While the effects of thoracolumbar deformity on disability have been well studied, ^1,​2,​3 recent research has aimed to describe the effect of cervical deformity on health status. ^4,​5 The analysis of outcomes and disability in cervical deformity patients have been less conclusive than those described in thoracolumbar literature, patients who undergo cervical deformity correction have been shown to have significant improvement. ⁶ However, the best methods to describe the morbidity associated with malalignment of the cervical spine have yet to be been fully elucidated. Given the recent importance placed on the assessments of both outcomes and disability in the American healthcare system, it is of paramount importance to identify the optimal methods to capture the disability inherent and specific to cervical deformity and quantify the benefits of correction.

In recent decades, the American healthcare climate has shifted from a volume based focus to a value-based model that emphasizes quality of care over quantity. ⁷ In the current healthcare landscape, the value of care is determined by the outcome obtained following intervention relative to cost. ⁸ In years past, outcome metrics were measured in the form of mortality rates but, more recently, health-related outcome questionnaires have gained popularity as a way to quantify patient outcomes. ⁸ Health-related outcome questionnaires ask relevant questions about a patient’s general health and how a patient’s life has been limited by his or her pathological symptoms.

A number of instruments have since been developed to measure pain, disability, and improvement following treatment in spine surgery, as well as other medical specialties, that have been used in both the clinical and research setting. ⁹ The goal of outcome assessment is to establish baseline disability, evaluate the effectiveness of various treatment options, and educate patients about their anticipated recovery following surgical or medical intervention. ^9,​10

In the spine surgery literature, many outcome metrics have been assessed for their ability to capture disability and postoperative improvement with varying success. In both cervical and thoracolumbar literature, outcome measures have been shown to improve following spine surgery. ^{11,​12,​13,​14} However, the disability inherent in patients with thoracolumbar deformity and cervical deformity is polyfactorial resulting from general frailty, neurologic compromise, as well as spinal malalignment; thus, an optimal outcome metric for sagittal malalignment has not been established.

In thoracolumbar spine literature, various health-related quality-of-life (HRQOL) measures have been shown to correlate with disability resulting from sagittal malalignment of the spine. Radiographic parameters to describe thoracolumbar malalignment have been shown to correlate with various HRQOL scores, with published correlation coefficients ranging from 0.3 to 0.5. ^{2,​3,​15,​16,​17} While several outcome metrics have shown significant correlation with sagittal malalignment of the thoracolumbar spine, studies that have managed to correlate sagittal malalignment with disability in the cervical spine have been much rarer and show more modest correlations. ^{18,​19,​20}

In a study of 113 patients undergoing posterior cervical fusion, Tang et al demonstrated that positive cervical sagittal alignment greater than 40 mm was associated with greater disability by the NDI and SF-36. ¹⁸ Additionally, in a cohort of 122 patients undergoing anterior cervical decompression and fusion (ACDF), Villavicencio et al demonstrated that improved or maintained cervical sagittal alignment was associated with superior outcomes in SF-36 and NDI. ¹⁹ Still, the association between outcome metrics and sagittal malalignment of the cervical spine remains elusive and an all-encompassing outcome metric to describe cervical deformity is yet to be found in the literature.

5.2 Challenges to Capturing Disability from Cervical Deformity

Surgical correction of cervical deformity is associated with increased potential for morbidity and neurologic injury. As such, the need for functional outcome assessment in patients with cervical deformity is great. ²¹ One contributing factor complicating the ability to capture disability from cervical deformity is the nature of neck pain itself. While neck pain is one of the most common complaints nationwide and one of the largest socioeconomic burdens on our current healthcare system, ²² several factors make assessing neck pain difficult. Neck pain is often a multifactorial condition and may be influenced by many factors outside of the cervical neck itself. ^23,​24 Indeed, most causes of common neck pain are unknown. ^9,​25 It has also been suggested that many complaints of primary neck pain have nonorganic causes, which may be associated with psychosocial factors. ²⁶

While neck pain can be difficult to capture with modern disability metrics, capturing disability from cervical deformity has proven to be even more difficult. Primary cervical deformity can result from a myriad of causes, including degenerative causes, congenital causes, or iatrogenic causes. ²¹ Additionally, cervical deformity can lead to a huge range of symptoms such as myelopathy, dysphagia, dyspnea, or severe chin-on-chest deformity which compromises horizontal gaze. ^{4,​27,​28} It is possible that the complex symptom profile of cervical deformity has contributed to the difficulty in developing an all-encompassing outcome metric to describe disability associated with this debilitating condition.

Finally, the applicability of currently utilized disability assessment measures in cervical spine literature has not been sufficiently investigated in cervical deformity populations. While many outcome assessment tools have been investigated for their correlation to various cervical pathologies and cervical malalignment, existing outcome questionnaires were not designed to capture cervical deformity specifically. Existing questionnaires are limited in that they do not currently address disabilities that are unique and debilitating to cervical deformity patients specifically, including symptoms like gaze limitation or swallowing.

It has been suggested that, eventually, a cervical deformity–specific outcome questionnaire may be necessary to assess baseline disability and the success of corrective surgery. Studies reporting on outcomes following cervical spine problems utilize several different outcome metrics in an attempt to best capture the patient’s pain and disability. In general, outcome metrics for patients undergoing cervical deformity correction should capture neck pain, arm pain, and compensatory back pain in addition to limitations in quality of life until more specific outcome metrics to capture the disability inherent in cervical deformity are developed.

5.3 Important Terminology in Describing Outcome Metrics

The utility and applicability of outcome metrics are often assessed based on validity, reliability, and ability to detect change in health. ²⁹ Validity refers to the agreeability of an outcome metric with previously reported outcome metrics. ³⁰ Assessment tools that correlate very strongly with other published measures (associated with Pearson correlations >0.8) are considered to have strong validity. ³⁰

Reliability is a term used to describe the reproducibility of an outcome metric in capturing a given pathology within the same study subject. ³¹ Outcome metrics with good reliability are associated with a great degree of consistency between questionnaire scores at two different time points in the same subject or when measured by two different observers. ³¹ Traditionally, a Pearson coefficient of 0.75 indicates acceptable reliability. ³²

The ability of an outcome metric to detect change in health status is measured using the minimum clinically important different (MCID). In the technical sense, the MCID was defined by Jaeschke et al in 1989 as “The smallest difference in score in the domain of interest which patients perceive as beneficial….” ^33,​34 While there are several methods of calculating the MCID for an outcome metric, calculations attempt to use the metric in question to measure a patient’s internal values and predict changes in medical management. ³³

5.4 Currently Utilized Outcome Metrics of Cervical Deformity

5.4.1 Pain Outcome Metrics

Numeric Rating Scale and Visual Analog Scale

The numeric rating scale (NRS) is one of the most commonly used outcome metrics to capture pain in the clinical setting. ³⁵ Using the NRS Neck, patients are asked to grade their neck pain on a scale of 0 to 10 with 0 being no pain and 10 being the worst pain possibly imaginable. ⁹ The visual analog scale (VAS) is similar to the NRS Neck, but instead pain is graded using visual cues rather than a numeric scale. Most commonly, patients are asked to mark their pain on a continuous 10-cm line, which the evaluator will then measure to obtain a VAS score ranging from 1 to 100 mm. ³⁶

The NRS and VAS are the most common pain metrics used clinically today due to ease of use and success in capturing pain. ⁹ The VAS Neck pain scale has been shown to have strong test–retest reliability, estimated to be 0.95. ³⁷ However, more recent studies have suggested that the reliability of VAS Neck and VAS Arm pain is closer to 0.874 and 0.810, respectively. ³⁶ In general, the VAS Neck pain is believed to be both valid and reliable in regard to assessing pain intensity in the neck and is often used as the gold standard when comparing new rating methods. ^9,​38 However, it is associated with several limitations.

It is well known that symptoms resulting from cervical neck pathologies, like cervical radiculopathy or myelopathy, may manifest as symptoms in the arms or subjacent spine structures. As such, when investigating pathologies of the cervical neck, it is important to capture pain in the neck, the back, and the limbs separately whenever possible. Despite ease of use, strong reliability, and good validity, the NRS and VAS pain scales are limited by the fact that they do not capture disability or symptoms other than pain. For a more comprehensive disability assessment, it is important to assess for motor pathologies, sensory pathologies, autonomic insufficiency, as well as how symptoms are affecting daily life.

5.4.2 Disability Outcome Metrics

Neck Disability Index

The Neck Disability Index (NDI) was developed in 1991 and is the oldest questionnaire developed to measure disability from neck pathologies. ^{39,​40,​41,​42} As of 2008, the NDI had been translated into 22 languages and had been cited in over 300 publications. ⁴³ Unlike the NRS Neck and VAS Neck pain assessment metrics, the NDI has the advantage of measuring cumulative disability rather than simply intensity of pain. The NDI was modeled after the Oswestry Disability Index (ODI), which is a validated and reliable outcome metric widely considered to be the gold standard in capturing disability from back pain. ^{39,​40,​41} Since its development, the NDI has become the most widely used scale in describing neck disability and is widely considered to be one of the most reliable neck disability metrics. ^{39,​42,​43}

Originally, the NDI was designed with six items from the ODI scale including pain intensity, lifting, sleeping, driving, personal care, and sex life. ⁴⁴ Later, four more items were added: headache, concentration, reading, and working resulting in a complete 10-item questionnaire. ^39,​44 The NDI questionnaire was then finalized after being specifically tailored to patients who had undergone whiplash injury to the cervical neck. ^39,​42 Each of the 10 questions in the NDI is evaluated on a scale of 0 to 5, with 0 being no disability and 5 being maximal disability. The total score, which ranges from 0 to 50, may then be doubled to calculate a disability percentage, which has been used to help account for missing questions that have not been answered. ^45,​46 An NDI score of 0 to 4 indicates no disability, a score of 5 to 24 indicates mild disability, a score of 15 to 25 is considered moderate disability, a score of 25 to 34 is considered severe disability, and a score greater than 35 is considered complete disability. ^42,​43

Despite being much more tailored to capturing total disability than the VAS and NRS Neck pain scores, the NDI has been shown to have a relatively small burden in terms of the difficulty in completing the questionnaire. Current literature has suggested that the time needed to complete the questionnaire is less than 10 minutes and the time needed to analyze the questionnaire is about five minutes. ^39,​47

In addition to being accessible and easy to complete, the NDI has also been shown to have strong validity. A systematic review conducted by Macdermid et al found that the NDI correlated strongly with VAS acute pain scores and moderately with the Short-Form 36 questionnaire as well as VAS chronic pain scores. ³⁹ However, measurements of the retest reliability of the NDI have been slightly more ambiguous. While several studies report reliability coefficients greater than 0.9, ^{46,​48,​49} more recent high-powered studies have shown lower reliability. ^45,​50 It has been postulated that the utility of the NDI is pathology specific and may be more reliable for chronic pain than acute pain. ^{39,​46,​49}

Given that disability assessment with the NDI has shown variability based on different pathological states, more research is needed to analyze how well the NDI captures disability in cases of cervical deformity. The NDI was validated in patients presenting to a chiropractic practice and has since been classically used to describe disability from whiplash injuries. ⁴² Unfortunately, the NDI does not account for several crucial pathologic symptoms relevant to cervical deformity, including arm pain, weakness, sensory pain, horizontal gaze, or difficulty swallowing. However, in recent years, an attempt has been made to apply the NDI to cervical deformity literature with mixed results.

While HRQOL outcomes generally fail to correlate substantially with malalignment of the cervical spine, NDI scores have been shown to be decreased at baseline in patients with substantial deformity. Increased cervical sagittal vertical axis in particular has been implicated in poor NDI scores. Tang et al used the NDI to define a threshold of disability of a cSVA greater than 4 cm. ¹⁸ Iyer et al reported that increasing T1S cervical lordosis mismatch (TS-CL) and increased cSVA were both independent risk factors for decreased NDI at baseline. ⁵¹

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