Neural Mechanisms In Idiopathic Scoliosis Aetiology - Current
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Neural Mechanisms In Idiopathic Scoliosis Aetiology - Current Hypotheses Mr Michael Edgar 149 Harley Street, London, W1N 2DE, Great Britain REVIEWED MATERIAL Medline 1986 - 1996 SRS Annual Meeting abstracts 1982 - 1996 BSS abstracts 1980 - 1997 ESDS and ESS abstracts 1986 - 1996 JBJS articles 1980 - 1997 Spine volumes 1 - 22 Relevant editions Clin Qrth 1973 - 1997 Zorab Symposium proceedings 1970 - 1993 Pathogenesis of Idiopathic Scoliosis Ed Jacobs R R (1984), Scoliosis Research Society NEURAL MECHANISMS IN IDIOPATHIC AETIOLOGY - CURRENT HYPOTHESES A Neurological Dysfunction 1. Dorsal Column Dysfunction i) Vibratory impairment ii) Proprioceptive impairment 2. Spinal Cord Monitoring 3. Equilibrial Dysfunction 4. Oculo-Vestibular Dysfunction 5. Dysfunction of Higher Centres B Abnomal Neuroanatomy 1. Spinal nerve root lesions 2. Spinal cord abnormalities 3. Brain stem and mid brain lesions C Neuro-hormonal Abnormality NEUROLOGICAL DYSFUNCTION 1 Dorsal Column Dysfunction i) Vibratory impairment Abnormal vibratory sensation has been looked for in controlled clinical studies as the most sensitive test of dorsal column function. 16,62,77. The PVD Bio-Thesiometer has been standard. Early findings showed a hypersensitivity to vibration compared with controls. The vibratory hypersensitivity can affect all 4 limbs but is not related to the magnitude of the curve18. No asymmetry of findings related to the side of the scoliosis was apparent. A more recent study suggests hyposensitivity confined to the great toe60. The key reference Maclnnes et al, 1991,39 publishes a thorough investigation in which the Bio-thesiometer is shown to be unreliable with considerable variation between patients without statistically provable vibratory dysfunction. ii) Proprioceptive impairment Again the literature on controlled clinical studies is split. Findings vary between no abnormality 23,32 and significant pathology affecting both upper and lower limbs 7,74,85. Tests include finger and elbow position matching 32,74 and knee angle reproducibility (7). The key reference is Yekutiel et al, 1981,85. In this study only the Sharpentier test, which involves comparisons of weights of different shaped objects held in each hand was significantly abnormal in MS patients compared to controls. Interestingly the blind-folded Sharpentier test revealed no difference. Impaired proprioception is not a constant feature. 2 Spinal Cord Monitoring Long tracts in the spinal cord have been monitored with SSEPs, MEPs and EMG studies. Only 2 papers consider aetiology. Machida et al, 1991,40 after experimental work in chickens performed cortical SSEPs in MS patients which compared with controls showed a right I left latency difference. Pathology in the thalamocortical pathways was proposed. MacGuire et all 37,38 using sophisticated EMG techniques has demonstrated long latency reflex activity in MS patients not present in controls or other scoliosis. The descending tracts from higher centres which control this were not investigated. 3 Equilibrial Dysfunction This concept was put forward by Yamada, Iakta and Yamamoto et al, 1969,80. Sahlstrand, 1977,64 describes the technique of stabilometry to assess sway pattern. The concept has been further developed 29,54,84 with confirmation of abnormal sway patterns using stabilometry 19,61,62, verticality perception and single limb standing 16,79. There is no difference between progressive and non-progressive curves 62,79. Postural sway reverts to normal at skeletal maturity 79,81. Lidstrom, 1989,34 considers the effect largely secondary supported by Gregoric et al, 1989,26. DeMuth et al, 1996,16 found considerable variability and suggests neurological heterogeneity in MS. 4 Oculo-vestibular Dysfunction Ocular and vestibular impulses are important equilibrial factors affecting the equilibrium centre in the brain stem 80. Vestibular dysfunction in MS was reported by Sahlstrand, 1977, 64. Abnormal nystagmus measured by electronystromography (ENG) using caloric tests or post-rotational state has been demonstrated 28,50,64,76,79. Abnormal nystagmus can herald the beginning of scoliosis among siblings of MS patients 51. A recent study found no difference between AIS patients and controls with post-rotatory nystagmus. The ocular influence in sway patterns using stabilometry (see section 3 above) has been investigated by Sahlstrand 65 and Herman 28,29 with opposite results. Sahlstrand found that lateral sway pattern was worse without visual input suggesting vestibular insufficiency. Herman found the lateral sway pattern more marked when visual information was available proposing an abnormal vestibulo-ocular reflex. This was found to be asymmetrical in MS patients and not present in congenital scoliosis. Further influence of the visual and somatosensory systems in producing abnormal sway has been recently produced by Byl and Gray 13. Congenital visual and inner ear defects produce a lower incidence of MS than expected 50,76. 6 Dysfunction of Higher Centres Herman, 1984,29 hypothesizes that vestibulo-ocular reflex asymmetry results from asymmetric maturation of higher cortical centres. No further work has been done. EEG studies have demonstrated some variation in MS patients compared with controls 55,62. Asymmetry was not related to the curve and there was no difference between progressive and non-progressive scoliosis. Goldberg and Dowling, 1988, 18 demonstrated that left or right handedness correlated statistically with the side of the MS convexity. An asymmetry of higher cortical function was proposed though the place of genetic determination was not considered. B ABNORMAL NEUROANATOMY 1 Spinal Nerve Root Lesions Experimental intradural posterior rhizotomy over at least 3 levels causes scoliosis, with the convexity ipsilateral to the lesion in the immature rabbit36,69,70, dogs35 and in monkeys57. There is controversy with rabbits that experimental scoliosis only develops when ventral horn cells are involved1,52,70. The only human anatomy study concerns the innervation of spinal ligaments in MS patients which although not asymmetrical is deficient in nerve density compared with controls4. This is probably a secondary feature. 2 Spinal Cord Abnormalities Pincott and Taffs, 1982, 58 classic work demonstrated unilateral destruction of posterior horn grey matter in association with the development of scoliosis. The asymmetrical destruction of Clarke's column and associated spinocerebellar tracks was the most consistent pathology with the convexity of the curve on that side. Curves are long and have a neuropathic rather than idiopathic appearance. Similar findings occur in the rabbit10. Children with juvenile or early adolescent curves have an incidence of cervicothoracic syrinx and Chiari I malformations occurring from 17.9% to 47% (see refs 3,6,12,14,15,17,22,27, 30,31,33,59,67,71,73,75,86). A controlled study 22 demonstrated that those curves are similar to 'normal' idiopathic deformities except they are more progressive and have a higher proportion of left-sided curves 22,31,44,67. Controversy exists between those who have found normal neurology 14,18,22,30,31,33,59,73 and those who state that subtle neurology involving asymmetrical abdominal reflexes or nystagmus is always present if carefully looked for 15,71,86. Abdominal reflex abnormality can be present without abnormal neuroanatomy 72. Drainage of the syrinx or decompression of the forama magnum can lead to improvement of the scoliosis as well as improvement of any neurology present 21,25,53,56. The debate is whether the syrinx leads to subclinical neurology and causes a "neuropathic" curve or whether the syrinx and scoliosis are both associated with a primary cause as yet undetected. 3 Stem and Mid Brain Lesions Stereotactic minute brain stem lesions in growing rats produced progressive scoliosis in 10% and this was associated with quite marked ataxia. Further work in rabbits with a unilateral lesion in the periaqueductal grey matter was carried out 82. Accurate asymmetrical lesions in the gracilis nucleus, the superior colleculus and the lateral vestibular nucleus was produced in rabbits11 and 25% developed scoliosis. Hypophysectomy with its inevitable damage to the roof of the third ventricle produces abnormal cortical evoked SSEPs 41,42. The hypothesis given by Machida, Dubousset, Yamada and colleagues is a destruction of sensory (thalamo-cortical) fibres rostral to the brain stem. MRI documentation of the mid and hind brain in MS subjects has demonstrated pathological asymmetry. Geissele et al, 1991,24 demonstrated asymmetry of the ventral portion of the pons and medulla oblongata conveying the corticospinal tracts. This occurred in 27% of MS patients but only 9% of controls. Asymmetry did not relate to the side of curve. Stevens et al, 1992,68 found a more common asymmetry of the cerebral peduncles with the larger side related to the convexity of the curve. This work has not been substantiated. C NEURO-HORMONAL ABNORMALITY Experimental pinealectomy in chickens is associated with abnormal cortical SSEPs 41,42,43. It is not known whether this impairment of conduction is due to damage of neurological tracts (thalamo-cortical) or grey matter in the roof of the third ventricle or whether it is due to an upset of the hormonal neurotransmitters as a result of the pinealectomy. Dubousset et al, 1982,83 demonstrated that pinealectomy does cause some neurological damage in this key area of the diencephalon. It is suggested that this could be the site for the equilibrium control centre. Re- implantation of the pineal gland or the administration of melatonin in chickens prevents the scoliosis 44,45,46,47,48. This would favour a neurotransmitter cause of neurological impairment. Post-pinealectomy scoliosis does not develop in normal quadriped mammals (rats, hamsters) 5 but young bipedal rats are vulnerable49. D COMMENT Impairment of the dorsal columns involving vibrationary and proprioceptive impulses is not a factor in idiopathic aetiology. Similarly experimental sensory deficiencies in immature animals produce neuropathic rather than idiopathic curves. Concept of equilibrial dysfunction related perhaps to vestibulo-ocular reflex asymmetry, as suggested by Herman and supported by the visual findings of Byl and Gray and consistent with the visual influence of co-ordination and proprioceptive noted by Yekutiel et al during the Sharpentier tests, is the most attractive area of neurological dysfunction. Further work is clearly needed. Asymmetrical maturation of higher cortical control of equilibrium fitting in with Goldberg and Dowling's work requires further work and confirmation. If abnormal neuro-anatomy exists it should come to light with better resolution of MR techniques which could demonstrate asymmetry of spinal tracts and grey matter in the brain stem and mid brain. Chiari I malformation and cervicothoracic syrinx in juvenile and early adolescent idiopathic scoliosis needs further investigation particularly with regard to its neuro-anatomy and neuro- physiology. This is the nearest syndrome to idiopathic scoliosis without a known neuropathic cause and may give the clue to the aetiological process. REFERENCES (key references in heavy type) Agadir M et al (1988). Induction of scoliosis in the growing rabbit. Spine, 13: 1065-9. Alexander M A, Bunch W H &Ebbesson S 0 E (1972). Can experimental dorsal rhizotomy produce scoliosis ? JBJS 54a:1509-13. Aral S et al (1992). Scoliosis associated with syringomyelia. Abstract of the 27th SRS Annual Meeting, Kansas City, 1992, p139. Bagnall K M et al (1997). Comparison of the innervation characteristics of the lateral spinal ligaments between normal subjects and patients with MS. Abstract of British Scoliosis Society meeting, Stratford 1997. Bagnall K M et al (1997). 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