Neurological Examination of the Lower Limbs — Clinical Case

DCE Short Case 1 — The Pes Cavus and the Upgoing Plantar

Patient brief (provided to trainee)

Patient: Mr James Nguyen, 24 years old, university student. ^[1]

Presenting complaint: Progressive difficulty walking and frequent falls over the last 5 years, since his late teens. His mother noted his high-arched feet and his clumsy gait from childhood. He has noticed his speech has become slurred over the last 2 years. ^[1]

Instruction: "Please examine this patient's lower limbs neurologically. Present your findings and offer a differential diagnosis." ^[1]

Examination findings (trainee elicits):

• Gait: A wide-based ataxic gait, irregular and staggering, worse on turning. Tandem walking is impossible.
• Inspection: Bilateral pes cavus with clawing of the toes. A thoracic scoliosis. Reduced distal muscle bulk in the calves. No scars.
• Tone: Normal in both lower limbs. No clonus.
• Power: MRC grade 4 in ankle dorsiflexion and plantarflexion bilaterally, with preserved proximal power. Inability to walk on the heels or the toes.
• Reflexes: The knee jerks and the ankle jerks are absent bilaterally.
• Plantar response: Upgoing (extensor) bilaterally.
• Coordination: The heel-shin test shows dysmetria and an intention tremor bilaterally. The foot tapping is dysrhythmic.
• Sensory: Vibration is absent at the medial malleoli and the great toe joint position sense is impaired bilaterally. Pinprick and light touch are reduced in a distal distribution. The Romberg test is positive.
• Speech: Scanning, staccato dysarthria. ^[1]

Candidate's structured presentation (model)

Opening statement: ^[1]

"I examined Mr Nguyen's lower limbs neurologically. On gait, there is a wide-based ataxic gait, irregular and staggering, worse on turning, with inability to tandem walk. On inspection, there is bilateral pes cavus with clawing of the toes, a thoracic scoliosis, and reduced distal calf muscle bulk. Tone is normal with no clonus. Power is reduced to MRC grade 4 in ankle dorsiflexion and plantarflexion bilaterally, with preserved proximal power. The reflexes — the knee jerks and the ankle jerks — are absent bilaterally. The plantar response is upgoing (extensor) bilaterally. Coordination — the heel-shin test — shows dysmetria and an intention tremor bilaterally, and the foot tapping is dysrhythmic. Sensation shows absent vibration at the medial malleoli, impaired joint position sense at the great toes, and reduced pinprick and light touch distally. The Romberg test is positive. The speech has a scanning, staccato quality. ^[1]

My summary is that this patient has the combination of absent lower limb reflexes (a dorsal root ganglion and peripheral neuropathy sign) with upgoing plantar responses (a corticospinal tract sign), pes cavus, scoliosis, a cerebellar ataxia, a scanning dysarthria, and a dorsal column sensory loss. This combination — specifically the areflexia with the extensor plantars — is pathognomonic of Friedreich ataxia, the most common hereditary ataxia. To complete my examination, I would examine the upper limbs for the ataxia and the dysdiadochokinesia, the cranial nerves for the nystagmus and the optic atrophy, the cardiovascular system for the cardiomyopathy, and I would arrange an echocardiogram and a fasting glucose." ^[1]

Examiner discussion questions

Q1: "How confident are you in the diagnosis of Friedreich ataxia, and what would you do to confirm it?" ^[1]

"I am confident on the clinical picture. The combination of long-standing pes cavus, absent lower limb reflexes, upgoing plantar responses, a cerebellar ataxia, a scanning dysarthria, and a dorsal column sensory loss is the classic clinical phenotype of Friedreich ataxia ^[4]. The pathognomonic feature is the areflexia with the extensor plantar — no other common condition produces this specific combination. To confirm, I would arrange the genetic test for the GAA trinucleotide repeat expansion in the FXN gene on chromosome 9 — homozygosity for the GAA expansion in intron 1 is found in about 96 per cent of affected individuals ^[3]. I would arrange an echocardiogram for the hypertrophic cardiomyopathy, which is present in about two-thirds of patients and is the leading cause of death. I would check a fasting glucose and an HbA1c for the diabetes or impaired glucose tolerance. I would examine the upper limbs and the cranial nerves, and I would take a three-generation family history and offer genetic counselling and testing to the parents and the siblings, because the inheritance is autosomal recessive."

Q2: "What is the single bedside discriminator between Friedreich ataxia and Charcot-Marie-Tooth disease?" ^[1]

"The single bedside discriminator is the plantar response. In Friedreich ataxia the plantar is upgoing — extensor — because the corticospinal tract is degenerating. In Charcot-Marie-Tooth the plantar is downgoing — flexor — because it is a pure peripheral neuropathy with no corticospinal tract involvement ^[5]. Both conditions cause pes cavus, both cause absent lower limb reflexes, and both cause a distal sensory loss. But only Friedreich ataxia causes the upgoing plantar, because only Friedreich ataxia has the central nervous system component — the corticospinal tract degeneration and the cerebellar degeneration — alongside the peripheral neuropathy. The other supporting features — the cerebellar ataxia, the scanning dysarthria, and the dorsal column loss with a positive Romberg — are absent in Charcot-Marie-Tooth. This is the high-yield teaching point for the short case: pes cavus with upgoing plantars is Friedreich ataxia, and pes cavus with downgoing plantars and a peripheral neuropathy is Charcot-Marie-Tooth."

Q3: "What is the prognosis, and how would you discuss it with the patient?" ^[1]

"The prognosis of Friedreich ataxia is unfortunately one of progressive disability. The age of onset is typically in the first or second decade (the mean is 10 to 15 years), and the condition progresses over 15 to 20 years, with loss of independent ambulation typically by the third or fourth decade ^[3]. The leading cause of death is the cardiomyopathy — the hypertrophic cardiomyopathy that progresses to heart failure and arrhythmia. The diabetes, the scoliosis, and the sensory neuropathy contribute to the morbidity. I would discuss the diagnosis with honesty, compassion, and appropriate pace, explaining that this is a hereditary neurological condition that affects the coordination and the sensation and the heart. I would explain that there is currently no cure, but that there are treatments that can manage the complications (the cardiomyopathy, the diabetes, the scoliosis) and that there is emerging evidence for omaveloxolone, an Nrf2 activator that slows the progression of the ataxia ^[3]. I would discuss the multidisciplinary team — the neurologist, the cardiologist, the endocrinologist, the physiotherapist, the occupational therapist, the speech pathologist, and the genetic counsellor. I would offer genetic counselling and testing to the family. The single most important principle is that the patient retains autonomy and dignity, and the role of the physician is to inform, to support, and to walk alongside."

Q4: "What is the significance of the cardiomyopathy, and how would you assess it?" ^[1]

"The hypertrophic cardiomyopathy is present in about two-thirds of patients with Friedreich ataxia and is the leading cause of death, from heart failure or arrhythmia ^[3][4]. It is a concentric or asymmetric left ventricular hypertrophy, often interstitial fibrosis, and it may be diagnosed before the neurological presentation. I would assess it with an echocardiogram (the left ventricular wall thickness, the ejection fraction, the diastolic function), a 12-lead ECG (the T-wave inversions, the conduction abnormalities, the arrhythmias), and a cardiology review. I would monitor it serially, because the cardiomyopathy may progress independently of the neurological progression, and the heart failure and the arrhythmia are the targets of the medical management (the ACE inhibitors, the beta-blockers, the diuretics, and the implantable cardioverter-defibrillator for the high-risk arrhythmia)."

Q5: "What is the single most important lesson from this case?" ^[1]

"The single most important lesson is that the plantar response is the cardinal sign and the recognition of the combined areflexia with the extensor plantar is the diagnosis. The registrar who sees the pes cavus and the ataxia and diagnoses a generic 'cerebellar syndrome' or 'hereditary neuropathy' has missed the specific combination that points to Friedreich ataxia. The corollary is that every step of the routine contributes: the gait framed the cerebellar component, the inspection revealed the hereditary foot deformity, the reflexes revealed the areflexia, and the plantar response revealed the corticospinal tract involvement. No single sign is sufficient — the synthesis of all the findings is the diagnosis." ^[1]