RSI or vibration? Proving causation in claims for Carpal Tunnel Syndrome
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There is a long established link between occupational activity and carpal tunnel syndrome.
Carpal tunnel syndrome has been a prescribed industrial disease since 1992 where the condition arises in association with the use of hand held vibrating tools and from 2006 where the work involves repeated flexion and extension of the wrist. A client who develops carpal tunnel syndrome in accordance with the prescription set down by the DWP is entitled to apply for Industrial Injuries Disablement Benefit and may qualify for a weekly state benefit depending on the level of disability.
Making a claim for carpal tunnel syndrome – 'the evidential burden'
When making a civil claim for personal injury a client or claimant must prove: that their work presented a foreseeable risk of injury; that their employer failed to take reasonable steps to avoid or reduce that risk (called 'negligence'); and that the work was causative of their carpal tunnel syndrome, on a balance of probability. The issue of causation is not always straightforward.
In addition to occupational activity there are a number of constitutional risk factors associated with carpal tunnel syndrome such as endocrine or hormonal disorders, fluid retention during pregnancy, rheumatoid arthritis and wrist fractures. Women are also more likely to develop the condition than men.
Despite this the condition presents with similar symptoms and signs irrespective of their origin. There is a standard set of diagnostic tests and clinical treatment.
Nerve conduction studies do not reveal significant differences between patients with a history of occupational exposure and those without. Where electro diagnostic testing is positive the usual outcome is surgery in the form of carpal tunnel release.
The lack of distinguishable clinical features to differentiate between a work related and idiopathic carpal tunnel syndrome creates uncertainty of attribution giving fertile ground for disagreement in the medico-legal arena.
How does a claimant seek to overcome this?
Where the condition has arisen due to compression of the median nerve occupation may be relevant where it involves manual work requiring repeated forceful gripping and deviation of the wrist (movement into flexion and/or extension). It is thought that the median nerve becomes compressed due to inflammation of the tendons inside the carpal tunnel.
In such cases expert evidence is often required from an ergonomist who is instructed to carry out an analysis of the work activity including the likely forces being applied, the wrist postures present and frequency of actions. Ergonomic expert evidence will not only assist the Court in determining the issue of negligence but also a medical expert's understanding of the risk factors present in the work when forming a view on causation.
Vibration induced CTS
The position is similar where the condition has arisen in association with the operation of hand held vibrating tools. Expert evidence is again required this time from a vibration expert who is instructed to assess and evaluate the claimant's likely exposure to hand transmitted vibration (this can often involve testing of equipment). Their report will assist the Court in determining the likely level of exposure; whether that exposure was unsafe and if the employer had complied with its duty to reduce the level of vibration to the lowest level reasonably practicable.
Causation in vibration induced carpal tunnel syndrome can be complex.
The mechanism by which vibration leads to carpal tunnel syndrome is not completely understood. It is thought that vibration induces structural change in the peripheral nerves including the median nerve in the form of demyelination and fibrosis. In a civil claim such changes are going to be extremely difficult to prove. You cannot take a biopsy from a client in order to try and prove your case! At best one can check the operation notes relating to the release surgery to ascertain whether the hand surgeon has commented on the state of the median nerve but this is unreliable and usually of limited value.
There are other signs and history which might help support a link to vibration.
It is thought that nerve damage is a reason for the mixed outcome of carpal tunnel release surgery for those with patients with a history of vibration exposure. Clients often report an improvement in symptoms whilst absent from work only for their symptoms to recur on their return. There are a number of possible explanations for this: the existence of scar tissue as a result of the surgery itself or peripheral nerve damage due to vibration (caused by exposure either before the operation and/or after the client has returned to work where the exposure has continued).
In addition to their neurological symptoms a client may also report episodes of blanching in the thumb or fingers on exposure to the cold indicating some vascular dysfunction consistent with secondary Raynaud's phenomenon or what is sometimes referred to as 'Vibration White Finger'.
In other cases the symptoms of tingling and numbness may not be confined to the median nerve distribution - the whole of the fourth and fifth fingers may also be affected. This may indicate the involvement of the ulnar nerve or a digital neuropathy associated with the sensorineural component of HAVS (Hand –Arm Vibration Syndrome).
HAVS comprises of three components: vascular, sensorineural and musculo-skeletal. A client may present with all three or may have the sensorineural and musculo-skeletal components only - which is becoming increasingly common as industry and processes have changed and vibrating tools become lighter.
It is not unusual for carpal tunnel syndrome and HAVS to co-exist.
Nerve conduction studies are useful for measurement of neuropathy proximal to the hand such as median or ulnar neuropathy at the wrist. However, they are limited in that they do not normally include measurement of the distal parts of the fingers which are initially affected by exposure to vibration and which are associated with HAVS.
There are quantitative sensory tests which can be undertaken in addition to nerve conduction studies which can measure the sensorineural abnormalities in the distal fingers. These tests can be conducted at certain referral sites (of which the University of Southampton is one). These tests include sensorineural tests called 'Vibrotactile Perception Threshold' (VPT) and 'Thermal (temperature) Perception Threshold' (TPT). There are a number of advantages in carrying out these quantitative sensory tests.
It is considered that testing the distal portions of the fingers are better predictors of the Stockholm Workshop sensorineural scale stages than nerve conduction findings across the wrists. The Stockholm Workshop scales are used to classify the severity of vascular and sensorineural symptoms of HAVS. This in turn assists the lawyer and the Court when seeking to quantify the value of a claim.
The tests are also important from an occupational health perspective when assessing an employee's fitness to work, evaluating changes in perception that relate to loss of function and monitoring progression of the disease.
The additional presence of these components of HAVS (vascular and sensorineural) in combination with a diagnosis of carpal tunnel syndrome serves to strengthen the case for a vibration induced syndrome.