Left-Sided Pure Trigeminal Motor Neuropathy With Radiologically Isolated Contralateral Subclinical Involvement: A Case Report

A 54-year-old woman presented with slowly progressive left facial asymmetry and jaw weakness over three years, without sensory disturbances or other cranial nerve deficits. Clinical examination revealed wasting of the left-sided muscles of mastication with preserved facial sensation. Magnetic resonance imaging demonstrated marked fatty atrophy involving the left temporalis, masseter, medial, and lateral pterygoid muscles, along with subtle contralateral fatty infiltration of the right medial pterygoid muscle, suggesting possible early subclinical involvement. Fat-suppressed and high-resolution MRI sequences demonstrated no active muscle edema, skull base lesion, or abnormality involving the cisternal segments of the trigeminal nerves. Electromyography supported chronic neurogenic denervation involving the left-sided muscles of mastication. Pure trigeminal motor neuropathy is an uncommon entity characterized by selective motor involvement of the mandibular division of the trigeminal nerve, most commonly presenting unilaterally. This case highlights the role of MRI in identifying characteristic chronic denervation changes, excluding secondary causes, and detecting subtle clinically occult contralateral abnormalities that may otherwise be overlooked. Early recognition of this rare neuropathy may help avoid unnecessary investigations and support appropriate clinical evaluation.