The 'Drift Without Pronation' Sign in Conversion Disorder
This prospective controlled study indicates that the bedside observation of a "drift without pronation" is a useful and reliable clinical sign to discriminate between functional and organic upper limb weakness. This 'positive sign' was observed in all (100%) Conversion Disorder subjects and in only 7.1% of organic patients. When looking for 'positive signs' of functional deficits, clinicians are interested in having highly specific tests, minimizing false-positive results. Our study revealed a specificity of 93%, suggesting that this test is indeed helpful in clinical practice to identify a functional disorder.
Pronator drift is understood as a sign of upper motor neuron disorder and is considered as an indicator of a structural cerebral lesion in subjects with moderate paresis. Of note, this sign is usually considered positive when either a drift or a pronation is observed and only one study carefully looked at both aspects separately, finding that amongst 38 patients with a pronator drift sign, 74% had both a downward drift and a pronation, when 26% had an isolated pronation. Our findings suggest that when performing the stabilisation arm test, one should carefully look at both components of the pronator drift; the presence of a pronation will favour an organic cause, either in isolation or with a downward drift, whereas the observation of a drift without pronation will be highly suggestive of a functional paresis.
Our study has some limitations. The examiners where not blinded to the subjects' diagnoses, the interpretation of a 'positive sign' could have been biased. In order to minimize the influence of such personal subjective judgement, the evaluation was dichotomised (present or absent) and even a slight pronation movement was reported as present. The interobserver reliability of bedside neurological signs of hemiparesis, Barré sign (downward drift) and pronator drift with fingers adducted has however been reported as good (Kappa scores ranging from 0.55 to 0.77) so it can reasonably be expected that our assessments were reliable, even though only a blinded design including an independent rater could confirm it.
As no gold standard to diagnose functional weakness exist, there is a potential risk for circular reasoning bias: if the studied sign is also used in the diagnosis process, the reported specificity and sensitivity are overestimated. We tried to minimize this bias by strictly using the DSM-IV criteria to establish the diagnosis of conversion disorder without specifically using the "drift without pronation". Moreover, the diagnosis and the testing were not performed by the same doctor.
Discussion
This prospective controlled study indicates that the bedside observation of a "drift without pronation" is a useful and reliable clinical sign to discriminate between functional and organic upper limb weakness. This 'positive sign' was observed in all (100%) Conversion Disorder subjects and in only 7.1% of organic patients. When looking for 'positive signs' of functional deficits, clinicians are interested in having highly specific tests, minimizing false-positive results. Our study revealed a specificity of 93%, suggesting that this test is indeed helpful in clinical practice to identify a functional disorder.
Pronator drift is understood as a sign of upper motor neuron disorder and is considered as an indicator of a structural cerebral lesion in subjects with moderate paresis. Of note, this sign is usually considered positive when either a drift or a pronation is observed and only one study carefully looked at both aspects separately, finding that amongst 38 patients with a pronator drift sign, 74% had both a downward drift and a pronation, when 26% had an isolated pronation. Our findings suggest that when performing the stabilisation arm test, one should carefully look at both components of the pronator drift; the presence of a pronation will favour an organic cause, either in isolation or with a downward drift, whereas the observation of a drift without pronation will be highly suggestive of a functional paresis.
Our study has some limitations. The examiners where not blinded to the subjects' diagnoses, the interpretation of a 'positive sign' could have been biased. In order to minimize the influence of such personal subjective judgement, the evaluation was dichotomised (present or absent) and even a slight pronation movement was reported as present. The interobserver reliability of bedside neurological signs of hemiparesis, Barré sign (downward drift) and pronator drift with fingers adducted has however been reported as good (Kappa scores ranging from 0.55 to 0.77) so it can reasonably be expected that our assessments were reliable, even though only a blinded design including an independent rater could confirm it.
As no gold standard to diagnose functional weakness exist, there is a potential risk for circular reasoning bias: if the studied sign is also used in the diagnosis process, the reported specificity and sensitivity are overestimated. We tried to minimize this bias by strictly using the DSM-IV criteria to establish the diagnosis of conversion disorder without specifically using the "drift without pronation". Moreover, the diagnosis and the testing were not performed by the same doctor.
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