Lower limb weakness: a challenge diagnosis

A 46-year-old male patient started a condition of lower back pain radiating to the lower limbs in 2016. He underwent lumbar arthrodesis surgery between L5-S1 due to a herniated disc and reports that after the surgery, his clinical condition improved, remaining asymptomatic over four years. In 2020, he started to experience hypoesthesia in the calf and dorsal region of his right foot, with progressive worsening throughout the year, with a tripping sensation associated with a numbness and decreased distal strength of the right lower limb, with subsequent difficulty in climbing stairs over the next year. He continued to follow up with orthopedics and began following up with neurosurgery, which did not identify a clear etiology for his clinical condition. In 2021, the patient was already complaining of worsening weakness in the right foot with foot drop and a decrease in trophism of the entire right lower limb. The patient denies sphincter alterations or sexual dysfunction throughout the course of the disease. In March 2022, the patient began using unilateral support due to falls and returned to the neurosurgery outpatient clinic with a referral to clinical neurology. He denies recent worsening of strength. In October 2022, during a consultation with neurology, treatment with prednisone 20 mg/day was started without improvement in the clinical condition, and during a followup visit in January 2023, the need for diagnostic elucidation led to the patient being hospitalized at the Hospital do Servidor Publico Estadual for etiological investigation. On neurological examination, the patient presented alterations in the right lower limb, with grade IV proximal strength and II distal strength of this limb, including plantar and dorsiflexion flexion, associated with global hypotrophy of the limb up to the gluteal region, with normoactive deep tendon reflexes and plantar reflex in flexion. With regard to sensitivity, the patient presented hypoesthesia in the lateral region of the right leg and the sole of the right foot, worse distally in the topography of L4 L5-S1, with preserved artresthesia and palesthesia. No other alterations were present on the somatic examination, except for a right-hand scraping gait with a component of proximal weakness. In complementary investigation, no signs of spinal cord compression lesions were found on magnetic resonance imaging, no inflammatory signs were found in cerebrospinal fluid results, and no systemic involvement was found in chest and abdominal computed tomography exams, except for asymptomatic hepatosplenomegaly. Electroneuromyography (ENMG) revealed motor impairment with preserved sensory parameters, leading to a preganglionic pattern of impairment, in addition to recent denervation findings with fibrillations and positive sharp waves, which were also visualized in the contralateral lower limb in a lesser degree of involvement. Despite the hypoesthesia on the physical exam, no sensory alterations were demonstrated on ENMG, which may be justified by the previous spinal surgery. Due to findings on the neurological and complementary exams, with impairment only in the right lower limb, substantial atrophy, and weakness with ENMG findings, the hypothesis of flail leg syndrome, a variant of amyotrophic lateral sclerosis (ALS) with lower limbs onset, was raised. Flail leg syndrome is a rare variant (6%) of atypical forms of amyotrophic lateral sclerosis, predominantly affecting males between 55–65 years of age. Its manifestation includes asymmetric and progressive distal onset weakness and wasting with restricted involvement of the lower limbs for at least 12 to 24 months. ENMG shows fibrillations and positive sharp waves, with little evidence of fasciculations. It has a significantly better prognosis in terms of median and 5-year survival rates compared to bulbar and limb onset ALS, due to its slower progression to other locations. FLS ir rare, but it should be considered as an alternative diagnosis when there is limited impairment of lower limbs after at least one year of clinical observation.