca and Ca-activated Cl channels. Specific block of Ca-activated Cl current (ICl(Ca)) by 100 microM niflumic acid (NFA) eliminated the prolonged depolarization, even though the negative slope conductance region in the I-V persisted and the cone could still produce the briefer Ca-dependent regenerative depolarizations. Application of 100 microM Cd2+ blocked both forms of depolarization. 4. Substitution of Ba2+, which among other actions did not activate ICl(Ca), usually supported regenerative depolarizations of shortened duration, demonstrating the role of Ca channels in the initial phase of these responses. 5. A difference was observed in the regenerative depolarization when ECl was shifted away from 0 mV, where it had been in the experiments described above. With ECl set to -40 or -60 mV by reduction of [Cl-] in the pipette, steady-state membrane bistability was eliminated and prolonged depolarization did not occur. Under these conditions, application of the Cl channel blocker NFA showed that ICl(Ca) contributes to membrane hyperpolarization.(ABSTRACT TRUNCATED AT 400 WORDS)