Neuromuscular Disorders in the Adult Spine

Pathophysiology

Alignment of the spine in the coronal and sagittal planes requires a complex balance of truncal and paraspinal muscle groups, including muscles that extend, flex, bend, and rotate the spine. Asymmetry or imbalance of muscle tone is an important cause of spinal malalignment or deformity. Spinal deformity as a result of muscle imbalance may include sagittal malalignments, including abnormal kyphosis or lordosis, or coronal and axial malalignments, including scoliosis. In children, alignment of the growing spine may be severely compromised by neuromuscular disorders, and neuromuscular disorders of the pediatric spine are reviewed in Chapter 54 . In the adult, de novo deformity is most commonly attributed to degenerative pathology, resulting in degenerative scoliosis or kyphosis. However, neurodegenerative conditions and primary myopathies are an important and often unrecognized cause of deformity in the adult spine. Identification and recognition of neuromuscular pathology in adults presenting with spinal deformity is important in guiding nonoperative and operative management decisions.

Spinal deformity is an important cause of disability in patients with neurodegenerative conditions and myopathies. Spinal deformity in the adult may be caused by a large number of neuromuscular disorders, including neurodegenerative conditions and primary myopathic disorders. Neurodegenerative disorders and movement disorders are both prevalent and significant in the aging population. In a population-based study of volunteers aged 50 to 89 years of age, the prevalence of parkinsonism was 7%, and the prevalence of primary and secondary dystonia was nearly 2%, with rates increasing significantly with age. Neuromuscular disorders in the child are recognized during prenatal evaluations, in the perinatal period, or in early childhood, with delay of developmental milestones and comorbidities. In the adult, neurodegenerative pathology may be present insidiously, which may delay a clinical diagnosis. A broad spectrum of neurodegenerative and primary myopathic conditions may have an impact on the adult spine. Neuromuscular deformity in the adult may be caused by primary neural conditions that lead to spasticity or flaccidity of the muscles affecting spinal alignment, or to primary muscle pathology. The cause of spinal deformity in adults with neu romuscular conditions remains incompletely understood, and may involve both central dystonia and myopathy. The occurrence of spinal deformity in conditions involving both spasticity and flaccid paralysis undermines a simple explanation of the generation of neuromuscular deformity. Factors that contribute to spinal deformity include asymmetric paraplegia, mechanical forces, intraspinal and congenital anomalies of the spine, sensory feedback, and the control of spinal balance by central pathways. Increased tone of the paraspinal muscles on one side of the spine with reduced tone or strength on the opposite side may generate a coronal or sagittal curvature of the spine over time. Recognition of the underlying neuromuscular disorder that leads to spinal deformity is important regarding prognosis and treatment options.

Neuromuscular conditions in the adult can be associated with significant and progressive deformity. An important characteristic of neuromuscular deformity in the adult is a rapid progression of an atypical, decompensated deformity. Degenerative scoliosis in the aging spine is characterized by primary thoracolumbar deformity, with compensatory thoracic curves. In contrast, deformity in patients with neuromuscular disorders characteristically presents with significant coronal or sagittal malalignment and decompensation, and these patients often have a long, sweeping major curve without a compensatory curve ( Fig. 55.1 ). Progression of deformity in the adult with degenerative scoliosis is variable but generally less rapid than deformity progression in the growing spine, and may average 1 to 3 degrees/year. Adults with neuromuscular deformity commonly present with a rapid onset of de novo deformity and a history of onset of deformity in late adulthood, with progression rates greater than 10 degrees/year. Patients with a rapid progression of an atypical, decompensated deformity should be evaluated for neuromuscular disease, including neurodegenerative disorders or primary myopathy.

Classification and Evaluation

Neurodegenerative diseases are progressive neuropathies that have characteristic loss of neurons (focally or globally), protein deposition, and altered physicochemical properties in the brain and in peripheral nerves. Neurodegenerative disorders present with progressive cognitive and motor dysfunction. Motor dysfunction may involve hyperkinetic and hypokinetic movement disorders, and symptoms are related to cerebellar dysfunction or involvement of the upper and/or lower motor neuron pathways. Focal neuronal degeneration involving the basal ganglia, thalamus, brainstem nuclei, cerebellar cortex and nuclei, motor cortical areas, and lower motor neurons of the spinal cord present with significant movement disorders. The diagnosis and clinical evaluation of a neurodegenerative condition in the etiology of deformity in the adult involves thorough history-taking and cognitive testing, physical examination, imaging, and nerve conduction studies. PD is an important focal neurodegenerative condition that is associated with spinal deformity.

Parkinson Disease

PD is the most common neurodegenerative disorder affecting elderly patients, and the burden of PD on the health care economy is growing rapidly. ^, Patients with PD characteristically present with difficulty with initiation of movement, shuffling gait, cogwheel spasticity on range of motion testing, and variable cognitive dysfunction. The clinical diagnosis of PD can be challenging because of the significant heterogeneity of symptoms, including motor findings. ^, Neuropsychological assessment and cognitive testing can be useful for the early detection of neurodegenerative conditions, including PD. The distinction between cognitive changes in normal aging and changes associated with neurodegenerative conditions is important for early diagnosis of neurodegenerative conditions.

Spinal deformities, including scoliosis and kyphosis, are an important cause of disability in patients with PD. Camptocormia (bent spine syndrome), Pisa syndrome, anterocollis, dropped head syndrome, and scoliosis are characteristic deformities in adult patients with neurodegenerative conditions, including PD. Adults with PD often present with rapid progression of standing imbalance, progressive deformity with standing, early fatigue, and improvement in the supine posture.

Up to one-third of patients with PD may have deformity of the limbs, neck, or trunk, and scoliosis is more prevalent in these patients (40%–90%) than in the general population (6%–30%). ^, Patients with PD have progressive postural imbalance and gait disturbance because of rigidity and bradykinesia, with spinal column changes that include increased flexion of the cervical spine, increased thoracic kyphosis, and decreased lumbar lordosis. For many patients, deformity of the spine is the most disabling disease manifestation of PD. The pathophysiology of spinal deformity in PD is multifactorial, with variations seen across the spectrum of deformities produced. Contributing factors may include: central mechanisms such as dystonia, proprioceptive deficits, or structural changes in the brain; peripheral mechanisms such as muscular rigidity, axial dystonia, muscle weakness, or structural changes of the spine; or drug-induced changes. Animal studies have supported central etiology from degeneration of the nigrostriatal pathway. Proprioceptive deficits or variations in spinopelvic balance could contribute by affecting the patient’s postural control, thus disturbing the patient’s sagittal balance. Patients with PD may have concurrent myopathy that may exacerbate spinal deformity and postural control, and there is significant evidence to support that spinal deformity in PD may be related to myopathy associated with neurodegeneration. Dopamine depletion has also been theorized to play a role in both Pisa syndrome and scoliosis, as studies have demonstrated that at early onset, up to 84% of those with PD and scoliosis lean contralateral to the side of symptoms. Further studies have noted that the concavity produced in both Pisa syndrome and scoliosis is more likely to be directed away from the side of predominant neurological dysfunction or early symptoms. A drug-induced cause has also historically been postulated to play a role in spinal deformity, as forward inclination of the body has been found to be associated with levodopa usage; however, evidence to support this has been weak.

Camptocormia (bent spine syndrome) is typically seen in patients with advanced PD, and has been associated with primary myopathy, a history of degenerative spine disease, or a history of spine surgery. ^, Antecollis involves forward flexion of the neck, and may be present in patients with prominent rigidity, leading to increased axial tone and reduced flexibility, and may be associated with the development of cervical myelopathy. The most common causes of dropped head syndrome are PD and primary myopathies, and an accurate diagnosis is important for appropriate nonoperative and operative management.