This 45-year old patient presented with numbness and weakness in her right upper limb with signs of mild weakness and wasting of the forearm and hand intrinsic muscles and decreased pinprick sensation over the limb extending to the thorax, anteriorly and posteriorly (C6 to T2). The reflexes in the upper limb were diminished; vibration sense was normal. Examination also revealed hyperreflexia in the lower limb accompanied by mildly increased tone bilaterally with preserved sensory functions. Cranial nerve and cognitive functions were normal.
Is the problem neurological ?
The problem appears to involve the nervous system, since sensory and motor findings were evident with abnormal limb reflexes. Functional inquiry regarding other systems was unremarkable.
Where is the lesion in the nervous system ?
This question is best answered by working from the output (muscle) to the input (CNS) and considering the correlative factors for each anatomical entity which make it more or less likely to be involved in the pathophysiological process:
Muscle - there was no symmetry or a characteristic distribution of muscle weakness. Also, the presence of sensory deficits and lower limb hyperreflexia makes this localization unlikely.
NMJ - unlikely, since only specific muscles were involved with no involvement of NMJs of cranial nerves. Absence of muscle fatigue and the presence of sensory findings and hyperreflexia rule against this location.
Peripheral Nerve - diffuse peripheral nerve disease would account for a simultaneous involvement of sensory and motor systems. However, hyperreflexia observed in the lower limb is not consistent with this hypothesis unless a concurrent myelopathy was present.
Spinal Cord - the spinal cord should always be considered a candidate given its function in relaying both input (sensory) and output (motor) between the periphery and the brain. It is also the place for production of local reflexes. Therefore, its pathology can give rise to abnormalities of reflexes, motor and sensory systems as observed in this case.
Brainstem - lesions of the brain stem often result in cranial nerve abnormalities which are absent in this patient. The clinical findings can help localize to one of the three levels: midbrain (CN 3,4), pons (5,6,7,8) or medulla (9,10,11,12).
Deep White Matter and Thalamus - there are no consistent hemibody sensory or motor findings
Cortex - cortical lesions tend to produce cognitive deficits; there were no cognitive deficits in this case
Cerebellum - cerebellar lesions typically produce deficits in coordination such as ataxia, dysmetria or dysdiadokinesia which are ipsilateral to the side of the lesion and not accompanied by sensory, LMN or UMN findings. Although in this patient some coordination deficits are evident, these are most likely secondary to existing sensory deficit and motor weakness.
Basal Ganglia - lesions of the basal ganglia are typically associated with movement disorders and produce characteristic signs such as muscle rigidity or tremor. No sensory deficit would be expected unless a comorbid lesion elsewhere were involved. Mildly increased tone in the lower limbs and spasticity of gait observed in this patient are more likely to be associated with local UMN disease.
Based on this analysis, the most likely site is the segmental involvement of the lower cervical spinal cord, given a sensory deficit which follows a dermatomal distribution (C6-T2), and localized motor weakness of the right upper limb. The lower limb signs are consistent with involvement of the descending tracts.
This case represents a chronic condition with progressive features. The most likely scenario is worsening of a long standing condition as opposed to a de novo etiology.