Neurological Examination of the Lower Limbs — Written Clinical Reasoning

SAQ 1 — Lower Limb Neurological Examination: Localisation, Diagnosis, and Differential (20 marks, 30 minutes)

Prompt: Outline your systematic approach to this patient's lower limb neurological examination, addressing: (a) the eight-step examination routine and how each step contributes to the localisation; (b) the interpretation of the combined absent reflexes and upgoing plantars and the most likely diagnosis; (c) the differential diagnosis and how you would discriminate between the key candidates (Friedreich ataxia versus Charcot-Marie-Tooth); (d) the investigations you would arrange; (e) the associated systemic findings you would specifically seek to complete the assessment; and (f) the common examination trap in this case. ^[1]

Model Answer

(a) The eight-step examination routine (4 marks): ^[1]

My systematic routine is gait and inspection, tone, power, reflexes, plantar response, coordination, sensory, and stance including Romberg — performed in the same order every time, because each step builds on the last to localise the lesion. ^[1]

On gait, the wide-based ataxic gait indicates cerebellar or dorsal column involvement — the candidate distinguishes by the Romberg test (a sensory ataxia is worse eyes closed; a cerebellar ataxia is present eyes open). On inspection, the bilateral pes cavus with claw toes indicates a long-standing hereditary neurological disorder, and the scoliosis supports a hereditary ataxia. On tone, I assess the hip, the knee, and the ankle, and I elicit clonus. On power, I test each myotome from L2 to S2 with MRC grading. On reflexes, the absent lower limb reflexes indicate a lower motor neuron or a dorsal root ganglion component. On the plantar response, the upgoing plantars indicate an upper motor neuron (corticospinal tract) component. On coordination, the heel-shin dysmetria and intention tremor indicate a cerebellar component. On sensory, the impaired vibration and joint position sense indicate a dorsal column component. On stance, the positive Romberg confirms the dorsal column loss. ^[1]

(b) Interpretation of the combined absent reflexes and upgoing plantars (4 marks): ^[1]

The combination of absent lower limb reflexes (a lower motor neuron or dorsal root ganglion sign) with upgoing (extensor) plantar responses (an upper motor neuron sign) is pathognomonic of Friedreich ataxia — the most common hereditary ataxia ^[3][4]. The mechanism is the simultaneous degeneration of the dorsal root ganglia (which abolishes the reflex arc and produces the areflexia) and the corticospinal tracts (which produces the extensor plantar response). No other common condition produces this specific combination: a peripheral neuropathy produces areflexia with flexor (downgoing) plantars; a cervical or thoracic myelopathy produces brisk reflexes with extensor plantars; a pure cerebellar lesion produces normal reflexes with flexor plantars. The upgoing plantar with the absent reflex is the signature, and it points to Friedreich ataxia until proven otherwise. The associated findings — the pes cavus, the scoliosis, the wide-based ataxic gait, the scanning dysarthria, and the dorsal column sensory loss — complete the clinical picture ^[4].

(c) Differential diagnosis and discrimination (4 marks): ^[1]

The two key candidates that present with pes cavus and a lower limb neurological disorder are Friedreich ataxia and Charcot-Marie-Tooth disease (hereditary motor sensory neuropathy). The single discriminator is the plantar response: in Friedreich ataxia the plantar is upgoing (extensor), and in Charcot-Marie-Tooth the plantar is downgoing (flexor) ^[5]. The other discriminators: Friedreich ataxia has a cerebellar component (the ataxic gait, the heel-shin dysmetria, the scanning dysarthria) and the dorsal column loss with a positive Romberg, whereas Charcot-Marie-Tooth is a pure peripheral neuropathy (distal wasting, the inverted champagne bottle legs, distal weakness, a stocking sensory loss) without cerebellar signs. The other differentials include hereditary spastic paraparesis (a pure spastic paraparesis with preserved reflexes and extensor plantars, but no areflexia, no cerebellar signs, and no pes cavus), subacute combined degeneration of the cord from vitamin B12 deficiency (dorsal column loss with a spastic paraparesis, but an acquired disorder of later onset without pes cavus), and multiple sclerosis (a relapsing-remitting demyelinating disorder in a young adult, with UMN signs, not areflexia).

(d) Investigations (3 marks): ^[1]

My investigation plan is:

1. Genetic testing for the GAA trinucleotide repeat expansion in the FXN gene — the definitive test. Homozygosity for the GAA expansion in intron 1 of FXN is found in about 96 per cent of affected individuals ^[3].
2. Echocardiogram — the hypertrophic cardiomyopathy is present in about two-thirds of patients and is the leading cause of death. I would assess the left ventricular wall thickness and the ejection fraction.
3. Blood glucose and HbA1c — the diabetes or impaired glucose tolerance (from the pancreatic beta-cell dysfunction) occurs in a subset.
4. MRI of the brain and cervical spine — to characterise the cerebellar and cord atrophy and to exclude alternative structural diagnoses.
5. Nerve conduction studies — to demonstrate the sensory axonal neuropathy (the reduced or absent sensory nerve action potentials) that accompanies the dorsal root ganglionopathy.

(e) Associated systemic findings (3 marks): ^[1]

To complete the assessment, I would specifically seek the cardiomyopathy (the hypertrophic cardiomyopathy on the echocardiogram and the ECG — the T-wave inversions and the conduction abnormalities), the diabetes or impaired glucose tolerance (the fasting glucose, the HbA1c, the oral glucose tolerance test), the scoliosis (on the clinical examination and the spine imaging), the optic atrophy (on the fundoscopy), the sensorineural hearing loss, and the autonomic disturbance. I would examine the upper limbs for the associated ataxia and dysdiadochokinesia, and the speech for the scanning dysarthria. I would take a three-generation family history (the inheritance is autosomal recessive) and offer genetic counselling and testing to the parents and the siblings. ^[1]

(f) The common examination trap (2 marks): ^[1]

The trap is diagnosing Charcot-Marie-Tooth on the basis of the pes cavus and the absent reflexes, and missing the upgoing plantars. The candidate who sees the pes cavus and the areflexia and stops at Charcot-Marie-Tooth has missed the cardinal distinguishing sign — the extensor plantar response. The teaching point is that pes cavus with upgoing plantars is Friedreich ataxia until proven otherwise, and pes cavus with downgoing plantars and a peripheral neuropathy is Charcot-Marie-Tooth until proven otherwise. The plantar direction is the single bedside discriminator between the two hereditary disorders, and its active testing and correct interpretation is the core competency the DCE short-case station is testing ^[4][5].