Scalenus syndrome (Naffziger syndrome): clinical features

Scalenus syndrome is traditionally considered a variant of thoracic outlet syndrome, in which compression occurs in the interscalene space, where the brachial plexus and subclavian artery pass side by side. The clinical interpretation is simple: depending on which is compressed more severely, neurogenic or vascular manifestations predominate. [1]

In modern terminology, three main forms of thoracic outlet syndrome are distinguished: neurogenic, venous, and arterial. These can coexist and partially overlap, so it is more accurate to think of "scalenus syndrome" as a specific anatomical compression zone, and determine the clinical form based on the leading symptoms and signs. [2]

Clinical "mosaic" occurs because the lesion does not necessarily confine itself to a single root or peripheral nerve. With the neurogenic variant, complaints may shift from the neck and shoulder to the arm and hand, fluctuate in intensity throughout the day, and worsen with strain on the upper limb, especially when raising the arm. [3]

In practice, it is important to distinguish between functional compression and compression with structural consequences. In the functional phase, provoked symptoms without persistent vascular changes often predominate, while with a prolonged or pronounced arterial component, signs of stenosis, poststenotic dilation, and distal emboli may appear, which clinically alters the "urgency threshold." [4]

Table 1. Where compression occurs and how it affects the clinical picture

Anatomical zone	What is most often compressed?	Typical clinical features
Interstair space	brachial plexus, subclavian artery	neurogenic symptoms plus possible arterial signs, often provoked by raising the arm [5]
Costoclavicular space	subclavian vein or artery, sometimes nerve structures	more vascular complaints in certain positions of the shoulder girdle [6]
Subcoracoid space of the pectoralis minor muscle	nerve structures and vessels	symptoms often worsen with prolonged work with a raised arm [7]

Neurogenic scalenus syndrome: leading complaints and "geography" of symptoms

The neurogenic variant is associated with compression of the brachial plexus and typically presents with pain, numbness, paresthesia, and weakness that begin in the neck and shoulder area and radiate to the arm and hand. It is often emphasized that complaints do not necessarily correspond to one peripheral nerve or one root and can fluctuate depending on the intensity of upper limb activity. [8]

A typical trigger is a shoulder abduction position and prolonged overhead work. For the patient, this manifests as "the arm quickly tires," "the fingers go numb when the arm is raised," "a pulling sensation from the neck to the arm," and "it gets worse when carrying a bag on the shoulder." This triggering is considered one of the clinical pillars of the neurogenic variant. [9]

The distribution of symptoms is often described in terms of involvement of the upper or lower trunks of the brachial plexus. With predominant compression of the lower trunk, the ulnar edge of the hand and fingers are more often involved, and the strength of the small muscles of the hand may be impaired. With involvement of the upper trunks, more complaints are felt in the neck, shoulder girdle, and lateral aspect of the arm. This is not an absolute rule, but it helps explain why the "same" scalenus syndrome can be experienced differently in different people. [10]

In cases of prolonged or severe neurogenic disease, objective motor signs may occur, including grip weakness and hand muscle atrophy. This is less common than isolated sensory complaints and pain, but the development of persistent weakness and atrophy requires a more rigorous diagnostic evaluation and the exclusion of alternative causes, including radicular compression and peripheral neuropathies. [11]

Table 2. Neurogenic manifestations: what the “upper” and “lower” profile looks like

Profile	Where does pain and numbness occur most often?	What is most often limiting?	Clinical clue
Mainly the upper trunks	neck, shoulder girdle, lateral surface of the arm	arm raise, overhead work	often more proximal pain [12]
Mainly lower trunks	forearm and hand, most often the ulnar edge of the fingers	fine motor skills, hand strength	weakness of the small muscles of the hand is possible [13]
Mixed profile	several zones at the same time	variable	often does not fit into 1 nerve or root [14]

Vascular clinical variants: venous and arterial component

The venous variant manifests itself with signs of venous congestion: arm swelling, pain, a feeling of heaviness, cyanotic skin discoloration, and dilation of the superficial veins. With subclavian vein thrombosis, symptoms can develop suddenly and appear dramatic, requiring urgent evaluation. [15]

The classic scenario of the thrombotic venous variant is known as Paget-Schroetter syndrome and is often associated with repeated use of the upper limb, particularly in young and active individuals. Clinically, it typically presents with sharp pain, rapid swelling, discoloration, and a feeling of fullness, sometimes with prominent venous markings.[16]

The arterial variant is less common, but clinically more dangerous due to the risk of ischemia and embolism. Coldness of the hand, pallor, decreased endurance during exertion, pain similar to ischemic "intermittent claudication" of the upper limb, weakening of the pulse, and, in severe cases, signs of distal digital embolism are possible. [17]

The concept of "functional" and "organic" stages, original to older descriptions, is clinically useful if interpreted correctly. Functional compression often produces evoked symptoms without permanent vascular damage, whereas prolonged arterial compression can lead to the formation of stenosis and poststenotic changes, creating a source of emboli. It is this transition that determines why some patients live with discomfort for years, while others develop an acute vascular episode. [18]

Table 3. Venous and arterial clinical differences

Sign	Venous variant	Arterial variant
Main complaint	swelling, heaviness, pain, cyanosis	coldness, pallor, weak pulse, ischemic pain [19]
Speed of development	often acute with thrombosis	may be gradual, but embolisms are dangerous [20]
Visible veins	collaterals may be expanded	not typical [21]
The main risk	deep vein thrombosis of the upper limb	distal embolization, ischemia [22]

Clinical triggers and variability: what worsens symptoms

Scalenus syndrome is characterized by pronounced posture and load dependence. The most common triggers are prolonged shoulder abduction, external rotation, overhead work, carrying heavy objects, prolonged hand placement on the steering wheel, and repetitive shoulder girdle movements. [23]

Neurogenic complaints often worsen towards the end of the day, after static loads and during prolonged computer work, if the shoulders are held in an elevated position. This is explained by the fact that the muscles of the scalene group and the muscles of the shoulder girdle can maintain a narrowing of the interscalene space and increase irritation of the plexus. [24]

In the venous variant, the trigger often appears as increased swelling and distension after hand activity and a reduction in symptoms at rest. In the arterial variant, the trigger is more often associated with ischemic stress and cold, causing coldness and rapid fatigue of the hand, and sometimes discoloration of the fingers. [25]

The presence of pronounced vegetative manifestations, such as sweating, acrocyanosis, and a feeling of "chillyness," is possible, but they are not specific. Such signs should be considered background manifestations of impaired microcirculation or neurovegetative response, and the clinical form should be determined by leading neurological or vascular criteria. [26]

Table 4. Typical provocateurs and which form reacts most often

Provocateur	What intensifies more often?	Why does this look like scalenus syndrome?
Long-term work with a raised arm	neurogenic symptoms	narrowing of the interscalene space in the pose [27]
Carrying a heavy load on the shoulder	pain and paresthesia	the load on the shoulder girdle changes the position of the clavicle and muscles [28]
Intense arm loading in athletes	venous symptoms, sometimes an acute episode	risk of vein compression and thrombosis [29]
Cold and prolonged strain on the wrist	arterial symptoms	manifestation of ischemia with an arterial component [30]

Examination and provocative tests: what is considered typical and what are the limitations?

In the neurogenic variant, pain and tension in the interscalene space are often detected, and palpation of the supraclavicular region can reproduce symptoms in the arm. This is considered an important clinical clue, but not an independent "proof" test. [31]

Provocative tests are widely used, but their diagnostic accuracy is limited. For example, a systematic review of the Adson test found a sensitivity of approximately 72–92%, but specificity remained low, at 9–53%, with frequent false-positive results. This means that a positive test without a clinical context can easily be misleading. [32]

The problem of false positivity is confirmed by earlier studies: provocative maneuvers often produce false-positive reactions in healthy individuals and even more frequently in patients with other upper limb conditions, such as carpal tunnel syndrome. Therefore, a combination of data is considered clinically more reliable: typical complaints, reproducibility of symptoms, objective signs, and a logical anatomical model of compression. [33]

Vascular signs during examination depend on the type of vascular event. In the venous variant, edema, color, superficial venous pattern, and tenderness along the veins are assessed, while in the arterial variant, skin temperature, capillary refill, pulse, and signs of distal ischemia are assessed. In the presence of such symptoms, clinical priority shifts from a "provocation test" to confirmation of a vascular event and assessment of its urgency. [34]

Table 5. Examination and testing: clinical meaning and limitations

Inspection or test element	What is considered significant	The main limitation
Palpation of the interscalene zone	reproduction of pain and paresthesia in the arm	not specific, requires context [35]
Adson's test	provocation of symptoms, sometimes change in pulse	low specificity, many false positives [36]
Roos test, EAST variant	provocation of symptoms by holding the arms in an abducted position	provokes fatigue and false positive responses [37]
Evaluation of vascular signs	edema, cyanosis, coldness, weak pulse	requires rapid vascular verification if thrombosis or ischemia is suspected [38]

Differential diagnosis and red flags

Scalenus syndrome most often must be differentiated from cervical radiculopathy, peripheral nerve compression syndromes (median and ulnar), shoulder joint and rotator cuff diseases, and neuralgic amyotrophy. A practical clue in favor of thoracic outlet syndrome is the provocation of symptoms by posture and load on the shoulder girdle and an "inaccurate" distribution of complaints that is poorly assigned to a single nerve or root. [39]

For vascular variants, the main mimics are thromboses and embolisms of other origins, as well as vasculitis and Raynaud's phenomenon. When in doubt, clinical safety is higher when treating the situation as vascular until exclusion, especially if arm swelling and discoloration appeared suddenly. [40]

An important feature of scalenus syndrome as a diagnosis is that it often coexists with other areas of thoracic outlet syndrome compression, such as costoclavicular tension or compression in the pectoralis minor muscle area. Therefore, the clinical picture can be mixed and variable, and an isolated "single muscle explanation" is sometimes an oversimplification. [41]

Red flags for urgent evaluation include sudden, severe swelling of the hand with pain and cyanosis, signs of acute hand ischemia, increasing objective weakness and atrophy of the hand muscles, and new ulcers or necrosis of the fingers. These signs elevate the situation from "chronic compartment syndrome" to a potentially urgent vascular or severe neurological condition. [42]

Table 6. What most often mimics scalenus syndrome

Simulator	What is similar	What is more common
Cervical radiculopathy	pain and paresthesia in the hand	more connection with neck movements and dermatomal
Carpal tunnel syndrome	numbness of fingers	more often nocturnal symptoms and the median nerve area, provocative tests for exit from the thoracic outlet are often false positive [43]
Ulnar neuropathy	symptoms in the ulnar edge of the hand	elbow connection, cubital tunnel tests
Shoulder pathology	pain when moving the shoulder	limited range of motion and localized pain in the shoulder
Venous thrombosis of other origin	edema and cyanosis	absence of typical postural dependence, urgent vascular assessment is important [44]