Primary lateral sclerosis in an adult patient with progressive evolution of upper motor neuron syndrome: a case report

Abstract

Case Presentation: A 45-year-old man had a four-year history of progressive gait impairment, stiffness, and slowness, first in the lower limbs and later in the upper limbs, followed by dysarthria and intermittent dysphagia. There was no relevant family history. Neurological examination showed spastic tetraparesis with asymmetry, diffuse hyperreflexia, bilateral Babinski and Hoffman signs, and clonus, without sensory loss or early muscle wasting. The initial differential diagnosis included myasthenia gravis, compressive myelopathy, hereditary spastic paraplegia, and other causes of progressive pyramidal syndrome. Brain and spinal magnetic resonance imaging showed only degenerative changes, without a structural explanation for the syndrome. Cerebrospinal fluid, infectious, autoimmune, and metabolic studies were unrevealing. Anti-acetylcholine receptor antibodies were negative. Repetitive nerve stimulation and electroneuromyography showed no evidence of neuromuscular junction dysfunction or lower motor neuron involvement. A transient subjective improvement with pyridostigmine did not change the overall clinical picture. Given the slowly progressive course, the persistent upper motor neuron phenotype, and the absence of lower motor neuron involvement on serial evaluations, primary lateral sclerosis was diagnosed. The patient was managed with baclofen, clonazepam, and multidisciplinary rehabilitation, with partial improvement in spasticity and spasms. Discussion: Primary lateral sclerosis is a rare motor neuron disease characterized by selective upper motor neuron degeneration and a typically slow clinical course. Diagnosis remains challenging and relies on careful exclusion of mimics and longitudinal follow-up. Contemporary consensus criteria acknowledge this diagnostic delay and recognize earlier categories for isolated upper motor neuron syndromes. This case is notable because bulbar symptoms and a misleading partial response to pyridostigmine created diagnostic uncertainty. However, the combination of progressive pyramidal syndrome, normal cerebrospinal fluid, negative neuromuscular junction studies, and repeated absence of lower motor neuron signs supported primary lateral sclerosis rather than amyotrophic lateral sclerosis or structural myelopathy. Final Comments: Primary lateral sclerosis should be considered in patients with progressive spastic tetraparesis and negative workup for mimics, even when bulbar symptoms or early therapeutic responses complicate the picture. Longitudinal reassessment is essential to avoid misdiagnosis and provide accurate prognostic counseling.