Definition:
This type of pathology pertaining to the family
of ocular surface diseases is characterized by sensitivity abnormalities / loss
of corneal sensation. It is often associated with a reduced metabolic activity
and vitality of the epithelium leading to inadequate healing (such as the loss
of transparency / neovascularisation of the cornea, which is usually exempt of
blood vessels thus ensuring its transparency) due to the smallest wound,
aggression or even lack thereof.
The loss of sensation can be total (anaesthetic
cornea) or partial (hypaesthesic
cornea). Sensation can be altered at several levels and vary according to
the different types of sensations (cold, hot, pain, reaction to eye drops,
etc).
Neurotrophic Keratitis was described at the
beginning of the XIXth century (Magendie, 1824). It usually begins by a
recurrent epithelial erosion, after a slight trauma or spontaneously, and then
evolves into more deep and serious erosions. These usually appear in the central
(or close to the centre of the) cornea without any major inflammatory symptoms
and limited pain (at least less than expected according to the doctors!).
Neurotrophic
Keratopathy is characterized by recurrent superficial corneal (epithelial)
erosions, often associated with a lachrymal deficit similar to dry eyes.
Diagnostic:
Beyond the ocular sequels more easily
observed, a simple test with a cotton swab or an esthesiometer (or eventually
the instillation of a pungent substance – irritating eye drops) can be very
useful to facilitate the diagnostic. Determining the medical history is also
crucial as any accident or surgery affecting the trigeminal nerve, some
degenerative disease, diabetes, or other neurological sequels could explain the
impairment of the ophthalmic branch of the trigeminal nerve (5th facial nerve).
Causes:
Different local (ocular) or systemic ocular
diseases may provoke a loss in sensitivity of the
cornea:
-
neurological sequels of the
trigeminal nerve (accident or surgery to relieve the patient from trigeminal
facial neuralgias rhizothomy or
even the removal of an acoustic neuroma)
-
diabetes mellitus,
-
chemical burns,
-
multiple sclerosis,
-
pharmaceutical causes (including the abuse of local
anaesthetics, anti-inflammatory medication, etc),
-
ocular viral infections such as herpes zoster or simplex,
-
corneal graft (keratoplasty) or other corneal surgery,
-
refractive surgery (LASIK), etc,
-
abuse of contact lenses,
-
Sjögren's syndrome...
Less frequently, it can occur in the context of a
hereditary or congenital neuropathic disease (such as familiar dysautonomia).
Neurological Context:
The cornea has the highest density of nerves
among all human tissues and the highest sensitivity. However, the innervation
(nerve) “network” is not well known. One of the main reasons for that is: almost
every cornea from a cadaver is targeted for corneal transplantation (and these
are certainly not enough to satisfy the needs) so very few are destined to be
researched. With the new confocal microscopes this crucial aspect of corneal
health is more likely to receive the attention it requires.
The cornea is under constant exterior aggressions
(wind, dust, lid friction, etc). Hence, the anaesthetic cornea is unlikely to
defend itself in an appropriate manner. But the simple fact that a person has
loss sensation or immediate pain perception is not enough to explain it all. The
integrity of the cornea is maintained through a complex system of regeneration
(even though the mechanism is not yet fully understood) of its tissues based on
corneal sensitivity. Therefore, appropriate sensation is crucial to heal or to
regenerate the corneal epithelium.
It is now known that the loss of sensitivity is
related to the diminution of stem cell proliferation (from the limbus), that it
delays the whole metabolic activity including corneal cell mitosis (division)
and reduces the levels of acetylcholine and other growth factors naturally
present in tears... to mention a few examples.
Symptoms,
neurological and visual consequences:
The main consequences are: frequent rupture of
the inadequate epithelium, the delay or absence in wound healing. The loss of
sensation is frequently accompanied by a reduction of the lachrymal function or
a reduced blinking of the lids. Hence, dryness further aggravates the condition.
In visual terms, loss of sensitivity may lead to
a completely opaque (non-transparent) cornea due to the frequent scars (left by
each successive erosion) or to a deep neurotrophic ulcer or even perforation of
the cornea. The latter may lead to blindness while the first is likely to cause
additional irregular astigmatism.
The main symptoms are : neurotrophic ulcers (non
inflammatory), frequent erosions, dry eyes, extreme photophobia, corneal
neovascularisation and loss of transparency.
Treatments
The usual treatments consist of aggressive
lubrication (as for severe
dry eyes), tarsorrhaphy (the eye is totally or
partially covered by the lids), but these treatments are often unsatisfactory.
In some cases, after a deep ulceration, it is necessary to perform a corneal
graft (keratoplasty). This is often done to save the eye but it cannot achieve
the recovery of a functional vision. Keratoplasty is always a great risk for the
patients suffering from neurotrophic keratitis.
Other Advices
Most advice is similar to that prescribed for
severe dry eyes both in terms of reducing the aggressions and in terms of
lubrication (lachrymal substitutes). It is very important to avoid eye drops
with preservatives because preservatives are abrasive.
These corneas are likely to benefit from the research carried out which aims to
reduce the consequences of dry eyes, and will benefit from
new medication and preservative-free eye drops.
New treatments
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Keratos 2005-2007
