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Keywords:

  • ectasia;
  • intraocular pressure;
  • keratectasias;
  • keratoconus

Abstract

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. REFERENCES

Background:  The aim was to describe the baropathic nature of the keratectasias and to examine the possible significance of intraocular pressure-elevating activities in the development and/or progression of these conditions.

Methods:  Articles were selected from 150 produced from a PubMed search for keratectasias and used to elucidate the biomechanics and dependence on intraocular pressure of those conditions.

Results:  The combination of viscoelastic and baropathic features of keratectasia indicate that elevations in intraocular pressure have the potential to contribute to the development and/or progression of these conditions. Acute events such as hydrops and perforation appear to be more or less dependent on IOP elevation.

Conclusion:  Development and/or progression of keratectasia might be slowed by patient counselling, which explains the dependence of keratectasia on intraocular pressure and recommends avoidance and/or moderation of activities that elevate intraocular pressure. Successful adoption of such advice could reduce rates of disease progression and the need for refitting with more expensive contact lens designs, which are often required for advanced cases, as well as reduce the need for grafting and the drain on limited availability of donor corneas.

Corneal shape appears to be determined by an interplay between intraocular pressure (IOP), the elastic properties of the corneal tissue and the amount and central-peripheral distribution of the corneal tissue mass.1 Keratectasia is most simply described as refractive instability associated with a corresponding progressive structural deformation of the corneal shape.2 The pathologicalIy abnormal central-peripheral distribution of corneal mass due to localised tissue loss is often a key feature of this condition. In keratoconus, for example, an area of the cornea thins and therefore becomes weakened and unable to support the IOP, the effect of which causes the cornea to bulge forward as a conical protrusion.3 Cone formation in a thin cornea is a response to IOP in keratoconus.4 Keratectasia is perhaps the most overtly biomechanical disease in all of ophthalmology and it continues to be poorly characterised in terms of its basic biomechanical behaviour.2 Corneal topography findings were studied before and after elevation of IOP to an average level of approximately double baseline (using an ophthalmodynamometer to applanate the inferior sclera through the lower lid).5 The baropathic nature of keratoconus was indicated in that study by the finding of an average response of 1.84 D of ‘simulated keratometric’ steepening in keratoconus subjects.5 Under similar conditions of approximately double baseline IOP, normal control corneas did not show any significant change.5,6 Keratoconic subjects less than 31 years of age showed significantly more steepening under these conditions than those over 31 years.5 Normal corneas stiffen with age7 and the related stiffening might explain the age-related responses to elevated IOP found in keratoconus.5

METHODS

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. REFERENCES

Articles were selected from the 150 found using a PubMed search for keratectasia and used to elucidate the biomechanics of the keratectasias including their baropathic nature.

RESULTS

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. REFERENCES

Corneal responses to elevated IOP are not independent of scleral responses. For example, compliant scleral tissue, which readily expands with IOP elevation, tends to reduce any responses from the cornea. Nevertheless, corneal responses to IOP distending stress are predominantly a function of corneal characteristics such as the elastic modulus. The corneal responses to elevated IOP vary inversely with the elastic modulus of the tissue,1 while corneal tissue compliance (tendency to yield to elevated IOP) also varies inversely with corneal thickness.1 In addition, bending of the cornea is facilitated by a reduction in the shear modulus2 and changes to the ‘glue’ function of the extracellular matrix might facilitate bending responses to distending stress.8

Another possible determinant of tissue compliance is the shape and/or length of the tissue element. For example, compliance increases (stiffness or rigidity reduces) with the length of a tissue element1 and large diameter corneas or, perhaps more likely, large diameter cones may be more likely to yield to IOP stress. As is the case for other living tissues,9 ocular tissues are constantly under stress and tissue remodelling may take place when homeostatic conditions of stable stress are disturbed.9 To explain progressive changes in the optic disc in glaucoma, one plausible hypothesis is that acute IOP spikes or transient elevations that are not detected during a routine clinical examination lead to disease progression.10 A similar hypothesis may be relevant for the development and/or progression of keratectasia when a region of the cornea thins and/or otherwise weakens and becomes more susceptible to the distending stress of episodes of elevated IOP.

IOP-elevating activities

IOP varies with a wide range of activities11 with the potential degree of elevation12–18 shown in Table 1. Other activities that might contribute to unsuspected elevation of IOP include wiping a watery eye, lid contact with a pillow during sleep, squinting in response to blur or glare and associated compressive lid forces due to increased tonus in the orbicularis muscle (CW McMonnies and B Russell, unpublished review). Like rubbing, these mechanisms have the potential to involve indention of the cornea and/or sclera, displace intraocular fluid and elevate IOP. Measurements of IOP during these activities do not appear to have been determined. Apart from the degree of elevation, the possible pathological significance of increased IOP, which occurs with such activities, might vary with the duration of each episode of elevation, the frequency of such episodes and the length of time over which such activities occur.

Table 1. Indication of intraocular pressure (IOP) levels that have been recorded during some activities known to result in IOP elevation
Eye rubbing: elevations up to 400 mmHg.13
Long duration prone sleeping: mean elevation of 40 mmHg.14
Short duration (30 minutes) supine positions: mean elevation of 4.4 mmHg.15
Inverted body position: mean elevation of 36 mmHg.16
Playing loud, high pitch notes on a trumpet: elevation up to 44 mmHg.17
Wearing swimming goggles: elevation up to 48 mmHg.18
Strenuous muscular effort (Valsalva response?): elevation up to 30 mmHg.19

While activities like playing a high wind resistance instrument such as a trumpet and assuming an inverted body position such as yoga exercises involving head stands could be classed as unusual activities, supine and prone sleeping are regularly occurring events, which are known to elevate IOP, as is eye rubbing for some people, especially those with keratoconus.8

Twenty-four experienced cataract surgeons were instructed to apply ‘typical’ pre-operative digital massage on an experimental phantom eye.12 A pressure transducer was used to monitor the IOP continuously.12 The peak pressures ranged up to 400 mmHg, with half the surgeons inducing peak IOP in excess of 100 mmHg.12 Knuckle rubbing by patients with keratoconus has been shown to involve forces that are up to 10 times greater than ‘normal’ rubbing in non-keratoconus patients.19 Compared with the force used during massage by a surgeon, such forceful knuckle rubbing might involve IOP elevation greater than 400 mmHg, with associated very high levels of distending stress on the cornea. Apart from elevated IOP, rubbing may be otherwise traumatic to the cornea with wound healing responses resulting in corneal changes that lower the mechanical strength (stiffness or rigidity) of the tissue and increase its susceptibility to elevated IOP.8

Keratectasia is a viscoelastic phenomenon

By virtue of its definition that it progresses over time, keratectasia is a viscoelastic event.2 Cone progression in keratoconus is not continuous and routine monitoringof patients with keratoconus reveals periods of stability separated by periods of progression, which frequently necessitate steepening of a contact lens fitting to accommodate increased cone curvature.20 Phased cone progression raises the possibility that such changes could correspond to periods of increased exposure to activities that elevate IOP. For example, in some cases the combination of atopy, keratoconus, spring season, increased ocular itch and increased eye rubbing could be coincident with periods of cone progression.

The changes in keratoconic corneas in response to approximately 15 seconds of exposure to elevated IOP are temporary5 and no significant variations from baseline values are evident within minutes of a return to normal IOP. Apparently, the steepening in curvature is predominantly an elastic response with an associated rapid recovery. Elastic steepening is related to the elastic modulus of the tissue and an elastic forward protuberance of the cornea must be distinguished from ectasia,21 which is a viscoelastic event; however, viscoelastic responses could include a gradual reduction in stress under constant strain (stress relaxation).22 Stress relaxation may be more evident when a cornea is thinned or otherwise weakened by genetic, wound healing or other mechanisms. Viscoelastic responses could also include episodes of gradual increase in strain under increased stress (creep).2 Such episodes of creep, which show full recovery over time, might not contribute to disease (cone) progression; however, creep in response to periods of exposure to elevated IOP might be associated with incomplete recovery and progression of ectasia. Hysteresis, in viscoelastic tissues such as the cornea, is the work or energy lost in the gradual recovery from deformation.23 Delayed recovery might increase susceptibility to a subsequent stress. For example, cone progression might be more likely if an episode of exposure to IOP elevation occurs before complete recovery from a previous episode has taken place. Thus, cone progression might be more likely to occur if a second episode of an IOP-elevating activity follows soon after a previous episode. Similarly, time-dependent viscoelastic responses and cone progression might occur more easily if the distending stress associated with IOP elevation is sustained in a single episode over several hours. Supine and especially prone sleeping positions could provide these conditions; however, rubbing in keratoconus typically occurs in prolonged episodes of up to five minutes,19 during which IOP appears likely to be elevated to much higher levels than during sleep (Table 1). It is not clear which condition, namely, rubbing or sleep, could have the greater pathological significance. Plastic responses such as yield and failure occur when a permanent strain is incurred and the tissue does not recover its original configuration on unloading.24 In the case of the cornea, unloading does not occur, because episodes of elevated IOP are followed by periods of normal IOP and normal distending stress.

IOP elevation and plastic responses such as hydrops and perforation

Rubbing or contact lens trauma have a role in hydrops but chiefly the weaker Desçemet's membrane in keratoconus responds to IOP distension.4 That hydrops is usually associated with rubbing25 is consistent with trauma due to the associated acute IOP elevation. Hydrops is more common in atopy and Down Syndrome children4 and both are associated with a higher prevalence of both keratoconus and abnormal rubbing habits.26 Similarly, corneal perforation in hydrops might be associated with even a single incident of rubbing trauma;25 however, case reports of hydrops and perforation might be associated with a denial of a history of eye rubbing.25,27 It is possible that a patient's denial of a history of rubbing in such cases could be based on guilt feelings of self harm. Alternatively, undisclosed rubbing might have occurred during sleep or without awareness. Spontaneous hydrops (or perforation) might be more likely to occur in advanced cases of corneal thinning and reduced tissue strength; however, it is possible that cases classified as spontaneous might have been precipitated by unsuspected IOP-elevating activities other than unconscious or undisclosed rubbing.

DISCUSSION

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. REFERENCES

Apart from keratoconus, the development and/or progression of other forms of corneal thinning disease such a Terrien's marginal dystrophy, pellucid marginal degeneration, keratoglobus and ectasia, which develops following laser-assisted in situ keratomileusis, might be similarly dependent to some degree of elevation of IOP. For example, the baropathic nature of ectasia that develops following laser in situ keratomileusis is indicated by the finding that such corneas can return to normal topography with treatment to lower the IOP.28,29 In cases in which the susceptibility of the cornea to shape change is high due to thinning and/or other forms of reduced stiffness or rigidity, the distending stress of the baseline/normal range of IOP might be a sufficient stimulus to the development and/or progression of keratectasia. In this sense, in both normal tension keratoconus and normal tension glaucoma, the corresponding tissue (cornea or optic nerve head) appears to respond adversely because it is susceptible to the distending forces of normal IOP. Keratoconus of this type could be called normal tension keratoconus, except that complete lack of exposure to at least some activities that elevate IOP (for example, supine and/or prone sleep positions) does not appear to be possible.

Longer duration, frequent episodes of above baseline/normal IOP appear more likely to contribute to shape changes, perhaps even in corneas with only marginal susceptibility to IOP distending stress. The risk might also increase with the level of IOP elevation. Consequently, given the baropathic nature of keratectasia, it is possible that avoidance or moderation of activities that elevate IOP to potentially pathological levels might contribute to the slowing of the development and/or progression of the condition. An IOP-lowering effect of a raised head position during sleep instead of a supine flat position30 or avoidance of or reduction in prone sleeping could be an advantage. Patients who understand the relationship between particular activities that elevate IOP and the mechanics of keratectasia might be more compliant with avoidance recommendations. Notwithstanding such understanding, the ability to follow advice to avoid eye rubbing might be lacking or only partially successful.31 Clinical trials intended to examine the relationships between activities that elevate IOP, such as rubbing and cone progression, by comparing changes in an intervention sample with those in controls could be fraught with problems. Matching the two samples for level of habitual rubbing habits would be difficult. There would be similar difficulty associated with assessing compliance with rubbing avoidance. In addition, compliance with treatment prescribed to reduce itch provocation for rubbing cannot be assured. Nevertheless, clinicians might prefer to counsel patients regarding the potential harm associated with activities that elevate IOP. Application of suggested changes to the behavioural patterns of patients might be considered if lifestyle alterations are not harmful, even if the benefit is not confirmed by stringent tests of study design and statistical significance.32 An understanding of the baropathic nature of keratoconus and common sources of elevated pressure might help to motivate patients to avoid or modify the relevant activities. Any delay in the progression of keratoconus subsequent to the suggested interventions might reduce the frequency of need to refit contact lenses. Refitting expenses often increase with the need for more sophisticated lens designs required for more advanced cases. Advanced keratoconus is a common indication for penetrating keratoplasty33 and other forms of corneal graft.34 Any delay in the progression of keratoconus subsequent to the interventions suggested might reduce the number of eyes that progress to a corneal graft with a concomitant reduction in the drain on the limited availability of donor corneas.

REFERENCES

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. REFERENCES