The Nervous System · Part Four · How It Fails and Recovers
Lesson 59 / 61
The Neurological and Metabolic Exam
A structured interrogation of the nervous system that reads behavior, movement, sensation, and the chemistry of the milieu in which neurons fire.
The neurological exam is the localizing instrument that converts a patient story into an anatomical address, asking where a lesion lives before it asks what the lesion is. Built on five components, the mental state, motor, sensory, cranial nerve, and cerebellar systems, it pairs with a metabolic survey because sodium, potassium, and calcium set the very membrane voltages the exam is trying to read. Clinical neuroscience treats the two as one act, since a neuron starved of glucose or floating in the wrong ionic bath cannot be examined in isolation from its chemistry.
Exam components
5: mental state, motor, sensory, cranial nerves, cerebellar
Power scale
0/5 no contraction to 5/5 full resistance
Serum potassium
normal 3.5 to 5.0 mmol/L; over 5.5 abnormal
Ionized calcium
roughly 50% free, 40% protein bound, 10% complexed
01LOCALIZE
Where Before What
The neurological exam is first a localizing instrument, designed to place a lesion in space before naming its pathology. The exam frames the whole sequence around key historical localizing points, clarifying mode of onset, chronological sequence, quality and severity, radiation, and the relieving or exacerbating factors that bracket a chief complaint. From that history the examiner forms a hypothesis about which neuraxis level is failing, then tests it. The exam consists of five components, the mental state exam, the motor system, the sensory system, the cranial nerves, and the cerebellar system, each a probe into a different stratum of the central integrative state the brain maintains moment to moment.
Localization is an act of subtraction. The examiner builds a map of intact function and lets the dark regions, the absent reflex, the lost vibration, the drifting limb, draw the outline of the silent tissue. A symptom is data, but a sign distributed across two successive dermatomes, or a weakness patterned to a single myotome, is an address. The art is refusing to leap to disease before the geography is fixed, because the same final complaint can arise from cortex, cord, root, junction, or muscle, and each demands a different next question.
02COGNITION
Reading The Mind
The mental state exam samples consciousness, orientation, memory, and language as the highest output of the nervous system. The Mini Mental State Exam offers the easiest structured screen, giving the examiner a rough guideline for the questions a full mental assessment requires, with scores below 24 suggestive of dementia. It is not run on every patient, only where cognitive decline is suspected, and for most encounters orientation to person, place, and time, the classic oriented times three, will suffice. The screen probes registration, attention, recall, and constructional praxis, each mapping loosely onto cortical territory.
Behind the bedside questions sits the reticular activating system and the cortex it arouses, the substrate of wakeful awareness. A flat affect, a perseverating answer, a hemineglected page, each points toward frontal, parietal, or limbic dysfunction rather than toward the limbs. The exam treats cognition as the thinnest, most metabolically expensive layer of nervous function, the first to dim when oxygen, glucose, or sodium drift out of range. That fragility is why a confused patient is examined neurologically and chemically at once, because altered mental status is a shared end point of cortical injury and of the metabolic disorders that silence healthy cortex.
03MOVEMENT
Motor And Reflex
The motor exam proceeds through a fixed order, inspection, tone, power, and reflexes, each step localizing differently. Inspection hunts wasting, fasciculation, and posture, the foot drop of pyramidal weakness or the claw hand of ulnar injury. Tone separates the spastic catch of an upper motor neuron lesion from the flaccidity of a lower one. Power is graded zero to five, from no contraction, through a flicker, through movement that cannot beat gravity, to full resistance, and mapped onto myotomes so a single weak segment betrays its root. Reflexes, scored zero to four, anchor the circuit between sensory input and motor output.
Reflex pattern is the cleanest localizer the exam owns. A hyperreflexic limb with an upgoing plantar, the Babinski sign, with a Hoffman sign and clonus, declares corticospinal release above the segment tested, the signature of myelopathy. Absent deep tendon reflexes instead point to root, peripheral nerve, junction, or muscle. In a Brown-Sequard hemicord pattern the exam reads ipsilateral motor and vibration loss against contralateral loss of pain and temperature, a dissociation that only careful side to side comparison reveals. The motor system, examined this way, becomes a ladder the examiner climbs to find the broken rung.
04SENSATION
Tracts In Parallel
The sensory exam answers two questions, whether sensation is lost and what shape that loss takes, by testing two parallel pathways. The spinothalamic system carries pain, temperature, and crude touch, while the dorsal column carries fine touch, joint position sense, and vibration. Because these tracts decussate at different levels, the cord ascending one and the medulla the other, their dissociation localizes with precision. The examiner shows the patient a normal stimulus first to set a baseline, then starts at the center of the complaint and compares each point to the identical spot on the opposite side, moving outward to map the border of the deficit.
Pattern is everything. A glove or stocking distribution suggests peripheral neuropathy, a dermatomal band suggests a root, a sensory level on the trunk suggests cord, and a hemisensory loss suggests thalamus or cortex. Vibration and proprioceptive loss flag dorsal column or large fiber disease, while preserved touch beside lost pin prick flags a spinothalamic lesion. This is the exam reading the Bell-Magendie division of dorsal sensory and ventral motor roots in living tissue, decades of anatomy compressed into a wisp of cotton and a pin moved deliberately across the skin.
The neurological examination does not ask what is wrong before it asks where, because localization is the question the nervous system can actually answer, and chemistry is the bath in which every answer is given.
05CHEMISTRY
The Metabolic Milieu
The metabolic exam recognizes that every sign the neurological exam reads depends on the ionic and metabolic state of the membrane. Serum potassium, normally 3.5 to 5.0 mmol/L and abnormal above 5.5, sets the resting membrane potential directly, and hyperkalemia raises that resting voltage closer to threshold, distorting excitability before any structural lesion exists. Calcium, of which roughly half is free, forty percent protein bound, and ten percent complexed, governs sodium conductance across the neuronal membrane, so hypocalcemia increases sodium entry and lowers the firing threshold, producing hyperexcitability. Sodium disorders, hypernatremia from fluid loss and hyponatremia from water excess, shrink or swell the very cells under examination.
This is why clinical neuroscience binds the two exams into one. A weak, hyporeflexic, confused patient may harbor a cord lesion, or may simply be floating in deranged electrolytes that silence healthy neurons through the central integrative state of each cell. Blood work, pulse oximetry, and glucose belong beside the reflex hammer because a neuron starved or poisoned by its milieu produces signs indistinguishable from anatomical injury. The examiner who reads only the wiring and ignores the chemistry will localize a lesion that does not exist, mistaking a reversible metabolic storm for permanent structural disease.