Clinical Reasoning: Adult Patient Presenting With Spine Pain Following a Motor Vehicle Accident

Section 1

A 52-year-old woman with depression, anxiety, noninsulin-dependent diabetes, obesity, asthma, obstructive sleep apnea, gastroesophageal reflux, left ventricular hypertrophy, right sciatica, and a medical history of carcinoma in situ of breast was referred for evaluation of unexplained and refractory spine pain since a motor vehicle accident 2 years earlier. She reported constant severe nonradiating lumbar spine pain, exacerbated with standing and walking, and had been unable to take walks or go hiking, as she did before. She denied limb weakness but endorsed shortness of breath with moderate exertion and occasional choking on food and episodic tingling and numbness of right leg and of both hands, all of which had an insidious onset around the time of the accident. She was independent for all basic activities of daily life. Her family history was significant for being the offspring of consanguineous parents. On physical examination, she had intact function of all cranial nerves. Body habitus precluded inspection of the proximal and trunk muscles, but distal limb muscles showed normal mass. Manual muscle testing showed hip flexors 4+/5 with fluctuating effort and otherwise normal muscle strength throughout. Reflexes were 1+ diffusely without postexercise facilitation. There was no abnormal response to muscle percussion or action myotonia. She had mildly decreased perception of pinprick and light touch sensation in right-sided limbs, without a defined anatomical pattern. Finger-to-nose and heel-to-shin tests were normal. The get up and go test was passed with minimal difficulty. Gait was slow and antalgic, with mild hyperlordosis of the lumbar spine.

Her serum creatine phosphokinase (CPK) level was mildly elevated (526 U/L), but an EMG study was normal (Table). Noncontrast MRI studies of the cervical and thoracic spines showed multilevel degenerative changes without cord or root compression. Noncontrast MRI studies of the lumbosacral spine showed a small disc protrusion at left L3-L4 space, severe narrowing of the right L3-L4 foramen, and atrophy and severe fat replacement of paraspinal musculature, also present with less severity at midthoracic level (Figure). Reporting was conducted according to local institutional review board guidelines and the declaration of Helsinki, and the patient provided informed consent.

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Figure Axial T1 MRI Images of Lower Thoracic (A), Upper Lumbar (B), Midlumbar (C), and Lower Lumbar (D) Spine

Note the severe atrophy/fatty replacement of the paraspinal musculature (red asterisks), with sparing of quadratus lumborus and psoas (white arrowheads).

Section 2

The patient in this vignette presents primarily with back pain. Spinal pain, and more specifically lumbar spinal pain, is highly prevalent and caused predominantly by structural musculoskeletal disorders, but its etiologic spectrum includes potentially harmful causes.¹ Neurologic causes of spine pain include pathologies affecting the spinal roots or paraspinal muscles. With spine pain of short duration, or in the absence of red flags, imaging studies are usually unnecessary.² In this case, a red flag in favor of imaging is her protracted history of refractory spine pain. Spine imaging may reveal atrophy (decreased cross-sectional area) and/or fatty replacement of paraspinal muscles, both of which can be a manifestation of normal, age-related loss in muscular mass and function (sarcopenia).³ Imaging changes related to sarcopenia are notorious in the spine muscles, affect all spine levels, and correlate with common structural pathologies.⁴ Fatty muscle replacement represents up to 20%–30% of muscle volume in older age groups.⁵ However, these changes may also be a manifestation of muscular disease.⁶ The imaging boundaries between age-related changes and pathologic conditions are insufficiently established, and distinction is highly based on the clinical context. In this case, the relatively young age of the patient is inconsistent with sarcopenia as cause of the extensive fatty replacement found on imaging studies.

CPK is a muscular enzyme central in the anaerobic production of adenosine triphosphate and is considered a useful marker of muscle disease. Mild serum CPK elevations are, however, nonspecific and may be due to nonmyopathic etiologies such as motor neuron disorders, cardiac causes, physical activity, or trauma. HyperCKemia is defined as a CPK level above 1.5 times the upper limit of normal according to ethnicity and gender.⁷ Women and people of non-Black ethnicity tend to have lower CPK levels. In this case, the combination of high CPK serum level, extensive fatty replacement of paraspinal muscles, and lumbar hyperlordosis suggested a potential neuromuscular cause.

Section 3

This patient's previous EMG showed normal nerve conduction studies and concentric needle examination of several limb muscles (Table). However, a follow-up concentric needle examination study aiming to include sampling of trunk muscles showed frequent myotonic discharges in those muscles along with profuse fibrillations and positive sharp waves. There were also myopathic motor unit action potentials without significant abnormalities in the motor unit recruitment pattern in trunk muscles. Of importance, there were no needle EMG abnormalities detected in proximal and distal limb muscles, corroborating the findings of the first EMG study (Table). The right median neuropathy at the wrist was considered an incidental finding. Her family history of consanguinity confers an increased risk to recessively inherited genetic disorders.

Section 4

EMG findings indicated the presence of a myopathic disorder with electrical myotonia and, in the context of this patient's family history of consanguinity, recessively inherited myopathies became a strong diagnostic consideration. Electrical myotonia without clinical myotonia, as neurophysiologic manifestation of a nondystrophic myopathy, points to a limited number of causes, such as necrotizing myopathies (i.e., immune-mediated necrotizing myopathies), several muscle channelopathies, and toxic myopathy caused by statins, chloroquine, cyclosporine, colchicine, and other agents. Electrical myotonia may also be seen in myofibrillar and myotubular myopathies, and it is also a typical feature of acid maltase deficiency (Pompe disease). In the clinical context of this patient's family history of consanguinity, late-onset Pompe disease (LOPD) became a strong diagnostic possibility. Panel genetic testing was sent to include DNA analysis of the GAA gene. Results showed 1 homozygous pathogenic variant (c.-32-13T>G) and 1 homozygous variant of uncertain significance (VUS) (c.510C>T) in GAA, and 2 additional VUS in different genes. Biochemical assays confirmed reduced activity of lysosomal acid alpha-glucosidase (GAA) (0.51 μmol/L/h; normal >2.10 μmol/L/h), confirming the diagnosis of Pompe disease. The patient was then referred for multidisciplinary care, including pulmonary, cardiac, ear, nose and throat evaluation, and genetic counselling, and was recommended to start enzyme replacement therapy (ERT) with avalglucosidase alfa.