Volume Flow of Tears Into and From the Eye
The volume of tears in the external eye at any moment is a balance between the rate of inflow of fluid from the lacrimal gland, from the accessory lacrimal tissue and by permeation of water from the corneal epithelium through aquaporin-controlled channels. Removal of fluid is principally by drainage through the puncta following each blink, and by evaporation from the open eye. When the lids close, the upper and lower puncta press on each other and prevent outflow, but as the lids open there is a drop in canalicular pressure and fluid is sucked into the puncta from the marginal lacrimal lake. Evidence for absorption of water by the corneal or conjunctival epithelium is lacking, although it is suggested that some or all of that passing down the canaliculi is absorbed before it reaches the nose .
Absorbent discs Fluorometry Angle-dependent fringes Wave-front fringes
There is considerable variation in the rate of inflow of tears. It has often been suggested that in the quiet eye there is a 'basal rate' of flow, augmented by different degrees of stimulation; one variant is that the basal secretion is produced by the accessory lacrimal tissue (about 10% of the total) and stimulated reflex or psychic tears by the main lacrimal gland, but there appears to be no firm evidence for this. Another view is that all secretion is stimulated, that in the quiet eye being produced simply in response to opening of the eye. Most clinical estimates of tear flow rate are based on the Schirmer test and its vari¬ants; these are described below in 'Clinical Tests'. Published values of the 'unstimulated' flow rate are usually around 1.2 |xl/min or roughly 1.2 ml/day (assuming a 16-hour waking cycle, since tear output is largely inhibited during sleep), with a turnover rate of 16%/min . However, using the Fluorotron Master instrument, a much lower value was found of 0.15 |xl/min (about 0.15ml/day from each eye) with a turnover rate of 8.2%/min [13]. Stimulated flow rates are much greater - up to 50 or 100 times more; 40-50 |xl in <1 min has been reported with nasal stimulus by ammonia [14]. Since the myoepithe¬lial cells which surround the acini of the lacrimal gland contract in this process, it seems possible that some of the released tears are preformed and the actual secretory process may be somewhat slower than at first appears. It is not clear whether there is a 'maximum' rate of secretion; sustained rates are generally less than the 40-50 |xl/min already mentioned.
Regulation of Tear Production
The innervation of the lacrimal gland is complex. The reflex arc is particu¬larly important, involving fibres from the fifth cranial nerve in the cornea, conjunc¬tiva or surrounding tissues. There is also innervation by both the parasympathetic and the sympathetic systems, inducing positive and negative control of secretion respectively. The parasympathetic route indicates some of the complexity: starting from the lacrimatory nucleus in the brainstem of the facial nerve , parasympathetic fibres follow the greater superficial petrosal nerve to the pterygopalatine ganglion; the conventional view is that from there the secretory fibres of the lacrimal nerve follow the zygomatico-cotemporal nerve and join the lacrimal nerve of the ophthalmic division of cranial nerve V and enter the lacrimal gland. However, there is evidence that a number of rami orbitales pass from the pterygopalatine ganglion and some of these travel directly to the lacrimal gland .
The innervation of the accessory lacrimal tissue is even less well known, but it is assumed that it is controlled in the same way as the main lacrimal gland, as they are histologically very similar.
The volume of tears in the external eye at any moment is a balance between the rate of inflow of fluid from the lacrimal gland, from the accessory lacrimal tissue and by permeation of water from the corneal epithelium through aquaporin-controlled channels. Removal of fluid is principally by drainage through the puncta following each blink, and by evaporation from the open eye. When the lids close, the upper and lower puncta press on each other and prevent outflow, but as the lids open there is a drop in canalicular pressure and fluid is sucked into the puncta from the marginal lacrimal lake. Evidence for absorption of water by the corneal or conjunctival epithelium is lacking, although it is suggested that some or all of that passing down the canaliculi is absorbed before it reaches the nose .
Absorbent discs Fluorometry Angle-dependent fringes Wave-front fringes
There is considerable variation in the rate of inflow of tears. It has often been suggested that in the quiet eye there is a 'basal rate' of flow, augmented by different degrees of stimulation; one variant is that the basal secretion is produced by the accessory lacrimal tissue (about 10% of the total) and stimulated reflex or psychic tears by the main lacrimal gland, but there appears to be no firm evidence for this. Another view is that all secretion is stimulated, that in the quiet eye being produced simply in response to opening of the eye. Most clinical estimates of tear flow rate are based on the Schirmer test and its vari¬ants; these are described below in 'Clinical Tests'. Published values of the 'unstimulated' flow rate are usually around 1.2 |xl/min or roughly 1.2 ml/day (assuming a 16-hour waking cycle, since tear output is largely inhibited during sleep), with a turnover rate of 16%/min . However, using the Fluorotron Master instrument, a much lower value was found of 0.15 |xl/min (about 0.15ml/day from each eye) with a turnover rate of 8.2%/min [13]. Stimulated flow rates are much greater - up to 50 or 100 times more; 40-50 |xl in <1 min has been reported with nasal stimulus by ammonia [14]. Since the myoepithe¬lial cells which surround the acini of the lacrimal gland contract in this process, it seems possible that some of the released tears are preformed and the actual secretory process may be somewhat slower than at first appears. It is not clear whether there is a 'maximum' rate of secretion; sustained rates are generally less than the 40-50 |xl/min already mentioned.
Regulation of Tear Production
The innervation of the lacrimal gland is complex. The reflex arc is particu¬larly important, involving fibres from the fifth cranial nerve in the cornea, conjunc¬tiva or surrounding tissues. There is also innervation by both the parasympathetic and the sympathetic systems, inducing positive and negative control of secretion respectively. The parasympathetic route indicates some of the complexity: starting from the lacrimatory nucleus in the brainstem of the facial nerve , parasympathetic fibres follow the greater superficial petrosal nerve to the pterygopalatine ganglion; the conventional view is that from there the secretory fibres of the lacrimal nerve follow the zygomatico-cotemporal nerve and join the lacrimal nerve of the ophthalmic division of cranial nerve V and enter the lacrimal gland. However, there is evidence that a number of rami orbitales pass from the pterygopalatine ganglion and some of these travel directly to the lacrimal gland .
The innervation of the accessory lacrimal tissue is even less well known, but it is assumed that it is controlled in the same way as the main lacrimal gland, as they are histologically very similar.
Posted in: Ophthalmology Articles
0 comments:
Post a Comment