Neurological Examination of the Lower Limbs — Viva

Short Case Viva Defence

Candidate's opening statement (model answer)

"I examined this patient's lower limbs neurologically using the systematic eight-step routine — gait and inspection, tone, power, reflexes, plantar response, coordination, sensory, and stance including Romberg. [1]

On gait, there is a wide-based ataxic gait, steady eyes open but unsteady on eye closure. On inspection, there is bilateral pes cavus with clawing of the toes and a thoracic scoliosis, with reduced distal muscle bulk in the calves. [1]

Tone is normal. [1]

Power is reduced to MRC grade 4 in ankle dorsiflexion and in the distal muscles. [1]

The reflexes are absent at the knee jerk and the ankle jerk bilaterally. [1]

The plantar response is upgoing (extensor) bilaterally. [1]

Coordination — the heel-shin test — shows dysmetria and an intention tremor bilaterally, and the speech has a scanning, staccato quality. [1]

Sensation — vibration is absent at the medial malleoli, and the joint position sense at the great toes is impaired. Pinprick and light touch are reduced distally in a stocking distribution. [1]

The Romberg test is positive — the patient is steady eyes open and unsteady eyes closed. [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. To complete my examination, I would examine the upper limbs for the ataxia and 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 for the diabetes." [1]

Examiner probing questions and model answers

Q1: "Why are you confident this is Friedreich ataxia and not Charcot-Marie-Tooth disease?" [1]

"The single discriminator is the plantar response. In Friedreich ataxia the plantar is upgoing — extensor — because the corticospinal tract is degenerating, producing the upper motor neuron sign. 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 corticospinal tract component alongside the peripheral neuropathy and the dorsal root ganglionopathy [4]. The other supporting features in this patient — the cerebellar ataxia (the heel-shin dysmetria, the wide-based gait), the scanning dysarthria, and the dorsal column loss with a positive Romberg — are absent in Charcot-Marie-Tooth, which is a pure peripheral neuropathy without central nervous system involvement. I would confirm with the genetic test for the GAA trinucleotide repeat expansion in the FXN gene, which is found in homozygosity in about 96 per cent of affected individuals [3]."

Q2: "Walk me through the eight steps of your examination and why you performed them in that order." [1]

"The order is gait and inspection, tone, power, reflexes, plantar, coordination, sensory, and stance. The order matters because each step builds on the last to localise the lesion. Gait and inspection frame the differential — the gait pattern (hemiplegic, parkinsonian, ataxic, high-stepping, waddling) and the inspection findings (wasting, pes cavus, foot deformity, scars) often generate the diagnosis before I touch the patient. Tone is the gateway — spastic tone with clonus means UMN, lead-pipe rigidity means basal ganglia, hypotonic tone means LMN or cerebellum. Power by myotome localises the weakness to a root or a nerve and grades it with the MRC scale. Reflexes confirm the UMN or LMN pattern and, by their level (knee jerk L3/4, ankle jerk S1/2), localise the root. The plantar response is the cardinal UMN sign of the lower limb — the Babinski. Coordination tests the cerebellum (the heel-shin test) and requires intact power. Sensory tests the two spinal cord pathways (the spinothalamic for pinprick, the dorsal column for vibration and joint position). Stance and the Romberg test the dorsal column and complete the functional assessment. Each step narrows the localisation, and the synthesis at the end — the UMN versus LMN framework, with the cerebellar and the dorsal column additions — delivers the diagnosis." [1]

Q3: "What is the significance of the absent ankle jerk with preserved knee jerk, and when do you see it?" [1]

"The dissociated reflex pattern of absent ankle jerks with preserved knee jerks is the classic bedside sign of a length-dependent (axonal) peripheral neuropathy. The longest nerve in the body is the sensory and motor supply to the gastrocnemius (S1, via the tibial nerve), and in an axonal neuropathy the longest nerves fail first. The ankle jerk (S1) is therefore lost before the knee jerk (L3/L4) as the neuropathy advances. The most common cause worldwide is diabetes mellitus, and in an elderly diabetic patient the isolated absence of the ankle jerks with preserved knee jerks is the clinical signature of diabetic peripheral neuropathy. The other causes include alcohol, vitamin B12 deficiency, and the toxic neuropathies. The teaching point is that the candidate who finds absent ankle jerks should test for a stocking sensory loss and should check a fasting glucose and an HbA1c. The contrast is the delayed-relaxation ankle jerk of hypothyroidism — the Woltman sign — where the reflex is present but the relaxation phase is visibly slowed, caused by the altered calcium re-uptake kinetics in the hypothyroid muscle." [1]

Q4: "How do you perform the Romberg test, and what is the difference between a sensory ataxia and a cerebellar ataxia?" [1]

"The Romberg test is performed by asking the patient to stand with their feet together, first with their eyes open and then with their eyes closed, while I stand close enough to catch them if they fall. A positive Romberg is the marked increase in unsteadiness or a fall with eye closure, in a patient who is steady with eyes open. The mechanism is that balance is maintained by the integration of vision, the vestibular system, and proprioception. The Romberg removes the visual input; if the patient then becomes unsteady, it means they were relying on vision to compensate for a proprioceptive (dorsal column) deficit. A positive Romberg therefore indicates a dorsal column lesion — a peripheral neuropathy, subacute combined degeneration from B12 deficiency, or tabes dorsalis. The critical distinction is the cerebellar ataxia: a cerebellar patient is unsteady even with eyes open, because the cerebellum is the organ of coordination, not of proprioception. A patient who is already unsteady eyes open cannot have the Romberg interpreted — the test is designed to detect the increase in unsteadiness with eye closure, and if the baseline is already unsteady, the test is not informative. The teaching point is that a positive Romberg is a dorsal column sign, not a cerebellar sign — the single most commonly confused distinction in neurological examination." [1]

Q5: "How do you distinguish a common peroneal nerve palsy from an L5 radiculopathy at the bedside?" [1]

"Both present with a foot drop, but they are distinguished by three bedside tests. First, inversion — the tibialis posterior. In a common peroneal nerve palsy, inversion is spared, because the tibialis posterior is supplied by the tibial nerve, which is a separate branch of the sciatic nerve and is unaffected by a common peroneal lesion at the fibular neck. In an L5 radiculopathy, inversion is weak, because the tibialis posterior is an L4/L5-supplied muscle and the L5 root is compressed. Second, great toe extension — the extensor hallucis longus. In a common peroneal nerve palsy the EHL is often spared; in an L5 radiculopathy the EHL is weak, because it is the classic L5 myotome. Third, the sensory distribution — in a common peroneal nerve palsy the loss is over the dorsum of the foot and the lateral lower leg (the superficial peroneal nerve territory); in an L5 radiculopathy the loss is dermatomal (L5 — the lateral calf and the dorsum of the foot). The single bedside discriminator is the inversion test: a common peroneal nerve palsy spares inversion, and an L5 radiculopathy does not. I would confirm with nerve conduction studies (which localise the common peroneal nerve lesion to the fibular neck by the conduction block) and an MRI of the lumbar spine (which excludes the L4/L5 disc)." [1]

Q6: "What is the single most important lesson from this case for a registrar performing the lower limb neurological examination?" [1]

"The single most important lesson is that the plantar response is the cardinal sign and the gait is the cardinal observation. In this patient, the diagnosis of Friedreich ataxia rested on the recognition of the upgoing plantar in the setting of areflexia and pes cavus — a combination found in no other common condition. The registrar who tests the plantar response and correctly interprets it has the discriminating sign. The corollary is the gait: the wide-based ataxic gait, observed from the moment the patient walked into the room, framed the differential before I touched the patient. The registrar who skips the gait, or who does not test the plantar, or who records 'positive Romberg' in a patient who was already unsteady eyes open, has missed the diagnosis. The systematic routine — every step, in the same order, every time — and the correct interpretation of each sign toward the localisation, is the core competency the DCE short-case station is testing." [1]