Letter: Congenitally Fused Cervical Spine is Associated With Adjacent-Level Degeneration in the Absence of Cervical Spine Surgery.

Abstract

To the Editor: I read the abovementioned article with interest.1 The authors identified 96 patients having “congenital” fusion at various levels of cervical spine and study the presence of adjacent-level degeneration. Our several related studies identify that fusion of spinal segments might not always be a result of primary error of embryogenetic process or related to error in segmentation.2-4 Our recent editorial on the subject observed that evidence of bone formation or bone fusion is an evidence of spinal instability that might be in the adjacent spinal segments or even remote to the level of spinal fusion.3 Instability in such cases is always longstanding. We observed that spinal segmental fusion is “always” secondary, has a naturally adaptive or protective role, and by itself is an indicator of spinal instability. Standing human posture entails life-long stress on the extensor muscles of the spine. The activity of large bulk of spinal muscles that facilitate standing position is focused on the facets. Weakness of these muscles related to disuse, misuse, or injury leads to “vertical” spinal instability in the subaxial spine and in atlantoaxial instability in the craniovertebral junction.5 In chronic situations, atlantoaxial instability can be of central or axial variety (CAAD).6-9 Our articles discuss that it is not the disk space reduction or loss of disk substance, but it is vertical spinal instability that is the point of genesis of spinal spondylotic disease.10-13 Both vertical spinal instability and CAAD might not be diagnosed on dynamic imaging with validated radiological parameters because of the lateral location of the facets, often subtleness of instability and frequently related to the absence of evidences of neural or dural compression.14 In our articles, we have mentioned that instability-related other secondary alterations such as reduction in the intervertebral disk space, osteophyte formation, bone fusions and buckling, or ossification of posterior longitudinal ligaments can be potentially or manifestly reversible after spinal instability is appropriately dealt with.10-13 The authors have not included Klippel-Feil syndrome in their study and have also excluded patients younger than 10 years.1 The essential element of Klippel-Feil syndrome is the presence of subaxial cervical spinal bone fusions of 2 or more levels. It is obvious to me (atleast from the figures provided) that atleast “some” of the patients in the authors study can certainly be included in the subset of Klippel-Feil syndrome.15 It would have been relevant if cases with assimilation of atlas were included. In the series, 47 patients had C2-3 fusion. Klippel-Feil alteration, assimilation of atlas, and C2-3 fusion when present discretely or in cohort are “strong” if not definite indications of the presence of atlantoaxial instability. The authors could have studied the neck length and neck deformity as short neck and torticollis can be frequent associations of bone fusion.16 The authors could have studied the spinal cord alterations in these cases. Atleast some of these (particularly in 47 patients having C2-3 fusion) must have associated Chiari formation with or without syringomyelia. Atlantoaxial stabilization in these cases can result in remarkable resolution of symptoms.9 Considering that most patients in the series were “elderly” (average age 51 years and maximum age 73 years), it is obvious that in atleast a number of these patients bone fusion is related to longstanding spinal degeneration. Our related articles mention that vertical spinal instability that leads to clinical and radiological manifestations of spinal degeneration is more often multisegmental rather than unisegmental.11-14 The presence of bone fusion as a manifestation of spinal degeneration by itself indicates the high possibility of the presence of adjacent segment/s spinal instability even in the absence of radiological evidences of neural compression or the presence of manifest instability on dynamic imaging. It is not unusual or strange that the authors identified evidences of disk degeneration in spinal segments adjoining the fused ones. In the Figure 1 shown by the authors, we are convinced that bone fusion in most patients were secondary to atlantoaxial instability and vertical spinal instability. If there were corresponding symptoms, most of these patients would have needed multisegmental spinal stabilization.11-14 Figures 1A, B, E and F have basilar invagination as per standard and described radiological parameters. In my view, labeling spinal segmental bone fusions as “congenital” takes away the opportunity to appropriately analyze or to rationally treat a patient. Obviously, when related symptoms are significant, surgery that involves spinal stabilization can be indicated and can be remarkably effective.