Neuroanatomical Localisation and Applied Anatomy

This section synthesises the structural and pathway anatomy covered throughout Sections 3-15 into a systematic framework for localising neurological lesions based on clinical findings, the core applied skill neuroanatomy exists to support.

• UMN lesion = spastic weakness, hyperreflexia, Babinski present, no early atrophy. LMN lesion = flaccid weakness, hyporeflexia, no Babinski, atrophy, fasciculations. This single distinction is the first branch point in localising any motor deficit, a lesion can occur anywhere along the descending pathway from motor cortex to muscle, and determining UMN vs LMN pattern immediately narrows the search to either "above the anterior horn/cranial nerve nucleus" (UMN) or "at or below it" (LMN).

• Cortical lesions produce deficits reflecting the specific functional area(s) involved, often combined with additional "cortical signs" not seen in subcortical lesions, aphasia (dominant hemisphere frontal/temporal involvement), neglect (non-dominant parietal involvement), apraxia (parietal/frontal involvement, inability to perform learned motor tasks despite intact strength/sensation), and cortical sensory signs (astereognosis, graphaesthesia loss, reflecting impaired sensory discrimination/integration despite intact primary sensation), the presence of these higher cortical function deficits, alongside a contralateral motor/sensory pattern that is often non-uniform across the face/arm/leg (reflecting the homunculus's disproportionate cortical territory allocation, so a small cortical lesion may affect the hand much more than the leg, or vice versa, depending on precisely where on the gyrus it sits), distinguishes a cortical lesion from a subcortical one.

• Produce a dense, uniform, complete contralateral hemiparesis/hemiplegia with hemisensory loss, given the compact concentration of somatotopically arranged corticospinal and thalamocortical fibres passing through this small bottleneck, critically, without cortical signs (no aphasia, neglect, or apraxia), since the cortex itself is untouched, this "complete but pure" motor/sensory pattern, uniform across face-arm-leg (unlike the patchy cortical pattern above), is the key distinguishing feature of a capsular (typically lacunar) stroke from a cortical stroke of similar vascular territory.

• The hallmark of a brainstem lesion is a "crossed" deficit pattern: ipsilateral cranial nerve findings (reflecting the cranial nerve nucleus/fascicle's level-specific location, damaged on the same side as the lesion, before its fibres have crossed) combined with contralateral long tract findings (motor via the corticospinal tract, and/or sensory via the medial lemniscus/spinothalamic tract, both of which have either already crossed (spinothalamic, low in the cord) or will cross further down (corticospinal, at the pyramidal decussation, and DCML, at the lower medulla) meaning at most brainstem levels above the lower medulla, these long tracts are already running contralateral to their cortex/body of origin).
• The specific combination of cranial nerves involved precisely localises the lesion to a particular brainstem level (midbrain = III, IV; pons = V, VI, VII, VIII; medulla = IX, X, XI, XII) and, further, to a particular medial/lateral/dorsal/ventral position within that level (reflecting the specific vascular territory and adjacent structures involved, as in the brainstem syndromes: Wallenberg, lateral pontine, medial medullary).

• Hemisphere (lateral) lesions → ipsilateral limb ataxia, dysmetria, intention tremor. Vermis (midline) lesions → truncal/gait ataxia with relatively preserved limb coordination. Cerebellar lesions are notable for producing incoordination despite preserved strength: a key distinguishing feature from both UMN and LMN weakness patterns, and for their ipsilateral laterality (unlike the contralateral pattern of cerebral hemisphere/internal capsule lesions), reflecting the cerebellum's predominantly uncrossed or double-crossed (net ipsilateral) connections.

• Deficits occur below the level of the lesion, reflecting interruption of ascending and/or descending pathways at that point, the specific pattern depends critically on which part of the cord's cross-sectional anatomy is involved, given the segregated, tract-specific organisation of spinal cord white matter, the following classic patterns each reflect a different pattern of cross-sectional involvement.

Hemisection of Spinal Cord (Brown-Séquard Syndrome)

• Damage to one half of the spinal cord (e.g., from penetrating trauma, or a laterally-placed tumour/disc), producing a characteristic dissociated pattern directly reflecting the differing decussation levels of the DCML and spinothalamic pathways:
• Ipsilateral (same side as the lesion), below the lesion level: loss of fine touch, vibration, and proprioception (DCML, these fibres are still ascending uncrossed at the level of the lesion, having not yet reached the medulla) and loss of voluntary motor function (corticospinal tract, already crossed at the pyramidal decussation, high above, so it is running ipsilateral to the body it controls at spinal cord level).
• Contralateral (opposite side to the lesion), below the lesion level: loss of pain and temperature (spinothalamic tract, these fibres crossed within 1-2 segments of their entry point, low in the cord, so by the time they reach a level below the lesion, they are already running on the side opposite their entry, meaning a lesion on one side of the cord interrupts the pain/temperature fibres that originated from the opposite side of the body and have already crossed to ascend on the lesioned side).
• Additionally, at the exact level of the lesion itself, there may be a narrow band of ipsilateral loss of all sensory modalities (from direct damage to the entering dorsal root/root entry zone at that specific segment) and LMN-type weakness (from direct anterior horn damage at that level), a useful additional localising detail distinguishing the lesion's exact segmental level from the pattern of loss below it.

Central Cord Patterns

• Damage centred on the spinal cord's central grey matter and the adjacent, centrally-crossing fibres (classically from syringomyelia: a fluid-filled cavity expanding within the central canal region, or from central cord contusion in cervical hyperextension trauma), produces a characteristic pattern reflecting which fibres pass through versus around this central region:
• Bilateral loss of pain and temperature in a "cape-like" or "suspended" distribution (typically affecting the upper limbs/shoulders in cervical syringomyelia), reflecting damage to the spinothalamic fibres crossing within the anterior white commissure at the affected levels (these fibres must cross directly through the central region to reach the contralateral ascending tract, making them uniquely vulnerable to central pathology, unlike the DCML fibres, which remain peripherally located in the dorsal columns and are relatively spared until the lesion becomes very large).
• Preserved DCML function (fine touch/proprioception/vibration) in early/moderate disease, given the dorsal columns' peripheral location relative to the expanding central lesion, a classic dissociated sensory loss pattern, but anatomically distinct in mechanism from Brown-Séquard syndrome (there, dissociation reflects a lateral hemisection affecting one side's uncrossed DCML vs the opposite side's already-crossed spinothalamic fibres; here, dissociation reflects a central lesion selectively damaging the crossing spinothalamic fibres while sparing the peripheral dorsal columns).
• As the lesion enlarges, it can extend to involve the anterior horns (producing LMN weakness/wasting at the affected levels, classically affecting the small hand muscles in cervical syringomyelia) and eventually the lateral corticospinal tracts (producing UMN weakness below the lesion) and even the peripherally-located dorsal columns in advanced disease.

Dorsal Column Lesions

• Isolated damage to the dorsal columns (e.g., from subacute combined degeneration, classically from vitamin B12 deficiency, which characteristically also affects the corticospinal tracts, producing a combined UMN + dorsal column pattern; or from tabes dorsalis, a manifestation of tertiary neurosyphilis, classically affecting the dorsal columns and dorsal roots relatively selectively), produces loss of fine touch, vibration, and proprioception with preserved pain and temperature sensation (a dissociated pattern, opposite in modality to central cord syndrome's pattern), clinically, proprioceptive loss produces a characteristic sensory ataxia (unsteady gait, worse with eyes closed or in the dark, since the patient loses the proprioceptive feedback normally substituting for visual guidance, a positive Romberg sign, distinguishing this ataxia from the visually-independent cerebellar ataxia).

Spinothalamic Lesions

• Isolated damage to the spinothalamic tract (e.g., from an anterolateral cordotomy, historically performed as a surgical pain-relief procedure, or from anterior spinal artery territory infarction), produces contralateral loss of pain and temperature below the lesion level, with preserved fine touch/proprioception/vibration (the reverse dissociation pattern from the dorsal column lesion above), anterior spinal artery syndrome additionally produces bilateral corticospinal tract involvement (UMN weakness below the lesion, bilateral given the anterior spinal artery's midline territory) alongside this bilateral spinothalamic loss, while sparing the dorsal columns entirely (posterior spinal artery territory), a distinctive combined pattern directly reflecting this specific vascular territory's anatomy.

• Ischaemic stroke reflects occlusion of a specific vessel, producing a deficit pattern corresponding to that vessel's known cortical/subcortical territory, the anatomical territory-to-deficit mapping and 13 is the direct clinical application underlying stroke localisation and the identification of the likely occluded vessel from the clinical examination alone, before imaging confirmation.

• Obstructive (non-communicating, most classically at the cerebral aqueduct) vs communicating (impaired arachnoid granulation reabsorption) causes, distinguished by which specific part of the ventricular system is disproportionately dilated on imaging.

• The Monro-Kellie doctrine (fixed cranial volume = brain + CSF + blood) and the resulting steep, disproportionate pressure rise once compensatory mechanisms are exhausted.

• Raised intracranial pressure, if severe/focal enough, can force brain tissue to shift across the rigid dural reflections (falx cerebri, tentorium cerebelli) or through the foramen magnum, causing direct mechanical compression of adjacent critical structures.

Subfalcine herniation:

• The cingulate gyrus is pushed beneath the falx cerebri, potentially compressing the anterior cerebral artery, often the earliest, most subtle herniation pattern, sometimes clinically silent initially.

Transtentorial (uncal) herniation:

• The medial temporal lobe (uncus) is pushed through the tentorial notch, compressing the ipsilateral CN III (producing an ipsilateral dilated, fixed pupil, the classic earliest clinical sign, given the superficial position of CN III's parasympathetic fibres, mirroring the same vulnerability pattern described for compressive CN III palsy) and, as it progresses, the midbrain itself (potentially producing a contralateral, or, if the herniation compresses the opposite cerebral peduncle against the tentorial edge, a false localising ipsilateral, hemiparesis, termed Kernohan's notch phenomenon), a neurosurgical emergency requiring immediate recognition and intervention.

Tonsillar herniation:

• The cerebellar tonsils are pushed through the foramen magnum, compressing the medulla, causing cardiorespiratory centre dysfunction (given the medulla's vital cardiorespiratory reticular formation nuclei) and rapid deterioration toward death if not immediately relieved, the most life-threatening herniation pattern, given its direct impact on brainstem vital centres.

• Middle meningeal artery rupture (classically following pterion fracture), lentiform haematoma, suture-limited.

• Bridging vein rupture, crescent-shaped haematoma, not suture-limited.

• Classically berry aneurysm rupture at the circle of Willis, sudden thunderclap headache, risk of delayed communicating hydrocephalus.

• First question: UMN or LMN pattern? This immediately divides the search into "above" vs "at/below" the anterior horn or cranial nerve nucleus.
• If UMN: Are there cortical signs (aphasia, neglect, apraxia, non-uniform face/arm/leg involvement)? → cortical lesion. Is the deficit dense, uniform, and pure (no cortical signs)? → internal capsule. Is there a crossed pattern (ipsilateral cranial nerve + contralateral long tract)? → brainstem. Is the deficit below a spinal level with a clear sensory level and/or dissociated sensory pattern? → spinal cord (further subdivide by the specific dissociation pattern: Brown-Séquard, central cord, dorsal column, spinothalamic/anterior spinal artery, each with its own distinctive combination).
• If ataxia without weakness: cerebellar (ipsilateral, hemisphere vs vermis) vs sensory/dorsal column (Romberg positive, worse with eyes closed), a key distinguishing pair.
• If LMN pattern alone: localise to the specific anterior horn/nerve root/peripheral nerve level based on the myotomal/dermatomal or specific nerve distribution (see the relevant regional Gross Anatomy content for peripheral nerve detail).
• This framework represents the practical clinical endpoint that all the preceding structural anatomy in this Neuroanatomy content exists to support.