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How far do you investigate the isolated Horner's syndrome? The sympathetic pathway to the pupil is long originating in the hypothalamus from where the central neuron descends to the intermediolateral column of the upper three dorsal spinal segments. The preganglionic neuron then ascends to the superior cervical ganglion and the postganglionic neuron travels from there via the internal carotid artery to the eye. There is little evidence of decussation and thus Horner's syndrome has long been known as a good lateralising but a poor localising sign.
Individual case reports relate isolated Horner's syndrome to various diseases including malignancy at the lung apex, dissection, arteritis or trauma of the internal carotid artery, jugular venous ectasia, and migraine or cluster headache.1 The paper by Kerrisonet al (this issue, pp131–132)2 adds syringomyelia to this list of possible causes of isolated Horner's syndrome and recommends that any such patient in which the lesion is thought to be of the central or preganglionic neuron should be investigated by MRI of the brain and cervical cord.
The available literature does not make it easy to develop a rational approach to the investigation of isolated Horner's syndrome. The incidence of central Horner's syndrome varies from 2.5% in ophthalmological reviews to 63% in the neurological literature. Preganglionic Horner's syndrome represented 21% of a neurological series and 67% of an ophthalmological series.3 Most series agree that vascular disease is the usual cause of central Horner's syndrome and that trauma or tumour explains most preganglionic lesions. Postganglionic lesions occur in between 0.5% and 57% of patients reported and arise from various pathologies including vascular disease and tumour.4
The localisation of the lesion by clinical and pharmacological means is important and influences the need to investigate. To neurologists clinical localisation is a familiar task, pharmacological localisation less so. In practice most cases of Horner's syndrome are not truly isolated and the association of neighbourhood symptoms or physical signs and the extent of any sweating impairment will enable the lesion to be placed with some precision. When this is not possible the use of cocaine and pholedrine eye drops5 can be helpful in determining the direction of investigation, the depth and pace of which is aimed at the rapid diagnosis of conditions in which intervention may alter the natural history. Thus diagnosing an apical bronchial carcinoma or dissection of the carotid artery is of potentially greater immediate importance than the diagnosis of syringomyelia.
Clinical assessment, pharmacological localisation, and imaging enable the full assessment of cases of isolated Horner's syndrome. How far and how fast we should undertake this process will be influenced by the clinical presentation and the local availability of investigative facilities.